Sound Level Meter

This nifty sound level meter is a perfect one chip replacement for the standard analog meters. It is completely solid state and will never wear out. The whole circuit is based on the LM3915 audio level IC and uses only a few external components. This circuit can also be integrated into audio amp projects.

Schematic

Parts

Part Total Qty. Description Substitutions C1 1 2.2uF 25V Electrolytic Capacitor C2, C3 1 0.1uF Ceramic Disc Capacitor R1, R3 2 1K 1/4W Resistor R2 1 10K 1/4W Resistor R4 1 100K 1/4W Resistor R5 1 1M 1/4W Resistor D1 1 1N914 Silicon Diode Q1 1 2N3906 PNP Transistor LED1-LED10 10 Standard LED or LED Array U1 1 LM3915 Audio Level IC MISC 1 Board, Wire, Socket For U1

Notes

1. V+ can be anywhere from 3V to 20V.
2. The input is designed for standard audio line voltage (1V P-P) and has a maximum input voltage of 1.3V.
3. Pin 9 can be disconnected from +V to make the circuit use a moving dot display instead of a bar graph display.
4. Thanks to help from the forum, this circuit has been improved from the original version to include a peak detector for a more stable and viewable output. This page has been updated with the new circuit.

Single Chip FM Radio Circuit

his extremely simple FM radio receiver circuit is made possible by the special purpose TDA7000 IC. It integrates nearly all the functions necessary to build an FM receiver needing only a few external capacitors and a tuning circuit. Using a simple active RC filter made of only a single inductor, a few resistors and a varicap, this FM receiver will pick up broadcast radio between about 88 and 108 MHz. The mono output signal can then be used to drive a set of high impedance headphones, or feed a power amplifier.

Schematic

Parts

Part Total Qty. Description Substitutions C1, C9, C12, C17 4 0.1uF Ceramic Disc Capacitor C2, C4, C5, C6, C13 5 0.01uF Ceramic Disc Capacitor C3 1 20pF Ceramic Disc Capacitor See Notes C7, C15, C16 3 0.001uF Ceramic Disc Capacitor C8, C10, C11 3 220pF Ceramic Disc Capacitor C14 1 100pF Ceramic Disc Capacitor C18 1 200pF Ceramic Disc Capacitor R1 1 22K 1/4W 5% Resistor R2 1 100K 1/4W 5% Resistor R3 1 100K Linear Taper Pot D1 1 Motorola MV209 Varicap VHF Tuning Diode U1 1 TDA7000 FM Radio IC See Notes L1 1 7 Turn Inductor (See Notes) MISC 1 Board, Wire, Socket For U1, Case

Notes

1. Unfortunately, the TDA7000 went obsolete a few years ago. The original version is made by Signetics (now Philips) and is getting a bit hard to find these days. There are, however, other options. Philips makes the TDA7010T which is a surface mount version. The TDA7021T is also available which includes stereo capability. They are electrically similar but are only 16 pin chips so you'll need to compare the datasheets to build the circuit.
2. The value of C3 is noncritical and can be between 10pF and 20pF. Minor adjustments to L1 may be required based on the value of C3.
3. L1 is made by winding 7 turns of 24 AWG wire around pencil. Remove the pencil and then space the turns slightly.
4. Frequency selection is via R3.

Single Chip AM Radio Circuit

The ZN414 IC contains an entire automatic gain controlled AM receiver in a small three pin package. With only a few external components, a simple radio with excellent selection and reception can be constructed. Since the chip also uses a low supply voltage of only 1.3V, 3V coin cell battery can make for a physically small circuit with many covert uses. The chip has a wide bandwidth of between 150KHz and 3MHz, so by playing with values in the tuning circuit you can pick up a wide variety of signals.

