WiTricity notes

WiTricity, a portmanteau for "wireless electricity", is a trademark of WiTricity corporation[1] referring to their devices and processes which use a form of wireless energy transfer including resonant energy transfer etc., the ability to provide electrical energy to remote objects without wires using oscillating magnetic fields. The term WiTricity was used for a project that took place at MIT, led by Prof. Marin Soljačić in 2007.
In April 27, 2011, car maker Toyota made an investment in WiTricity.

Technical background

Overview
WiTricity is based on strong coupling between electromagnetic resonant objects to transfer energy wirelessly between them. This differs from other methods like simple induction, microwaves, or air ionization. The system consists of transmitters and receivers that contain magnetic loop antennas critically tuned to the same frequency. Due to operating in the electromagnetic near field, the receiving devices must be no more than about a quarter wavelength from the transmitter (which is a few meters at the frequency used by the example system). In their first paper, the group also simulated GHz dielectric resonators. The WiTricity devices are coupled almost entirely with magnetic fields (the electric fields are largely confined within capacitors inside the devices), which is argued to make them safer than resonant energy transfer using electric fields (most famously in Tesla coils, whose high electric fields allow them to be used as lightning generators), since most materials couple weakly to magnetic fields (Kurs, 2007). The WiTricity devices are also claimed to be unusual in that they support efficient energy transfer for "mid-range" distances several times larger than the diameter of the resonant objects (Karalis, 2007).
Unlike the far field wireless power transmission systems based on traveling electro-magnetic waves, WiTricity employs near field resonant inductive coupling through magnetic fields similar to those found in transformers except that the primary coil and secondary winding are physically separated, and tuned to resonate to increase their magnetic coupling. These tuned magnetic fields generated by the primary coil can be arranged to interact vigorously with matched secondary windings in distant equipment but far more weakly with any surrounding objects or materials such as radio signals or biological tissue.
In particular, WiTricity is based on using 'strongly-coupled' resonances to achieve a high power-transmission efficiency. Aristeidis Karalis, referring to the team's experimental demonstration, says that "the usual non-resonant magnetic induction would be almost 1 million times less efficient in this particular systemThe researchers suggest that the exposure levels will be below the threshold for FCC safety regulations, and the radiated-power levels will also comply with the FCC radio interference regulations.
Researchers attribute the delay in developing wireless-power technology to limitations of well-known physical laws and a simple lack of need. Only recently have modern consumers obtained a high number of portable electronic devices which currently require batteries and plug-in chargers.[3]
[edit]Experimental demonstration
The MIT researchers successfully demonstrated the ability to power a 60 watt light bulb wirelessly, using two 5-turn copper coils of 60 cm (24 in) diameter, that were 2 m (7 ft) away, at roughly 45% efficiency.[6] The coils were designed to resonate together at 9.9 MHz (≈ wavelength 30 m) and were oriented along the same axis. One was connected inductively to a power source, and the other one to a bulb. The setup powered the bulb on, even when the direct line of sight was blocked using a wooden panel. Currently, researchers have been able to power a 60 watt light bulb at roughly 90% efficiency at a distance of 3 feet[citation needed].
The emerging technology was demonstrated by Eric Giler, CEO of the US firm WiTricity, at the TED Global Conference held at Oxford in July 2009.[7][8] In this demonstration, Giler shows a WiTricity power unit powering a television as well as three different cell phones, the initial problem which inspired Soljacic to get involved with the project.
[edit]Radiation levels
See also: Electromagnetic radiation and health
The company's FAQ [1] claims that it uses a "non-radiative mode of energy transfer, relying instead on the magnetic near field. Magnetic fields interact very weakly with biological organisms—people and animals—and are scientifically regarded to be safe." No actual studies or reports are claimed of the specific technology, power levels and use in home environments but it does claim that "WiTricity products are being designed to comply with applicable safety standards and regulations." No clinical field study radiation levels from in-home tests are reported on the WiTricity web site as of November 2010.

Who Invented Touch Screen Technology?

Touch Screen Technology - How It Works

There are three components used in touch screen technology:

• The touch sensor is a panel with a touch responsive surface. Systems are built based on different types of sensors: resistive (most common), surface acoustic wave, and capacitive (most smart phones). However, in general sensors have an electrical current running through them and touching the screen causes a voltage change. The voltage change signals the location of the touching.
• The controller, is the hardware that converts the voltage changes on the sensor into signals the computer or other device can receive.
• Software tells the computer, smartphone, game device, etc, what's happening on the sensor and the information coming from the controller. Who's touching what where; and allows the computer or smart phone to react accordingly.

Of course, the technology works in combination with a computer, smart phone, or other type of device.

Resistive & Capacitive Explained

According to Malik Sharrieff, an eHow Contributor, "the resistive system is comprised of five components, including the CRT (cathode ray tube) or screen base, the glass panel, the resistive coating, a separator dot, a conductive cover sheet and a durable top coating."

When a finger or stylus presses down on the top surface, the two metallic layers become connected (they touch), the surface acts as a pair of voltage dividers with connected outputs. This causes a change in the electrical current. The pressure from your finger causes conductive and resistive layers of circuitry to touch each other, changing the circuits' resistance, which registers as a touch screen event that is sent to the computer controller for processing.

apacitive touch screens use a layer of capacitive material to hold an electrical charge; touching the screen changes the amount of charge at a specific point of contact.

History of Touch Screen Technology

1960s

Historians consider the first touch screen to be a capacitive touch screen invented by E.A. Johnson at the Royal Radar Establishment, Malvern, UK, around 1965 - 1967. The inventor published a full description of touch screen technology for air traffic control in an article published in 1968.

