Single chip metal detector circuit

Description.
This is a simple single chip metal detector circuit based on IC CS209A from the Cherry Semiconductors. A 100uH coil is used to sense the presence of metal. The IC CS209A has a built in oscillator circuit and the coil L1 forms a part of its external LC circuit which determines the frequency of oscillation. The inductance of the coil change in the presence of metals and the resultant change in oscillation is demodulated to create an alarm. The LED gives a visual indication too. This circuit can sense metals up to a distance of few inches.

Circuit diagram with Parts list.

Notes.

• Assemble the circuit on a general purpose PCB.
• The switch S1 can be a slide type ON/OFF switch.
• The IC must be mounted on a holder.
• The POT R1 can be used to adjust the sensitivity of the circuit.

Temperature controlled LEDs

Description.
The circuit is nothing but two LEDs (D1 and D2), whose status are controlled by the temperature of the surroundings. The famous IC LM35 is used as the temperature sensor here. Output of LM35 increases by 10mV per degree rise in temperature. Output of LM35 is connected to the non inverting input of the opamp CA3130.The inverting input of the same opamp can be given with the required reference voltage using POT R2. If the reference voltage is 0.8V, then the voltage at the non inverting input (output of LM35) becomes 0.8V when the temperature is 80 degree Celsius. At this point the output of IC3 goes to positive saturation. This makes the transistor Q1 On and LED D1 glows. Since the base of Q2 is connected to the collector of Q1, Q2 will be switched OFF and LED D2 remains OFF. When the temperature is below 80 degree Celsius the reverse happens.IC1 produces a stable 5V DC working voltage from the available9V DC supply. If you already have a 5V DC supply then you can use it directly.

Circuit diagram.

Notes.

• The circuit can be assembled on a Vero board.
• IC3 must be mounted on a holder.
• The temperature trip point can be set by adjusting POTR2.
• Type no of Q1 and Q2 are not very critical. Any general purpose NPN transistors will do it.

Liquid level indicator

LM1830 based liquid level indicator circuit.

LM1830 is a monolithic integrated circuit that can be used in liquid level indicator / control systems. Manufactured by National Semiconductors, the LM1830 can detect the presence or absence of polar fluids . Circuits based on this IC requires minimum number of external components and AC signal is passed through the sensing probe immersed in the fluid. Usage of AC signal for detection prevents electrolysis and this makes the probes long lasting. The IC is capable of driving a LED, high impedance tweeter or a low power relay at its output.

Low liquid level indicator (LED)

The circuit of a low liquid level indicator with LED is shown above. Capacitor Ct sets the frequency of the internal oscillator. With the give value of C1 the frequency will be around 6KHz. Capacitor Cb couples the oscillator output to the probe and it ensures that no DC signal is applied to the probe. The circuit detects the fluid level by comparing the probe to ground resistance with the internal reference resistor Rref.

When the probe to ground resistance goes above the Rref the oscillator output is coupled to the base of the internal output transistor making it conducting. The LED connected to the collector (between pin 12 and Vcc) is driven. Since the base of the transistor is driven using the oscillator, actually the transistor is being switched at the oscillator’s output frequency @50% duty cycle. There is no problem in driving the LED using AC signal and this method is very useful when it comes to use a loud speaker as the indicator. Loud speakers can be driven only by using AC signals and a DC signal will not produce any sound out of the speaker. The circuit diagram of a liquid level indicator using loud speaker at its output is shown below. The circuit is similar to the first circuit except that the LED is replaced by a loud speaker and the load current limiting resistor is changed from 1.2K to 1.5K.

Low liquid level warning (audio)

Low liquid level indicator with external reference resistor.

Some time there may be situations when the fluid resistance (probe to ground) do not match with the 13K internal reference resistor. In such cases there is option to avoid the internal reference resistor and an external reference resistor of your choice (Rx in the circuit) can be employed. The oscillators output is directly available at pin 5 and the Rx connects the oscillator output to the probe through the blocking capacitor. The circuit diagram for the above said scheme is shown below.