Schematic

Parts

Part Total Qty. Description Substitutions R1 1 100K 1/4W Resistor R2 1 470 Ohm 1/4W Resistor R3 1 1K 1/4W Resistor C1 1 0.04uF Ceramic Disc Capacitor C2 1 365pF Variable Tuning Capacitor C3 1 0.01uF Ceramic Disc Capacitor D1, D2 2 1N4148 Signal Diode U1 1 ZN414 Radio IC MK484 L1 1 See Notes MISC 1 Board, Holder For Batteries, Wire, Coil Form For L1

Notes

1. The ZN414 is obsolete. The MK484 is it's replacement, but like all special purpose ICs, it can be hard to find. At the time of this writing, there are many suppliers online that carry the IC. The ZN416 is functionally the same but with the additiion of a built in headphone amplifier.
2. L1 is made by winding 40 turns of 28 AWG magnet wire around a 4" non-magnetic coil form. Cardboard tubes or paper cups are ideal forms.
3. The audio output of the circuit is about 0.1V peak to peak, which will drive a set of crystal earphones or other very high impedance phones. A small audio amplifier made with an LM386 will allow you to use modern dynamic headphones or a small speaker.
4. L1, C1 and C2 are the tuning circuit. By changing them around, you can change the range of frequencies the radio is capable of. Easiest to change is L1. Simply by altering the number of coils or moving the windings farther apart, you can shift the circuit to different frequency ranges.

Op Amp Radio

Here is a simple radio that is easy to build and inexpensive. In fact, you probobly have all the parts you need in your junk box. You'll be suprised at the great reception with this little set.

Schematic

Parts

Part Total Qty. Description Substitutions C1 1 Tuning Capacitor C2 1 0.1uf Disc Capacitor R1 1 10 Meg 1/4 W Resistor D1 1 1N34 Germanium Diode U1 1 741 Or Similar Op Amp L1 1 "Loopstick" Antenna MISC 1 IC Socket, Crystal Earphone, Wire, Antenna

Notes

1. The antenna can be a piece of wire or a telescoping antenna. 18 inches is a good length for in the city.
2. The tuning capacitor is a regular broadcast band tuning capacitor. I got mine from a junked AM radio. I got the loopstick antenna from that same radio.
3. You can change L1 and C1 to recieve different bands (eg. Shortwave). To recieve shortwave, try this: Make L1 30 turns of 30 guage wire wound on a film can and make C1 a 10-365pf capacitor.
4. Combine this circuit and the 8 Watt Audio Amp for a really neat radio.

Mono To Stereo Synthesizer

This circuit attempts to liven up mono sound sources by simulating a stereo signal. It does this by shifting certain frequencies between left and right to fool the ear. It can often produce a passable mock stereo sound to bring some depth to otherwise flat recordings. Of course, there is no way to produce real stereo sound from a purely mono source unless the synthesizer had a way to tell which direction the original sound came from, but an illusion is generally enough for all but the hard core audiophile.

Schematic

Parts

Part Total Qty. Description Substitutions R1 1 4.7K 1/4W Resistor R2 1 10K 1/4W Resistor R3 1 12K 1/4W Resistor R4, R6 2 22K 1/4W Resistor R5 1 11.K 1% 1/4W Resistor R7 1 16K 1/4W Resistor R8 1 100K 1/4W Resistor R9 1 24K 1/4W Resistor R10 1 18K 1/4W Resistor C1 1 0.1uF Ceramic Disc Capacitor C2, C3 2 0.47uF Ceramic Disc Capacitor C4, C5, C7 3 0.01uF Ceramic Disc Capacitor C6 1 0.013uF Ceramic Disc Capacitor C8, C9 2 4.7uF 25V Electrolytic Capacitor C10 1 0.22uF Capacitor C11 1 47uF 25V Electrolytic Capacitor C12 1 100uF 25V Electrolytic Capacitor D1 1 LED (Red, Green or Orange) U1 1 TBA3810 (442-794) S1 1 SPDT Switch S2 1 SPST Switch J1, J2, J3 3 RCA Jack Other suitable jack MISC 1 PC Board, Wire, Case, Holder for D1, Socket for U1

Notes

1. This circuit was sent in by oRbEq (email address unavailable).
2. S2 controls power to the circuit. Switching S1 to ground bypasses the circuit and outputs mono sound to both output jacks. Setting S1 to R1 enables the synthesized stereo sound at the outputs.
3. The circuit draws very little current and can thus be powered by most any well filtered 9V-12V source (including a standard 9V radio battery).

Microphone Mixer

This relatively simple mixer was designed for three dynamic microphones, but can be re-designed for more or less. Level and tone controls are available to tailor the sound to your needs.