1970s

In 1971, a "touch sensor" was developed by Doctor Sam Hurst (founder of Elographics) while he was an instructor at the University of Kentucky. This sensor called the "Elograph" was patented by The University of Kentucky Research Foundation. The "Elograph" was not transparent like modern touch screens, however, it was a significant milestone in touch screen technology. The Elograph was selected by Industrial Research as one of the 100 Most Significant New Technical Products of the Year 1973.

In 1974, the first true touch screen incorporating a transparent surface came on the scene developed by Sam Hurst and Elographics. In 1977, Elographics developed and patented a resistive touch screen technology, the most popular touch screen technology in use today.

In 1977, Siemens Corporation financed an effort by Elographics to produce the first curved glass touch sensor interface, which became the first device to have the name "touch screen" attached to it. On February 24, 1994, the company officially changed its name from Elographics to Elo TouchSystems.

• Elographics Patents
• US3662105: Electrical Sensor Of Plane Coordinates
  Inventor(s)Hurst; George S., Lexington, KY - Parks; James E., Lexington, KY
  Issued/Filed Dates:May 9, 1972 / May 21, 1970
• US3798370: Electrographic Sensor For Determining Planar Coordinates
  Inventor(s)Hurst; George S. , Oak Ridge, TN
  Issued/Filed Dates:March 19, 1974 / April 17, 1972

1980s

In 1983, the computer manufacturing company, Hewlett-Packard introduced the HP-150, a home computer with touch screen technology. The HP-150 had a built in grid of infrared beams across the front of the monitor which detected finger movements. However, the infrared sensors would collect dust and require frequent cleanings.

1990s

The nineties introduced smart phones and handhelds with touch screen technology. In 1993, Apple released the Newton PDA, equipped with handwriting recognition; and IBM released the first smart phone called Simon, which featured a calendar, note pad, and fax function, and a touch screen interface that allowed users to dial phone numbers. In 1996, Palm entered the PDA market and advanced touch screen technology with its Pilot series.

2000s

In 2002, Microsoft introduced the Windows XP Tablet edition and started its entry into touch technology. However, you could say that the increase in the popularity of touch screen smart phones defined the 2000s. In 2007, Apple introduced the king of smart phones, the iPhone, with nothing but touch screen technology

X-Ray Defender

Backscatter X-Ray Detector

Backscatter X-ray imaging devices have been in the news of late, primarily due to their use in U.S. airports and other public facilities. But, not everyone is aware that the technology is also being deployed in ordinary vans that can image the insides of passing cars and trucks, and even peer into the interior of homes and businesses. In order to penetrate the exteriors of vehicles and buildings, the x-ray beam is substantially more intense. In the wrong hands, the beam could even serve as an undetectable weapon. That is, until now. the X-Ray Defender is designed to detect the short, powerful X-ray beam from such scanners, giving the owner time to hightail it out of there, before his hair catches on fire. Remember, the X-rays will zip right through aluminum foil hats, too!

The X-ray Defender employs an ordinary PIN photodiode to detect x-rays, instead of light. The photodiode must be kept in the dark, hence the heatshrink tubing sealed with "liquid tape" cap. The uncovered diode can be seen in the insert on top of the battery. It isn't clear how powerful the scanning beam is, so it's possible that the photodiode could directly drive the SCR, without any amplification. But, it was decided to enhance the sensitivity, stretch the response to short pulses, and make the device respond only to sudden field changes. The prototype also has a small block of scintillation plastic sitting on top of the photodiode to further enhance the sensitivity, but the beam will almost certainly trigger this circuit without it. The amplifier is intentionally "starved" for current so that it doesn't respond to smaller signals, causing false triggering and the standby current is virtually zero.

Here's how it works: The powerful pencil beam of x-rays sweeps across the photodiode, causing a sudden current flow from the diode into the base of the MPSW45. The collector pulls low and current flows up through the 1N5711, clamping the gate of the SCR near zero volts. The dwell time of the pulse isn't known, but this circuit will respond rather quickly. When the x-ray beam moves away, the transistor turns off and the voltage on the collector swings up. The other side of the coupling capacitor also swings up and current flows into the gate of the SCR, triggering it. This slower waveform has plenty of width and energy to trigger a sensitive-gate SCR. The buzzer is simply reset by cycling power, a fairly easy operation to perform while running. When power is first applied, the circuit will trigger. Let it buzz for a few seconds then turn the power off and back on quickly. Try again if it continues to buzz. When the power is on and the unit is not triggered and buzzing, the power consumption is practically zero, so the unit may be left on. The tendency to trigger when power is first applied serves as a circuit/battery check.

Component notes: The PIN photodiode in the prototype is an Advanced Photonics, SD200-11-31-241 which has a 0.2" active area. Most other types should work. The high-value 62 megohm resistor provides a path to ground for any leakage and it could probably be left out for even more sensitivity, if there is no significant leakage from the photodiode. Or, a 22 megohm could be substituted. The NPN darlington isn't critical, but it should be a modern small-signal type. The 1N5711 is a small-signal schottky diode but an ordinary silicon diode will also work. The SCR should be a sensitive-gate type, but substitutions are fine. The buzzer is a low-current type designed to operate on 9 volts. The capacitor across it keeps the intermittent current it consumes from resetting the SCR.

Keep the unit nearby or build several.

Continental USA Breakfast

Class-A Audio Amplifiers

A class-A audio amplifier is pretty wasteful of power but when plenty of power is available the simplicity is attractive. Here is a simple darlington transistor example intended for use with a 5 volt power supply:

This circuit and the following aren't for beginners; they are of limited usefulness and require an understanding of the underlying principles and potential applications. They all pass DC through the speaker which is wasteful and can cause problems for the inexperienced builder. If built without variation, they should perform as described but make sure to read the text.