Level indicator using external reference resistor

High liquid level activated switch.

A method for activating a relay when the liquid level exceeds a predetermined level is shown here. DC voltage is required for driving the relay and AC voltage cannot be used here just like what we did in the case of LED and speaker. Pin 9 of the IC can be used to solve this problem. A capacitor connected from this pin to ground will keep the internal output transistor steadily ON whenever the probe resistance goes higher than the reference resistor. External transistor Q1 is connected to the collector of the internal transistor. The load that is the relay is connected at the collector of Q1. When the probe is not touching the water it equal to an open circuit situation and surely the probe resistance will be many M ohms and it is greater than the Rref(13K). The internal transistor will be switched ON and Q1 whose base is connected to the collector of the internal transistor will be in OFF condition keeping the relay inactive. When the reverse scenario occurs (fluid level touches the probe) the internal transistor is switched OFF and this in turn makes the transistor Q1 ON resulting in the activation. The load connected through the relay whether pump, lamp, alarm, solenoid valve or anything is driven. Resistor R3 limits the collector current of the internal transistor while resistor R4 provides protection to the IC from transients.

High liquid level activated switch

Probe: The probe used here can be any metal rod with size and shape of your choice. The tank must be made of metal and it should be properly grounded. For non metal tanks fix a small metal contactor at its bottom level and ground it. The probe must be placed at the level you want to monitor.

Notes.

• The circuit can be assembled on a Perf board.
• I used 12V DC for powering the circuit.
• Maximum supply voltage LM1830 can handle is 28V.
• The tweeter I used was of a 16 ohm type.
• The relay I used is a 200 ohm/12V type.
• Maximum load current Q1 (2N2222) can handle is 800mA.
• The switching current/voltage ratings of the relay must be according to the load you want to drive using it.
• It is recommended to mount the IC on a holder.

Simple UPS

Description.

This is the circuit diagram of a simple UPS that can deliver 12V unregulated and 5V regulated DC. The transformer T1 steps down the mains voltage to 12V AC and then the bridge B1 rectifies it. The rectified signal is smoothed by the capacitor C1.When the mains supply is available the battery will be charged via diode D3 and the regulator IC gets supply via diode D5. 12V and 5V DC will be available at the output terminals. When mains supply is not available the battery supplies current to the regulator IC and to the 12V DC terminal through diode D4.Also, the diode D3 blocks reverse flow of current during battery mode. Capacitors C2 and C3 acts as filters.

Circuit diagram with Parts list.

Notes.

• Assemble the circuit on a good quality PCB.
• The transformer T1 can be a 230V AC primary, 12V secondary,3A step-down transformer.
• The bridge B1 can be a 2A bridge. If such a bridge is not available, make one using four 1N4007 diodes.
• The capacitor C1 must be rated at least 25V.

UPS-Uninterruptable Power Supplies

Most of us take the mains ac supply for granted and use it almost casually without giving the slightest thought to its inherent shortcomings and the danger posed to sophisticated and sensitive electronic instruments/equipments. For ordinary household appliances such as incandencent lamps, tubes, fans, TV and fridge, the mains ac supply does not make much difference, but when used for computers, medical equipments and telecommunica¬tion systems, a clean, stable interruption free power supply is of the utmost importance. Of the myriad of devices, processes and systems which rely on ac power, computers are probably the most sensitive to power disturbances and failures. Interruptions in power supply may cause the contents of a memory to be lost or corrupted, the entire system to malfunction or fail, or even variety of components failures to occur, all of which not only result in inconvenience but also loss of money.

As more and more PCs, word processors and data terminals find their way into small business, UPS systems that meet the power requirements and price range needs of even the small business organizations and offices are being manufactured.

Uninterruptible Power Supply Systems.