Schematic

Parts

Part Total Qty. Description Substitutions R1, R2, R3 3 1K 1/4W Resistor R4, R5, R6 3 10K Logarithmic Pot R7 1 1Meg 1/4W Resistor R8, R10 2 10K 1/4W Resistor R9, R11 2 100K Linear Pot C1, C2, C5, C6 4 0.1uF Ceramic Disc Capacitor C3, C4 2 22nF Ceramic Disc Capacitor U1, U2 2 741 Op Amp J1, J2, J3 3 Input Jacks Of Your Choice MISC 1 Board, Wire, Knobs, IC Sockets

Notes

1. Email Anatoly I. Shikhatov with questions, comments, etc.
2. R1-R3 are level controls. R9 and R11 control bass and treble, respectively.
3. Since the circuit draws such low current, two 9V batteries can be used for a power supply.
4. Building the mixer in a metal case will cut down on noise.

Guitar Fuzz Effect

Fuzz is one of the classic guitar effects, and this simple circuit generates it quite well. The circuit is so compact that it can be built into guitars or amps that do not have built in fuzz to add that capability to the instrument. The circuit does not use much battery power, so a standard alkaline battery will last many years even with daily use.

Schematic

Parts

Part Total Qty. Description Substitutions R1, R2 2 100K 1/4W Resistor R3 1 1K 1/4W Resistor R4 1 1M 1/4W Resistor C1 1 100uF 16V Electrolytic Capacitor C2, C3 2 0.47uF Ceramic Disc Capcitor D1, D2 2 1N4148 Diode U1 1 741 Op Amp MISC 1 Socket for U1, Jacks for Input/Output, Wire, Board, 9V Battery Snap, Case

Notes

1. You can use any input/output jacks you want, but the standard for musical instruments such as guitars is a mono 1/4" headphone plug and jack.

One Tube Regenerative Radio Circuit

A regenerative radio works by feeding back a small amount of amplified output of the detector back into the input. Thus it achieves sensitivity far beyond what only a detector could alone. This simple regen radio uses a single tube as it's detector and amplifier; the "Audion". It's a great first project for those wishing to bring back some nostalgia by building one of the first amplified radio sets. Built on a board using point to point wiring and a set of period headphones, it can be a great functional conversation piece.

Schematic

Parts

Part Total Qty. Description Substitutions R1 1 50K Linear Taper Pot R2 1 2.2 Meg 1/4W Resistor R3 1 10K 1/4W Resistor C1 1 250pF 100V Ceramic Disc Capacitor C2 1 365pF Air Variable Tuning Capacitor C3, C4 2 120pF 100V Ceramic Disc Capacitor C5 1 0.1uF 100V Ceramic Disc Capacitor V1 1 3A4 Audion Tube 3S4, 3Q4 (See Notes) L1 1 30 Turns 26 AWG Magnet Wire (See Notes) L2 1 80 Turns 26 AWG Magnet Wire (See Notes) L3 1 20 Turns 26 AWG Magnet Wire (See Notes) S1 1 SPST Switch HEADPHONE 1 High Impedance Headphones (2K or Greater) ANT1, ANT2 2 See Notes MISC 1 Board, Wire, Sockets For V1, Case, Knob for R1, Clips for Antenna and Batteries

Notes

1. L1 - L3 are constructed on the same coil form. A toilet paper tube will be the coil form. Secure the 26 AWG wire to the form by punching two holes close together and winding the wire once around the "bridge" between them. Alternately, just a drop of hot glue can be used. Leave about 6" of wire. Wind on twenty turns close together but never overlapping. Make a tap by securing the wire with a drop of glue and twisting a loop. This is L3. Wind 80 more turns in the same direction and then secure the end, leaving about 6" of wire at the end. This is L2. Now secure the 26 AWG wire 1/8" from the end of L2 and wind 30 turns in the same direction as the other coils, making sure that this coil is spaced 1/8" evenly from L2. Secure the end and leave about 6" lead. This is coil L1. Now trim the extra length of the coil form and spray the coil with several coats of lacquer to hold the wires in place.
2. R1 and S1 can be one unit if you purchase a pot with a built in switch. This will avoid turning the set on with full regen and causing a nasty squeal from the headphones.
3. The 3S4 or 3Q4 can be substituted for the 3A4 in the connections of pin 3 and pin 4 are swapped.
4. An antenna of only a few feet can be used for ANT1 for easy indoor installation. Far better is an antenna of 50 feet or more outside, which would be connected to ANT2.
5. Note that this circuit requires a good Earth ground. A suitable location is a metal cold water pipe.
6. To use the set, place C2 at about midway, set R1 fully counterclockwise and then turn on S1 and allow the tubes several seconds to warm up. Increase R1 until the headphones squeal, then back off. That is when the set is most sensitive. Now tune C2 to the desired frequency. You may have to readjust R1 as you tune different frequencies. With some practice you will figure out how much regen you need as you change stations.
7. If R1 seems to have no effect, swap the connections to L1.