The 5 volts should be provided by a regulated power supply. The efficiency is below 25% and significant DC current flows in the speaker and that additional power should be figured in to the power rating of the speaker. But look how simple it is! The voltage gain is only about 20 and the input impedance is about 12k. The schematic shows two values of bias resistor to be used with the corresponding speaker impedance. With the 150k bias resistor and 8 ohm speaker, the circuit draws about 210mA (1 watt) and can deliver about 250 mW to the speaker which is plenty of volume for most small projects.   The speaker should be rated at 500 mW or more and should exhibit a DC resistance near 8 ohms (perhaps 7 ohms). Check the candidate speaker with an ohmmeter; much below 7 ohms will cause excessive current draw. With the 220k resistor and 16 ohm speaker, the circuit draws about 100 mA (500 mW) and delivers about 125 mW to the speaker. The 16 ohms speaker should be rated at 200 mW or more and exhibit nearly 16 ohms of DC resistance. (Most small speakers have a DC resistance near the rated impedance and that resistance is used to set the quiescent current level in this circuit.) Other NPN darlington transistors will work but choose one that can dissipate 1 watt minimum. Most power types don't need a heatsink but tiny TO92's might overheat.

If the inefficiency of the class-A hasn't dissuaded you yet, here is a 4-transistor amplifier suitable for small signals:

The input impedance is about 5000 ohms and the frequency response is flat from 30 Hz to over 20,000 Hz. With the 8 ohm speaker the current drain is about 215 mA and the gain is about 1700 (64 dB). With the 16 ohm speaker the current gain is about 110 mA and the gain is about 2500 (68 dB).  A volume control may be added by connecting one end of a 5k potentiometer to ground, the wiper to the amplifier input. The other end of the pot becomes the input.

Lets face it; just about any of the various IC audio amplifiers make more sense than this inefficient design. But, this circuit uses parts with only 3 legs. Umm, it doesn't use large capacitors except for the power supply bypassing. Lets see, its more fun-ariffic.  Well, lets see if we can come up with a project that takes advantage of the inefficiency:

So, what is it?

It is a modulated light sender! Connect the input to an audio source or microphone (a speaker will work) and the audio will amplitude modulate the light intensity. The inefficiency of the class-A works in our favor now, lighting the lamp to mid-brightness with no audio present. Actually, with a 4.7 volt bulb, the lamp will be near full brightness and will be "overdriven" on sound peaks. A higher voltage bulb will last longer but will be dimmer. Try a 6.8 volt bulb as a compromise. With a sensitive detector like a phototransistor, this communicator will work several hundred feet (at night). Best range is realized if the bulb is mounted in a typical flashlight reflector and the detector is similarly mounted. The input capacitor is reduced to .01 uF to give the amplifier a high-pass character to compensate for the slow response of the bulb. The audio will sound a bit muffled, anyway. The clever designer could use this amplifier for the receiver, too, switching the speaker to the input for transmitting and to the output for listening. If you choose a detector with good infrared response, like a pin photo diode, you can add plastic IR filters to block out ambient light and make the communicator harder to see at night.

Increasing the voltage to 12 VDC, replacing the bulb with a  3 watt, 16 ohm speaker and replacing the .01uF with a 1uF gives an audio amp that will deliver nearly 1 watt of audio power. The speaker will get warm, however! (Due to the nearly 2 watts of DC power in the speaker coil.)

how to make Crystal Radio (and other purpose) Audio Amplifier

Crystal Radio (and other purpose) Audio Amplifier

Here is a simple audio amplifier using a TL431 shunt regulator. The amplifier will provide room-filling volume from an ordinary crystal radio outfitted with a long-wire antenna and good ground. The circuitry is similar in complexity to a simple one-transistor radio but the performance is superior (with the exception of the amazingone-transistor reflex ). The TL431 is available in a TO-92 package and it looks like an ordinary transistor so your hobbyist friends will be impressed by the volume you are getting with only one transistor and the amplifier may be used for other projects, too. Higher impedance headphones and speakers may also be used. An earphone from an old telephone will give ear-splitting volume and great sensitivity! The 68 ohm resistor may be increased to several hundred ohms when using high impedance earphones to save battery power.

Here is the amplifier used to boost the output from a simple crystal radio. The volume control is at the bottom left and the other components are on the terminal strip at the bottom of the picture. This is a really quick and easy audio amplifier!

how to make Op-Amp Audio Amplifier

Op-Amp Audio Amplifier

The above circuit is a versatile audio amplifier employing a low cost LM358 op-amp. The differential inputs give the amplifier excellent immunity to common-mode signals which are a common cause of amplifier instability. The dotted ground connection represents the wiring in a typical project illustrating how the ground sensing input can be connected to the ground at the source of the audio instead of at the amplifier where high currents are present. If the source is a power supply referenced signal then one of the amplifier inputs is connected to the positive supply. For example, an NPN common-emitter preamplifier may be added for very high gain and by connecting the differential inputs across the collector resistor instead of from collector to ground, destabilizing feedback via the power supply is greatly reduced.

My utility amplifier was built into an aluminum Bud box and eventually ended up bolted to the bottom of a shelf as shown. The well-behaved and ready-to-go amplifier is really handy.