There are three distinct types of uninter­rupted power supplies, namely, (£) on-line UPS (ii) off-line UPS, and (Hi) electronic gen­erators. In the on-line UPS, whether the mains power is on or off, the battery operated inverter is on all the time and supplies the ac output voltage. When the mains power supply goes off, the UPS will be on only until the battery gets discharged. When the main power resumes, the battery will get charged again. In off-line UPS and electronic genera­tors, ther inverter is off when the mains power is present and the output voltage derived directly from the mains is the same as the mains supply voltage. The inverter turns on only when the mains supply goes off.

The block diagrams of on-line UPS, off-line UPS and electronic generators are given in figs

The ever increasing importance of computers in industry and commerce will increase the need for quality, high stability and interruption free power supplies.

A clean ac power source is the fundamental to the operation of most sensitive elec­tronic equipment, and many new and sophisticated circuits are designed to overcome the effects of disturbances normally found in the mains ac supply.

In order to protect a sensitive system from power losses and blackouts, an alternative power source is required that can switch into operation immediately when disruption occurs. An interruptible power supply (UPS) is just such an alternative source. A UPS generally consists of a rectifier, battery charger, a battery bank and inverter circuit which converts the commercial ac input into dc suitable for input to the battery bank and the inverter. The rectifier should have its input protected and should be capable of supplying power to the inverter when the commercial supply is either slightly below the normal voltage or slightly above.

Online UPS:

Online UPS Block Diagram

In case of On-line UPS, the battery operated inverter works continuously whether the mains supply is present or not. Triac T₁ is on for all the times while Triac T₂ has been provided to bypass the UPS inverter, only when a fault develops in the UPS inverter. When the mains supply fails, the UPS supplies power only until the batteries get dis­charged. However, once the mains power resumes, the batteries will get charged again. The switching times of these supplies is considered to be zero. Usually sealed maintenance free batteries are used and the running time of the inverter is low (approximately 10 to 30 minutes).

Off Line UPS:

Offline UPS Block Diagram

In the case of Off-Line UPS, the inverter is off when the mains power is on and the output voltage is derived directly from the mains. The inverter turns on only when the mains supply fails. Its switching time is less than 5 ms. These UPS are generally used with PCs or computers or other appliances where a small duration (5 ms or less) interrup­tion in power supply can be tolerated. Usually, sealed batteries or lead-acid batteries are used. The running time of these supplies is also low (about 10 to 30 minutes).

Electronic Generators:

Electronic Generator

An electronic generator is the same as the off-line UPS system except for one difference that switching time from the mains supply to battery driven inverter supply will not be small (over 10 ms) for the electronic generator. Also, the electronic generators will run for longer time (1 to 4 hours) than off-line UPS systems because, usually large size lead-acid batteries are used with^/electronic generators. These are meant for household applications to run fans, coolers, fridge, lights, TV and VCR.

The demand is the highest for the electronic generators meant for house hold applica­tions, followed by the off-line UPS, and then the on-line UPS systems. The off-line or on­line UPS systems are mainly used in places where PCs or computers are used. The de­mand for on-line UPS systems is less than for off-line UPS systems because the price of the on-line UPS systems is higher.

Fire alarm using thermistor & NE555

Description.

Many fire alarm circuits are presented here,but this time a new circuit using a thermistor and a timer to do the trick. The circuit is as simple and straight forward so that , it can be easily implemented.The thermistor offers a low resistance at high temperature and high resistance at low temperature. This phenomenon is employed here for sensing the fire.

The IC1 (NE555) is configured as a free running oscillator at audio frequency. The transistors T1 and T2 drive IC1. The output(pin 3) of IC1 is couples to base of transistor T3(SL100), which drives the speaker to generate alarm sound. The frequency of NE555 depends on the values of resistances R5 and R6 and capacitance C2.When thermistor becomes hot, it gives a low-resistance path for the positive voltage to the base of transistor T1 through diode D1 and resistance R2. Capacitor C1 charges up to the positive supply voltage and increases the the time for which the alarm is ON. The larger the value of C1, the larger the positive bias applied to the base of transistor T1 (BC548). As the collector of T1 is coupled to the base of transistor T2, the transistor T2 provides a positive voltage to pin 4 (reset) of IC1 (NE555). Resistor R4 is selected s0 that NE555 keeps inactive in the absence of the positive voltage. Diode D1 stops discharging of capacitor C1 when the thermistor is in connection with the positive supply voltage cools out and provides a high resistance path. It also inhibits the forward biasing of transistor T1.