FET Audio Mixer

This simple circuit mixes two or more channels into one channel (eg. stereo into mono). The circuit can mix as many or as few channels as you like and consumes very little power. The mixer is shown with two inputs, but you can add as many as you want by just duplicating the "sections" which are clearly visible on the schematic.

Schematic

Parts

Part Total Qty. Description Substitutions R1, R3 2 10K Pot R2, R4 2 100K 1/4 W Resistor R5 1 6.8K 1/4 W Resistor C1, C2, C3 3 0.1uF Capacitor Q1 1 2N3819 Junction FET MISC 1 Wire, Shielded (Metal) Case, Phono Or Other Plug For Output

Notes

1. As many or as few channels as are required can be added to the mixer. Do this by just duplicating the input "sections" which are shown on the schematic.
2. A shielded case is probably needed to reduce hum and help stop oscillations.
3. The circuit can be powered by a single 9 volt battery.

Electronic Stethoscope

Stethoscopes are not only useful for doctors, but home mechanics, exterminators, spying and any number of other uses. Standard stethoscopes provide no amplification which limits their use. This circuit uses op-amps to greatly amplify a standard stethoscope, and includes a low pass filter to remove background noise.

Schematic

Parts

Part Total Qty. Description Substitutions R1 1 10K 1/4W Resistor R2 1 2.2K 1/4W Resistor R4 1 47K 1/4W Resistor R5, R6, R7 3 33K 1/4W Resistor R8 1 56K 1/4W Resistor R10 1 4.7K 1/4W Resistor R11 1 2.2K to 10K Audio Taper Pot R12 1 330K 1/4W Resistor R13, R15, R16 3 1K 1/4W Resistor R14 1 3.9 Ohm 1/4W Resistor C1, C8 2 470uF 16V Electrolytic Capacitor C2 1 4.7uF 16V Electrolytic Capacitor C3, C4 2 0.047uF 50V Metalized Plastic Film Capacitor C5 1 0.1uF 50V Ceramic Disc Capacitor C6, C7 2 1000uF 16V Electrolytic Capacitor U1 1 TL072 Low Noise Dual Op-Amp U4 1 741 Op-Amp U5 1 LM386 Audio Power Amp MIC 1 Two Wire Electret Microphone J1 1 1/8" Stereo Headphone Jack Batt1, Batt2 2 9V Alkaline Battery LED 1 Red/Green Dual Colour Two Wire LED SW 1 DPST Switch MISC 1 Stethoscope head or jar lid, rubber sleeve for microphone, board, wire, battery clips, knob for R11

Notes

1. MIC is an assembly made out of a stethoscope head and electret mic. Cut the head off the stethoscope and use a small piece of rubber tube to join the nipple on the head to the mic.
2. Be careful with the volume, as excess noise levels may damage your ears.
3. R11 is the volume control.
4. The circuit marked as optional is not required for the main circuit to function. The optional circuit blinks an LED to the heartbeat as it is heard by the microphone. Even if the optional circuit is not included, sound will still be heard via the headphone jack.
5. Thanks to audioguru for correcting and improving this original schematic. He has essentially redesigned the circuit as a much improved version. Read about it in this forum topic.

Aircraft Radio Communications Receiver

The communications between commercial aircraft and the ground can be interesting, amusing and sometimes even disturbing. However radios that receive the approximately 220MHz to 400MHz band commonly used for aircraft (both military and commercial) are not easily found. And scanners can be complicated, large and expensive. With an easy to build circuit such as this one, everyone can enjoy listening in on these conversations.