As is often the case, the circuit values are not critical. Other op-amps will usually work but a bit of experience may be necessary if problems develop. The two 4.7 ohm resistors in the emitters may be replaced with a single 10 ohm resistor in either position - I just like the symmetry!

how to make 4-Transistor Amplifier for Small Speaker Applications

4-Transistor Amplifier for Small Speaker Applications

The circuit above shows a 4-transistor utility amplifier suitable for a variety of projects including receivers, intercoms, microphones, telephone pick-up coils, and general audio monitoring. The amplifier has a power isolation circuit and bandwidth limiting to reduce oscillations and "motorboating". The values are not particularly critical and modest deviations from the indicated values will not significantly degrade the performance.

Three cell battery packs giving about 4.5 volts are recommended for most transformerless audio amplifiers driving small 8 ohm speakers. The battery life will be considerably longer than a 9 volt rectangular battery and the cell resistance will remain lower over the life of the battery resulting in less distortion and stability problems.

The amplifier may be modified to work with a 9 volt battery if desired by moving the output transistors' bias point. Lowering the 33k resistor connected from the second transistor's base to ground to about 10k will move the voltage on the output electrolytic capacitor to about 1/2 the supply voltage. This bias change gives more signal swing before clipping occurs and this change is not necessary if the volume is adequate.

As before, the two 4.7 ohm resistors may be replaced with a single 10 ohm resistor in series with either emitter.

how to makeComputer Audio Booster

Here is a simple amplifier for boosting the audio level from low-power sound cards or other audio sources driving small speakers like toys or small transistor radios. The circuit will deliver about 2 watts as shown.  The parts are not critical and substitutions will usually work.  The two 2.2 ohm resistors may be replaced with one 3.9 ohm resistor in either emitter.

Curiously Low Noise Amplifier

Curiously Low Noise Amplifier

The Curiously Low Noise Amplifier takes advantage of the wonderful noise characteristics of the 2SK170 JFET that boasts a noise voltage below 1 nV/root-Hz and virtually no noise current. The noise voltage of the amplifier is only 1.4 nV/root-Hz at 1 kHz, increasing to only 2.7 nV/root-Hz at 10 Hz. The noise current is difficult to measure, so this simple utility amplifier can see the noise from a 50 ohm resistor and a 100k resistor, too. (The 1.4 nV input-referred noise will increase to about 1.7 nV with a 50 ohm resistor, instead of a short, and a 100k resistor will give an input-referred noise near 40 nV, with very little contribution from the amplifier.)

This amplifier is a "utility" amplifier with a gain of 100, that would typically be used in a lab setting to boost tiny signals for measurement or further processing. It isn't intended to drive a speaker or headphones directly. (It could drive the LM386 quite nicely.) The circuit is a simple discrete transistor feedback circuit with two gain stages and a unique class-A output buffer:

• The 2sk117 is from the "BL" Idss current range and is selected for an Idss near 7 mA. The drain resistor is adjusted to achieve about 4 volts on the drain and the value depends on the Idss of the JFET.
• Most of the resistors aren't critical, but precision values are shown because the resistors should be metal film types for best noise performance. Approximate DC voltages are shown for helping with resistor selection. Deviating from the shown voltages will reduce the available output voltage swing, but the amplifier might work fine for smaller signals. Unloaded swing should be about 6 volts, p-p with about 60 mV p-p input, before distortion is observed.
• The MPSA18 acts as a noise filter. High gain is desirable here to keep the value of the base filter capacitor reasonable, but a 2N4401 could be substituted by reducing the 10k and 120k by a factor of 5. The filter will still be rolling off the noise voltage from the 15 volts supply above about 0.2 Hz. But some power supplies can be really noisy!
• The 0.1 uF capacitors serve as bypass capacitors but mainly as terminals for holding the components. These are the white rectangles seen in the photo.
• The feedback resistor is selected for a gain of exactly 100 and the value is well above the expected 1k, due to the limited open-loop gain of the simple circuit.
• A small resistor is included in series with the output for stability and that resistor can reduce the gain a bit when driving a lower resistance load. The designer may choose to set the gain for that particular load, say 75 ohms, or for a high impedance load. The circuit can drive a lower resistance than 100 ohms,  but the swing will be somewhat limited. It may be possible to leave out the 33 ohm resistor without stability issues. (Usually, such a utility amplifier is driving a much higher resistance load, typically 600 ohms or above.) Note: To give you an idea of how you can play with the output resistance, I just changed my unit's series output resistor to 55 ohms and adjusted the gain for 55 dB, when driving 75 ohm loads. Unloaded, the gain is exactly 5 dB higher at 60 dB. This way, I have even number gains whether driving a 75 ohm instrument or a high-Z device. The output buffer has no trouble driving the total 125 ohm load, with a swing limit of about 3.5 volts, p-p.
• The output stage is an unusual self-biasing arrangement where the PNP holds the gate-source voltage near 0.6 volts, running the JFET somewhat below its Idss. The 2N5486 was chosen to not waste too much current, but a higher Idss JFET will give more drive capability, if desired.

• Input Impedance: 47 megohm (set by bias resistor), shunted by 20 pF
• Output Impedance: 36 ohms, set by series resistor plus about 3 ohms from the circuit. My 55 ohm resistor mentioned above gives an output Z of about 58 ohms and exactly 5 dB of gain loss from no load to 75 ohms.
• Output voltage swing: 6 volts p-p into a high impedance load.
• Gain: 100 (40 dB) set by feedback resistor. Lower gain could be selected for wider bandwidth.
• Frequency Response: flat from below 1 Hz to above 2 MHz.
• Input Noise: 1.4 nV, rising to 2.7 nV at 10 Hz. Noise current has eluded measurement so far, but it's really low. With a 97.3 k resistor (100k in parallel with 3.6 meg) connected across the input, the noise voltage measures within a tiny fraction of a dB of 40 nV, so little to no noise current is seen. In fact, this amp and a selected resistor make an inherently accurate noise source. Connect a 152k across the input (in a shielded box), and you have a precise 5 uV/root-Hz noise source throughout the audio spectrum (50 nV times 100). A quick measurement at 40 Hz gives 770 nV/root-Hz with nothing connected; the 47 megohm is expected to contribute 867 nV. That's pretty close and still little noise current from the FET.