Circuit diagram with Parts list.

Notes.

• The circuit can be powered from a 6V battery or a 6V power supply.
• Click Here ! for the circuit diagram of a power supply circuit for this project.
• The thermistor can be mounted on a heat resistant material like mica to prevent it from damage due to excessive heat.
• The LED acts as an indication when the power supply is switched ON.

2 km FM transmitter

Description.
With a matching antenna, the FM transmitter circuit shown here can transmit signals up to a range of 2 kilo meters. The transistor Q1 and Q2 forms a classic high sensitive preamplifier stage. The audio signal to be transmitted is coupled to the base of Q1 through capacitor C2. R1, R3, R4, R6, R5 and R9 are the biasing resistors for the preamplifier stage comprising of Q1 and Q2. Transistor Q3 performs the collective job of oscillator, mixer and final power amplifier.C9 and L1 forms the tank circuit which is essential for creating oscillations. Inductor L2 couples the FM signal to the antenna.

Circuit diagram.

Notes.

• Assemble the circuit on a good quality PCB.
• The circuit can be powered from anything between 9 to 24V DC.
• Inductor L3 can be a VK220J type RFC.
• For L1 make 3 turns of 1mm enamelled copper wire on a 10mm diameter plastic former. On the same core make 2 turns of 1 mm enamelled copper wire close to L1 and that will be L2.
• Frequency can be adjusted by varying C9.
• R9 can be used to adjust the gain.
• For optimum performance, value of C8 must be also adjusted.
• Using a battery for powering the circuit will reduce noise.

Long duration timer circuit.

Description.

This timer circuit can be used to switch OFF a particular device after around 35 minutes. The circuit can be used to switch OFF devices like radio, TV, fan, pump etc after a preset time of 35 minutes. Such a circuit can surely save a lot of power.

The circuit is based on quad 2 input CMOS IC 4011 (U1).The resistor R1 and capacitor C1 produces the required long time delay. When pushbutton switch S2 is pressed, capacitor C1 discharges and input of the four NAND gates are pulled to zero. The four shorted outputs of U1 go high and activate the transistor Q1 to drive the relay. The appliance connected via the relay is switched ON. When S2 is released the C1 starts charging and when the voltage at its positive pin becomes equal to ½ the supply voltage the outputs of U1 becomes zero and the transistor is switched OFF. This makes the relay deactivated and the appliance connected via the relay is turned OFF. The timer can be made to stop when required by pressing switch S1.

Circuit diagram with Parts list.

Notes.

• Assemble the circuit on a good quality PCB or common board.
  
• The circuit can be powered from a 9V PP3 battery or 12V DC power supply.
  
• The time delay can be varied by varying the values of C1&R1.
  
• The push button switch S2 is for starting the timer and S1 for stopping the time.
  
• The appliance can be connected via contacts N1 & N2 of the relay RL1.
• The IC U1 is 2 input quad NAND gate 4011.

USB sound card

Description.
Designing and building a USB sound card is no longer a head ache because we have got the PCM 2702 integrated circuit from Texas Instruments. The PCM2702 is an integrated 16 bit digital to analog converter that has two digital to analog output channels. The integrated interface controller of PCM2702 is compliant to the USB 1.0 standards. The IC can handle sampling rates of 48 KHz, 44.1 KHz and 32 KHz. The IC also has a number of useful features like on-chip clock generator, digital attenuator, play back flag, suspend flag, zero flag, mute function etc. The most interesting thing is that this circuit is plug & play and doesn’t need any driver software for Windows XP and Windows Vista operating systems.