Schematic

Parts

Part Total Qty. Description Substitutions R1, R3 2 47K 1/4W Resistor R2 1 10K 1/4W Resistor R4 1 4.7K 1/4W Resistor R5 1 5K Linear Taper Pot R6 1 2.2K 1/4W Resistor C1, C2, C3, C6 4 0.001uF Ceramic Disc Capacitor C4 1 2.2pF Ceramic Disc Capacitor C5 1 1pF Ceramic Disc Capacitor C7 1 15uF 15V Electrolytic Capacitor C8 1 18pF Variable Capacitor D1 1 1N82 Diode Q1 1 2N918 NPN Transistor L1 1 See Notes L2 1 1.8uH Inductor ANT1 1 Approx. 18 Inch Wire Antenna MISC 1 PC Board, Wire, Knob For C8

Notes

1. The circuit originally appeared in the Think Tank column of the Sept. 1995 issue of Popular Electronics.
2. L1 is made by winding 2 turns of 22 AWG magnet wire on a 5/32 drill bit. This inductor can be modified to shift the frequency range of the circuit.
3. The antenna can also be placed at the anode of D1 if overload is a problem with it connected to the emitter of Q1
4. R5 adjusts regen and thus sensitivity.

Digital Volume Control

This digital volume control has no pot to wear out and introduces almost no noise in the circuit. Instead, the volume is controlled by pressing UP and DOWN buttons. This simple circuit would be a great touch to any home audio project.

Schematic

Parts

Part Total Qty. Description Substitutions C1 1 0.1uf Ceramic Disc Capacitor U1 1 DS1669 Digital Pot IC (See Notes) S1, S2 2 Momentary Push Button Switch MISC 1 Board, Wire, Socket For U1

Notes

1. U1 is available from Dallas Semiconductor.
2. S1 turns the volume up, S2 turns it down.
3. The input signal should not fall below -0.2 volts.
4. Using a dual polariity power supply (+-5V works fine) will cure most clipping problems. You will have to check the data sheet for the correct pins to connect your voltages.

Crystal Radio

I have received a number of emails regarding schematics for crystal radios. After about the third email, I figured that I may as well put one on my page. So here it is. The circuit is very simple with only 5 parts, but performs very nicely when used with the right size antenna.

Schematic

Parts

Part Total Qty. Description Substitutions C1 1 Tuning Capacitor (See Notes) D1 1 1N34 Germanium Diode L1 1 Loopstick Antenna (See Notes) SPKR1 1 Crystal Earphone MISC 1 Wire, Board, Wire For Antenna, Knob For C1

Notes

1. C1 and L1 can be bought, or salvaged from an old AM radio (which is where I got mine). You may need to experiment with the connections on L1 in order to get the best (or any) signal.
2. You may or may not need the ground connection. I never use it and the radio usually works fine without it.
3. The bigger the antenna, the more stations you pick up and the louder you hear them. On my radio, I get about 10 stations, 3 of which are very loud in the earphone. Of course, not everyone has room for a 60' antenna...
4. A 47K resistor in parallel with the earphone will help properly load the detector (This suggestion from Kb8tej1@aol.com)

8 Watt Audio Amp

Here is the schematic for an 8 watt audio amp. This amp can be used as a simple booster, the heart of a more complicated amplifier or used as a guitar amp.

Schematic

LM383 Pinout

Parts

Part Total Qty. Description Substitutions C1 1 10uf Electrolytic Capacitor C2 1 470uf Electrolytic Capacitor C3 1 0.1uF Disc Capacitor C4 1 2000uf Electrolytic Capacitor 2200uF R1 1 2.2 Ohm Resistor Anything Within 10% R2 1 220 Ohm Resistor Anything Within 10% IC1 1 LM383 8 Watt Amp IC ECG1232, TDA2002

Notes

1. IC1 MUST be installed on a heat sink.
2. C3 is for filtering and to prevent oscillation and should not be omitted.
3. The circuit can be built on a perf board, universal solder board or PC board, the PC board is preferred. I built the circuit on a perf board and had to add a low value inductor at the input to prevent oscillation.
4. The circuit draws about 880 ma at 12 V.
5. By swapping the values of R1 and R3, you can turn this amplifier into a guitar amp with no preamp required.
6. If you can't find 2000uF, then replace C4 with a 2200uF unit.
7. If you add a 0.2uF capaciitor in series with a 1 ohm resistor to the output you can prevent oscillation of the circuit under certain conditions.
8. Thanks to James Maloway for redrawing the schematic. The mew image is much easier to read then the old one.