For even better performance, the bipolar stages could be replaced with a low noise op-amp. The input noise would drop a little, perhaps to 1 nV, as would the input capacitance, perhaps below 10 pf. Compensating the op-amp might be a bit of a challenge.

Simple LM386 Audio Amplifier

This simple amplifier shows the LM386 in a high-gain configuration (A = 200). For a maximum gain of only 20, leave out the 10 uF connected from pin 1 to pin 8. Maximum gains between 20 and 200 may be realized by adding a selected resistor in series with the same 10 uF capacitor. The 10k potentiometer will give the amplifier a variable gain from zero up to the maximum

How Cell Phone Jammers Work

Introduction to How Cell Phone Jammers Work

 Cell phones are everywhere these days. According to the Cellular Telecommunications and Internet Association, almost 195 million people in the United States had cell-phone service in October 2005. And cell phones are even more ubiquitous in Europe.
It's great to be able to call anyone at anytime. Unfortunately, restaurants, movie theaters, concerts, shopping malls and churches all suffer from the spread of cell phones because not all cell-phone users know when to stop talking. Who hasn't seethed through one side of a conversation about an incredibly personal situation as the talker shares intimate details with his friend as well as everyone else in the area?
­While most of us just grumble and move on, some people are actually going to extremes to retaliate. Cell phones are basically handheld two-way radios. And like any radio, the signal can be disrupted, or jammed.
In this article, you'll see how cell-phone jammers work and learn about the legality of their use.


Cell Phone Jamming Basics

Disrupting a cell phone is the same as jamming any other type of radio communication. A cell phone works by communicating with its service network through a cell tower or base station. Cell towers divide a city into small areas, or cells. As a cell-phone user drives down the street, the signal is handed from tower to tower.A jamming device transmits on the same radio frequencies as the cell phone, disrupting the communication between the phone and the cell-phone base station in the tower.

It's a called a denial-of-service attack. The jammer denies service of the radio spectrum to the cell-phone users within range of the jamming device.

 

Cell Phone Jamming Device

Jamming devices overpower the cell phone by transmitting a signal on the same frequency and at a high enough power that the two signals collide and cancel each other out. Cell phones are designed to add power if they experience low-level interference, so the jammer must recognize and match the power increase from the phone.
Cell phones are full-duplex devices, which means they use two separate frequencies, one for talking and one for listening simultaneously. Some jammers block only one of the frequencies used by cell phones, which has the effect of blocking both. The phone is tricked into thinking there is no service because it can receive only one of the frequencies.
Less complex devices block only one group of frequencies, while sophisticated jammers can block several types of networks at once to head off dual-mode or tri-mode phones that automatically switch among different network types to find an open signal. Some of the high-end devices block all frequencies at once, and others can be tuned to specific frequencies.
To jam a cell phone, all you need is a device that broadcasts on the correct frequencies. Although different cellular systems process signals differently, all cell-phone networks use radio signals that can be interrupted. GSM, used in digital cellular and PCS-based systems, operates in the 900-MHz and 1800-MHz bands in Europe and Asia and in the 1900-MHz (sometimes referred to as 1.9-GHz) band in the United States. Jammers can broadcast on any frequency and are effective against AMPS, CDMA, TDMA, GSM, PCS, DCS, iDEN and Nextel systems. Old-fashioned analog cell phones and today's digital devices are equally susceptible to jamming.
The actual range of the jammer depends on its power and the local environment, which may include hills or walls of a building that block the jamming signal. Low-powered jammers block calls in a range of about 30 feet (9 m). Higher-powered units create a cell-free zone as large as a football field. Units used by law enforcement can shut down service up to 1 mile (1.6 km) from the device.

Inside Cell Phone Jammers

Electronically speaking, cell-phone jammers are very basic devices. The simplest just have an on/off switch and a light that indicates it's on. More complex devices have switches to activate jamming at different frequencies. Components of a jammer include:

Antenna

Every jamming device has an antenna to send the signal. Some are contained within an electrical cabinet. On stronger devices, antennas are external to provide longer range and may be tuned for individual frequencies.

Circuitry

The main electronic components of a jammer are:

• Voltage-controlled oscillator - Generates the radio signal that will interfere with the cell phone signal
• Tuning circuit - Controls the frequency at which the jammer broadcasts its signal by sending a particular voltage to the oscillator
• Noise generator - Produces random electronic output in a specified frequency range to jam the cell-phone network signal (part of the tuning circuit)
• RF amplification (gain stage) - Boosts the power of the radio frequency output to high enough levels to jam a signal

Power supply

Smaller jamming devices are battery operated. Some look like cell phone and use cell-phone batteries. Stronger devices can be plugged into a standard outlet or wired into a vehicle's electrical system.

• ­ Check your phone - If the battery on your phone is okay, and you'd like to continue your conversation, try walking away from the area. You may be able to get out of the jammer's range with just a few steps.