The circuit gets control data and audio data from the USB through the D+ and D- pins of the PCM2702 all the data transferring is carried out at full speed. The decoded audio signals will be available at the VOUTL and VOUTR pins of the IC. The 12MHz crystal is connected between the XT0 and XT1 pins of the IC. The VBUS (USB bus power) pin and DGND (digital ground) pins of the IC are connected to the +5V and ground pins of the USB respectively. The circuit requires +5V DC and +3.3V DC for operation and both of these voltages can be derived from the USB port using LDO (low drop out) voltage regulators (not shown in circuit).

Circuit diagram.

Block diagram of PCM2702.

Notes.

• +5V DC supply can be derived from the USB port using a +5V LDO regulator.
• +3.3V DC supply can be derived from the USB port using a +3.3V LDO regulator.
• Audio signals (output) available at VOUTL and VOUTR requires further amplification for driving low impedance head phones or loud speakers.
• The PCM2702 is available only in SSOP28 package and requires special care while assembling.
• Before attempting this circuit please go through the datasheet of PCM2702 and get a clear idea about the device.

12 volt DC Power Supply from USB port

12 volt DC Power Supply from USB port

The circuit given below infact is a 5 volt to 12 volt converter. It uses the 5 Volts from USB port and converts it to 12 volts DC with the help of IC LT1618.

Description :

Using this circuit we can convert 5V DC from the computer USB port to 12V DC and a circuit like this will find a lot of application in USB powered systems. The heart of this circuit is IC LT1618 which is a constant current, constant voltage boost converter. The IC has a wide input voltage range of 1.8 to 18V DC and output voltage can be up to 35V DC.

In the circuit resistors R1, R2 sets the output voltage. Pin number 9 is the shutdown pin, less than 0.3V to this pin will shut down the IC. Pin number four is the current sense adjust pin. The current sense voltage can be reduced by applying a DC voltage to this pin. If this adjustment is not needed connect this pin to ground and you can omit components R3, R5 and Q1.

Circuit diagram of 12 volts dc power supply:

Circuit Diagram-12Volts DC power supply from USB port

Notes :

• C2 and C3 must be rated at least 15V.
• Less than 0.3V at the shutdown pin will shutdown the IC.
• Output voltage is governed by the following equation R1 = R2 (  (Vout /1.263V) -1).

How to make a Rheostat

I have already explained in detail the working of a rheostat. To know more about the component click on the link below.

TAKE A LOOK : POTENTIOMETER AND RHEOSTAT – WORKING AND COMPARISON

Although the below explained process is not applicable for any electronic circuits, you will clearly get a clear idea on how a rheostat works.

Components

1. The components needed for the connection are
2. Flashlight bulb and socket [1]
3. Dry cell lantern battery/D-cell battery [2]
4. Wire [About 15 to 17 inches and another one 2 inches]
5. Spring [1]
6. Wire Clippers [A pair]
7. A typical spring can be obtained from a widow roll up. You can even get to buy one at a cheap rate.

Procedure

• Connect the two Dry cell lantern/D-cell batteries tail-to-tail, so that the positive polarity of one battery is connected to the negative polarity of the other.
• Using a wire cutter, cut the wire in equal lengths. One wire should be at least 8 centimetres long.
• Connect the wires onto the open ends of both the batteries.
• The end of one wire must be connected to the bulb socket with the bulb in it.
• Connect the second wire to one end of the long spring.
• Connect the free end of one wire to one terminal of the light socket.
• Connect the other free wire to one end of the spring.
• Take the two inch wire and connect it to the second terminal of bulb socket.
• Connect the other end of the two inch wire onto the other end of the spring.

How to make a Rheostat

How to make a Rheostat

What happens?

As soon as the circuit is in closed loop, the bulb begins to glow. Although the glow intensity is less, when you move the wire through the spring onto the other end where the wire is connected, the bulb starts to glow more brightly. When both the wires are nearby the glow will be in its maximum.

The spring is mainly made of steel wire. Steel wires are not very good conductors of electricity. Thus the resistance of the circuit also increases. If the spring length is long enough you will get to see different stages of the glow. Thus you will get to see the working of a rheostat.