8 Note Tune Player

This neat little circuit can play 8 note tunes at any speed you want. You select the notes with 8 trim pots. The speed in selected through a ninth pot. The circuit draws very little power and can be powered by a 9 volt battery, with the addition of a 7805 regulator to power the TTL logic used.

Schematic

Parts

Part Total Qty. Description Substitutions R1 1 10K Trim Pot R2 1 1K 1/4 W Resistor R3, R4, R5, R6, R7, R8, R9, R10 8 500 Ohm Trim Pot C1, C3 2 10uF 12V Electrolytic Capacitor C2 1 1uF 12V Electrolytic Capacitor U1 1 555 Timer U2 1 7490 Binary Counter 74LS90, 74HC90 U3 1 7445 BCD To Decimal Decoder 74LS45, 74HC45 U4, U6 2 7404 Hex Inverter 74LS04, 74HC04 U5, U7 2 4016 Quad Bilateral Switch U8 1 3909 LED Flasher/Oscillator SPKR1 1 8 Ohm 500mW Speaker MISC 1 Board, Wire, IC Sockets

Notes

1. R1 controls the speed at which the notes are played. R3-R10 control the pitch of those notes.
2. The circuit will operate fine on a 9V battery, with the addition of a 7805 regulator to power the TTL logic used.
3. The missing pin on U2 is pin 14.

50 Watt Amplifier

This is a handy, easy to build general purpose 50 watt amp. The amp has an input for a radio, TV, stereo or other line level device. It also has a phono input for a record player, guitar, microphone or other un-amplified source. With the addition of a low pass filter at the input, it makes a great amp for a small subwoofer.

Schematic

Parts

Part Total Qty. Description Substitutions R1 1 200 Ohm 1/4 W Resistor R2 1 200K 1/4 W Resistor R3 1 30K 1/4 W Resistor R5 1 1K 1/4 W Resistor R6 1 5K 1/4 W Resistor R7,R10 2 1 Meg (5%) 1/2 W Resistor R8,R9 2 0.4 Ohm 5 W Resistor R11 1 10K Pot R12,R13 2 51K 1/4 W Resistor R14 1 47K 1/4 W Resistor C1 1 100uF 35V Electrolytic Capacitor C2 1 0.011uF Capacitor C3 1 3750pF Capacitor C4,C6 2 1000pF Capacitor C5,C7,C8 3 0.001uF Capacitor C9 1 50pF Capacitor C10 1 0.3uF Capacitor C11,C12 2 10,000uF 50V Electrolytic Capacitor U1,U2 2 741 Op Amp U3 1 ICL8063 Audio Amp Transister Driver thingy Q1 1 2N3055 NPN Power Transistor Q2 1 2N3791 PNP Power Transistor BR1 1 250 V 6 Amp Bridge Rectifier T1 1 50V Center Tapped 5 Amp Transformer S1 1 SPST 3 Amp Switch S2 1 DPDT Switch F1 1 2 Amp Fuse SPKR1 1 8 Ohm 50W Speaker MISC 1 Case, Knobs, Line Cord, Binding Posts Or Phono Plugs (For Input And Output), Heatsinks For Q1 And Q2

Notes

1. I know I skipped R4. That is not a problem :-)
2. Distortion is less than 0.1% up to 100HZ and increases to about 1% at 20kHz.
3. I haven't been able to find anyone who sells a suitable T1. You can always use two 24V 5A units in series. If you are building two amps (for stereo), then I would suggest using an old microwave transformer and rewinding it. Follow the instructions in the 12V To 120V Inverter, execpt wind 26 turns, twist a loop (center tap) and wind 26 more turns. That should work out to around 50 volts. You may need to add or remove turns depending on your transformer.
4. Q1 and Q2 will require heatsinks.
5. You may have trouble finding U3 because it is discontinued. Please don't email me about sources...I can't find it either. See if any of the sources in Where To Get Parts has it. A possible source was sent in by JBWilliams

3 Watt FM Transmitter

This is the schematic for an FM transmitter with 3 to 3.5 W output power that can be used between 90 and 110 MHz. Although the stability isn't so bad, a PLL can be used on this circuit.This is a circuit that I've build a few years ago for a friend, who used it in combination with the BLY88 amplifier to obtain 20 W output power. From the notes that I made at the original schematic, it worked fine with a SWR of 1 : 1.05 (quite normal at my place with my antenna).