Cell Phone Jammer Applications

Cell phone jamming devices were originally developed for law enforcement and the military to interrupt communications by criminals and terrorists. The bombs that blew up commuter trains in Spain in March 2004, as well as blasts in Bali in October 2002 and Jakarta in August 2003, all relied on cell phones to trigger explosives. It has been widely reported that a cell-phone jammer thwarted an assassination attempt on Pakistani President Musharraf in December 2003. When President Bush visited London in November 2004, it was reported that British police considered using jammers to protect the president's motorcade through London.
During a hostage situation, police can control when and where a captor can make a phone call. Police can block phone calls during a drug raid so suspects can't communicate outside the area. Cell-phone jammers can be used in areas where radio transmissions are dangerous, (areas with a potentially explosive atmosphere), such as chemical storage facilities or grain elevators. The TRJ-89 jammer from Antenna System & Supplies Inc. carries its own electrical generator and can block cellular communications in a 5-mile (8-km) radius.
Corporations use jammers to stop corporate espionage by blocking voice transmissions and photo transmissions from camera phones. On the more questionable end of the legitimacy spectrum, there are rumors that hotel chains install jammers to block guests' cell-phone usage and force them to use in-room phones at high rates.

Photo courtesy Netline Communications Technologies (NCT) Ltd.

Cell Phone Jamming Legal Issues

In the United States, United Kingdom, Australia and many other countries, blocking cell-phone services (as well as any other electronic transmissions) is against the law. In the United States, cell-phone jamming is covered under the Communications Act of 1934, which prohibits people from "willfully or maliciously interfering with the radio communications of any station licensed or authorized" to operate. In fact, the "manufacture, importation, sale or offer for sale, including advertising, of devices designed to block or jam wireless transmissions is prohibited" as well.
Jamming is seen as property theft, because a private company has purchased the rights to the radio spectrum, and jamming the spectrum is akin to stealing the property the company has purchased. It also represents a safety hazard because jamming blocks all calls in the area, not just the annoying ones. Jamming a signal could block the call of a babysitter frantically trying to contact a parent or a someone trying to call for an ambulance.
The Federal Communications Commission is charged with enforcing jamming laws. However, the agency has not yet prosecuted anyone for cell-phone jamming. Under the U.S. rules, fines for a first offense ca­n range as high as $11,000 for each violation or imprisonment for up to one year, and the device used may also be seized and forfeited to the government.
In most countries, it is illegal for private citizens to jam cell-phone transmission, but some countries are allowing businesses and government organizations to install jammers in areas where cell-phone use is seen as a public nuisance. In December 2004, France legalized cell-phone jammers in movie theaters, concert halls and other places with performances. France is finalizing technology that will let calls to emergency services go through. India has installed jammers in parliament and some prisons. It has been reported that universities in Italy have adopted the technology to prevent cheating. Students were taking photos of tests with their camera phones and sending them to classmates.
With phones ringing in movies, weddings and classrooms, it's no wonder people want to tone down the intrusion. So what legally can be done to stop annoying cell-phone use?

Alternatives to Cell Phone Jamming

While the law clearly prohibits using a device to actively disrupt a cell-phone signal, there are no rules against passive cell-phone blocking. That means using things like wallpaper or building materials embedded with metal fragments to prevent cell-phone signals from reaching inside or outside the room. Some buildings have designs that block radio signals by accident due to thick concrete walls or a steel skeleton.
Companies are working on devices that control a cell phone but do not "jam the signal." One device sends incoming calls to voicemail and blocks outgoing calls. The argument is that the phone still works, so it is technically not being jammed. It is a legal gray area that has not been ruled on by the FCC as of April 2005.
Cell-phone alerters are available that indicate the presence of a cell-phone signal. These have been used in hospitals where cell-phone signals could interfere with sensitive medical equipment. When a signal is detected, users are asked to turn off their phones.
For a less technical solution, Coudal Partners, a design firm in Chicago, has launched the SHHH, the Society for HandHeld Hushing. At its Web site, you can download a note to hand to people conducting annoying cell-phone conversations, expressing your lack of interest in what they're talking about.
For more information on cell-phone jamming and related topics, check out the links on the next page.

learm about Mobile phone jammer

A mobile phone jammer is an instrument used to prevent cellular phones from receiving signals from base stations. When used, the jammer effectively disables cellular phones. These devices can be used in practically any location, but are found primarily in places where a phone call would be particularly disruptive because silence is expected.

History

The rapid proliferation of cell phones at the beginning of the 21st century to near ubiquitous status eventually raised problems such as their potential use to invade privacy or contribute to rampant and egregious academic cheating. In addition public backlash was growing against the intrusive disruption cell phones introduced in daily life. While older analog cell phones often suffered from chronically poor reception and could even be disconnected by simple interference such as high frequency noise, increasingly sophisticated digital phones have led to more elaborate counters. Cell phone jamming devices are an alternative to more expensive measures against cell phones, such as Faraday cages, which are mostly suitable as built in protection for structures. They were originally developed for law enforcement and the military to interrupt communications by criminals and terrorists. Some were also designed to foil the use of certain remotely detonated explosives. The civilian applications were apparent, so over time many companies originally contracted to design jammers for government use switched over to sell these devices to private entities. Since then, there has been a slow but steady increase in their purchase and use, especially in major metropolitan areas.

Operation

As with other radio jamming, cell phone jammers block cell phone use by sending out radio waves along the same frequencies that cellular phones use. This causes enough interference with the communication between cell phones and towers to render the phones unusable. On most retail phones, the network would simply appear out of range. Most cell phones use different bands to send and receive communications from towers (called frequency division duplexing, FDD). Jammers can work by either disrupting phone to tower frequencies or tower to phone frequencies. Smaller handheld models block all bands from 800MHz to 1900MHz within a 30-foot range (9 meters). Small devices tend to use the former method, while larger more expensive models may interfere directly with the tower. The radius of cell phone jammers can range from a dozen feet for pocket models to kilometers for more dedicated units. The TRJ-89 jammer can block cellular communications for a 5-mile (8 km) radius.
Less energy is required to disrupt signal from tower to mobile phone, than the signal from mobile phone to the tower (also called base station), because the base station is located at larger distance from the jammer than the mobile phone and that is why the signal from the tower is not as strong.
Older jammers sometimes were limited to working on phones using only analog or older digital mobile phone standards. Newer models such as the double and triple band jammers can block all widely used systems (CDMA, iDEN, GSM, et al.) and are even very effective against newer phones which hop to different frequencies and systems when interfered with. As the dominant network technology and frequencies used for mobile phones vary worldwide, some work only in specific regions such as Europe or North America.
Components of a jammer include:

Antenna

Every jamming device has an antenna to send the signal. Some are contained within an electrical cabinet. On stronger devices, antennas are external to provide longer range and may be tuned for individual frequencies.

Circuitry

The main electronic components of a jammer are:

• Voltage-controlled oscillator — Generates the radio signal that will interfere with the cell phone signal
• Tuning circuit — Controls the frequency at which the jammer broadcasts its signal by sending a particular voltage to the oscillator
• Noise generator — Produces random electronic output in a specified frequency range to jam the cell-phone network signal (part of the tuning circuit)
• RF amplification (gain stage) — Boosts the power of the radio frequency output to high enough levels to jam a signal

Power supply

Smaller jamming devices are battery operated. Some look like cellphones and use cellphone batteries. Stronger devices can be plugged into a standard outlet or wired into a vehicle's electrical system.
The jammer's effect can vary widely based on factors such as proximity to towers, indoor & outdoor settings, presence of buildings and landscape, even temperature and humidity play a role.
There are concerns that crudely designed jammers may disrupt the functioning of medical devices such as pacemakers.^{[citation needed]}However, like cellphones, most of the devices in common use operate at low enough power output (<1W) to avoid causing any problems.^{[citation needed]}

Resistors

Question 1:

Shown here is the schematic symbol for a resistor: 
 

What is the purpose of a resistor? What function does it perform? Also, draw an illustration of what a real resistor looks like. 

The purpose of a resistor is to provide a precise amount of electrical resistance in a circuit. Here is an illustration of a small (1/8 or 1/4 watt) resistor: 
 

It is also good to know that the zig-zag symbol shown in the question is not the only symbol used to represent resistors. Another common resistor symbol is shown here: 
 

Notes:
Students may (properly) ask, "Why is there such a thing as a component whose sole purpose is to impede the flow of electrons?" While resistors may seem rather pointless at first, they end up being extremely valuable electrical/electronic components. If asked, you may cite several uses of resistors in circuits: 

•

To limit maximum circuit current to a safe value.

•

To ßplit" a voltage into proportions.

•

To ßcale" meter movements, for precise measurement of current and voltage.

•

To provide a non-shorting path to discharge static electricity.

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Question 2:

Resistors are sometimes represented in electrical and electronic schematic diagrams by a symbol other than this: 
 

Draw this other symbol next to the one shown above. 

 

Notes:
It might be a good idea to occasionally draw schematic diagrams for your students using the öther" resistor symbol, just so they are not taken by surprise when they see this symbol in real schematics. Just be sure to remain consistent in your symbolism within each diagram: never mix the two different symbols within the same schematic! 

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Question 3:

A primitive resistor may be formed by sketching a thick line on a piece of paper, using a pencil (not an ink pen!): 
 

How may the end-to-end electrical resistance of this pencil mark be increased? How may it be decreased? Explain your answers. 

The electrical resistance of a pencil mark may be increased by increasing its length. It may be decreased by increasing its width. 

Notes:
Creating a resistor on paper using a pencil is a very easy experiment to perform, the resistance of which may be measured with an ohmmeter. I strongly recommend this as a classroom exercise! 

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Question 4:

When a resistor conducts electric current, its temperature increases. Explain how this phenomenon is significant to the application of resistors in electric circuits. In other words, why would we care about a resistor's temperature increasing? 
Also, what does this indicate about the technical ratings of resistors? Aside from having a specific resistance rating (i.e. a certain number ofohms), what other rating is important for proper selection of resistors in electric circuits? 

The heating effect of electricity through a resistance is significant because that resistance may be damaged by excessive temperature. To avoid damage, resistors must be selected to be able to withstand a certain amount of heating. 

Notes:
Students need to understand that resistance alone does not fully dictate the selection of a resistor for electrical service. Failure to heed the dissipation ratings of a resistor can result in catastrophic failure! 
A good follow-up question to this is to ask what the unit of measurement is for this kind of thermal rating. 

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Question 5:

Many resistors have their electrical resistance shown by a set of color codes, or "bands," imprinted around their circumference. A standard color code associates each color with a specific decimal digit (0 through 9). Associate each of the following digits with its respective color: 

0

=

1

=

2

=

3

=

4

=

5

=

6

=

7

=

8

=

9

=

0

= Black

1

= Brown

2

= Red

3

= Orange

4

= Yellow

5

= Green

6

= Blue

7

= Violet

8

= Grey

9

= White

Notes:
Several limericks have been invented to remember this color code, most of them "politically incorrect." I often challenge students to invent their own limericks for remembering this color code, and screen the inappropriate creations from general class discussion. 

Hide Answer

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Question 6:

Observe the following "4-band" resistors, their color codes, and corresponding resistance values (note that the last color band is omitted, since it deals with precision and not nominal value): 
 

What patterns do you notice between the color codes (given as three-letter abbreviations, so as to avoid interpretational errors resulting from variations in print quality), the resistance values, and the physical sizes of the resistors? 

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Question 7:

There is more than one answer to this question! On some resistors, the last band represents the tolerance (also known as precision) for that resistor, expressed as a percentage. On other resistors, the last band represents a reliability rating for that resistor. 

Notes:
This question is worded simply and directly enough that students might think there is only one right answer. However, upon doing some research they should find that there is more involved than one simple answer can encompass! Discuss with your students the different color code types, and what applications one might find resistors with "reliability" color codes in. 
Regarding precision, nothing in life is perfectly accurate. However, the absence of perfect accuracy does not necessarily imply total uncertainty. In science, especially, it is important that all data be qualified by a statement of precision (or tolerance). Your students may be familiar with "margins of error" stated for public opinion polls. With resistors, this "margin of error" (expression of uncertainty) is explicitly given in the form of a separate color band. 

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Question 8:

Determine the nominal resistance values of these resistors, given their band colors, and also express the allowable tolerance in ohms. 
For example, a 25 kΩ resistor with a 10% tolerance rating would have an allowable tolerance of +/- 2.5 kΩ. 

•

Red, Org, Blu, Gld =

•

Brn, Blk, Grn, Sil =

•

Blu, Blk, Brn, Gld =

•

Yel, Vio, Red, Sil =

•

Grn, Brn, Yel =

•

Wht, Blu, Blk, Sil =

•

Gry, Grn, Org, Gld =

•

Org, Org, Gld =

•

Vio, Red, Sil, Gld =

•

Brn, Red, Blk, Sil =

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Question 9:

Observe the following "5-band" precision resistors, their color codes, and corresponding resistance values (note that the last color band is omitted, since it deals with precision and not nominal value): 
 

What patterns do you notice between the color codes (given as three-letter abbreviations, so as to avoid interpretational errors resulting from variations in print quality) and the resistance values of each resistor? Why do precision resistors use a "5-band" color code instead of a "4-band" color code? 

The first three color "bands" for precision five-band resistors denote three digits and a "multiplier" value, respectively. A five-band color code is necessary to express resistance with a greater number of significant digits than a four-band code. 

Notes:
The normal way to teach students the resistor color code is to show them the code first, then show them some resistors. Here, the sequence is reversed: show the students some resistors, and have them figure out the code. An important cognitive skill is the ability to detect and apply patterns in sets of data. Exercises such as this help build that skill. 
It should be noted that there is a 5-band color code for non-precision resistors as well, with the first four bands serving the same purpose as in a 4-band code, the extra band indicating resistor reliability. This scheme was developed for military purposes and is seldom seen in civilian circuitry. 

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Question 10:

Determine whether or not the following resistors measure within the resistance range specified by their color codes: 

•

(Org, Org, Red, Blk, Blu) Measured resistance = 332.5 Ω

•

(Brn, Blk, Blk, Gld, Red) Measured resistance = 9.7 Ω

•

(Blu, Vio, Brn, Red, Grn) Measured resistance = 67.43 kΩ

•

(Red, Wht, Grn, Yel, Vio) Measured resistance = 2.949 MΩ

•

(Yel, Vio, Org, Gld) Measured resistance = 44.68 kΩ

•

(Gry, Red, Brn, Sil) Measured resistance = 905 Ω

•

(Grn, Blu, Gld) Measured resistance = 6.73 Ω

•

(Vio, Brn, Red, Gld, Brn) Measured resistance = 70.82 Ω

•

(Wht, Org, Blu, Brn, Grn) Measured resistance = 9.38 kΩ

•

(Red, Blk, Wht, Grn, Vio) Measured resistance = 20.86 MΩ

Assume that all five-band resistors listed here use the precision color code as opposed to the military 5-band code where the fifth band indicates resistor reliability. 

•

(Org, Org, Red, Blk, Blu) Measured resistance = 332.5 Ω Within tolerance

•

(Brn, Blk, Blk, Gld, Red) Measured resistance = 9.7 Ω Out of tolerance!

•

(Blu, Vio, Brn, Red, Grn) Measured resistance = 67.43 kΩ Within tolerance

•

(Red, Wht, Grn, Yel, Vio) Measured resistance = 2.949 MΩ Within tolerance

•

(Yel, Vio, Org, Gld) Measured resistance = 44.68 kΩ Within tolerance

•

(Gry, Red, Brn, Sil) Measured resistance = 905 Ω Out of tolerance!

•

(Grn, Blu, Gld) Measured resistance = 6.73 Ω Out of tolerance!

•

(Vio, Brn, Red, Gld, Brn) Measured resistance = 70.82 Ω Within tolerance

•

(Wht, Org, Blu, Brn, Grn) Measured resistance = 9.38 kΩ Within tolerance

•

(Red, Blk, Wht, Grn, Vio) Measured resistance = 20.86 MΩ Out of tolerance!

Notes:
This question serves as a great review for the mathematical concepts of scientific notation and percentages. They will have to calculate the allowable range of resistance values for each resistor in order to determine whether or not the measured value falls within that range. 

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Question 11:

Find one or two real resistors and bring them with you to class for discussion. Identify as much information as you can about your resistors prior to discussion: 

•

Resistance (ideal)

•

Resistance (actual)

•

Power rating

•

Type (carbon composition, metal film, wire-wound, etc.)

If possible, find a manufacturer's datasheet for your components (or at least a datasheet for a similar component) to discuss with your classmates. 
Be prepared to prove the actual resistance of your resistors in class, by using a multimeter! 

Notes:
The purpose of this question is to get students to kinesthetically interact with the subject matter. It may seem silly to have students engage in a ßhow and tell" exercise, but I have found that activities such as this greatly help some students. For those learners who are kinesthetic in nature, it is a great help to actually touch real components while they're learning about their function. Of course, this question also provides an excellent opportunity for them to practice interpreting color codes and/or component markings, use a multimeter, access datasheets, etc.