Schematic

Parts

Part Total Qty. Description Substitutions R1,R4,R14,R15 4 10K 1/4W Resistor R2,R3 2 22K 1/4W Resistor R5,R13 2 3.9K 1/4W Resistor R6,R11 2 680 Ohm 1/4W Resistor R7 1 150 Ohm 1/4W Resistor R8,R12 2 100 Ohm 1/4W Resistor R9 1 68 Ohm 1/4W Resistor R10 1 6.8K 1/4W Resistor C1 1 4.7pF Ceramic Disc Capacitor C2,C3,C4,C5,C7,C11,C12 7 100nF Ceramic Disc Capacitor C6,C9,C10 3 10nF Ceramic Disc Capacitor C8,C14 2 60pF Trimmer Capacitor C13 1 82pF Ceramic Disc Capacitor C15 1 27pF Ceramic Disc Capacitor C16 1 22pF Ceramic Disc Capacitor C17 1 10uF 25V Electrolytic Capacitor C18 1 33pF Ceramic Disc Capacitor C19 1 18pF Ceramic Disc Capacitor C20 1 12pF Ceramic Disc Capacitor C21,C22,C23,C24 4 40pF Trimmer Capacitor C25 1 5pF Ceramic Disc Capacitor L1 1 5 WDG, Dia 6 mm, 1 mm CuAg, Space 1 mm L2,L3,L5,L7,L9 5 6-hole Ferroxcube Wide band HF Choke (5 WDG) L4,L6,L8 3 1.5 WDG, Dia 6 mm, 1 mm CuAg, Space 1 mm L10 1 8 WDG, Dia 5 mm, 1 mm CuAg, Space 1 mm D1 1 BB405 BB102 or equal (most varicaps with C = 2-20 pF [approx.] will do) Q1 1 2N3866 Q2,Q4 2 2N2219A Q3 1 BF115 Q5 1 2N3553 U1 1 7810 Regulator MIC 1 Electret Microphone MISC 1 PC Board, Wire For Antenna, Heatsinks

Notes

1. Email Rae XL Tkacik with questions, comments, etc.
2. The circuit has been tested on a normal RF-testing breadboard (with one side copper). Make some connections between the two sides. Build the transmitter in a RF-proof casing, use good connectors and cable, make a shielding between the different stages, and be aware of all the other RF rules of building.
3. Q1 and Q5 should be cooled with a heat sink. The case-pin of Q4 should be grounded.
4. C24 is for the frequency adjustment. The other trimmers must be adjusted to maximum output power with minimum SWR and input current.
5. Local laws in some states, provinces or countries may prohibit the operation of this transmitter. Check with the local authorities.

3 Watt FM Transmitter

3 Watt FM Transmitter

22 Watt Audio Amplifier

The 22 watt amp is easy to build, and very inexpensive. The circuit can be used as a booster in a car audio system, an amp for satellite speakers in a surround sound or home theater system, or as an amp for computer speakers. The circuit is quite compact and uses only about 60 watts. The circuit is not mine, it came from Popular Electronics.

Schematic

Parts

Part Total Qty. Description Substitutions R1 1 39K 1/4 Watt Resistor C1,C2 2 10uf 25V Electrolytic Capacitor C3 1 100uf 25V Electrolytic Capacitor C4 1 47uf 25V Electrolytic Capacitor C5 1 0.1uf 25V Ceramic Capacitor C6 1 2200uf 25V Electrolytic Capacitor U1 1 TDA1554 Two Channel Audio Amp Chip MISC 1 Heatsink For U1, Binding Posts (For Output), RCA Jacks (For Input), Wire, Board

Notes

1. The circuit works best with 4 ohm speakers, but 8 ohm units will do.
2. The circuit dissipates roughly 28 watts of heat, so a good heatsink is necessary. The chip should run cool enough to touch with the proper heatsink installed.
3. The circuit operates at 12 Volts at about 5 Amps at full volume. Lower volumes use less current, and therefore produce less heat.
4. Printed circuit board is preferred, but universal solder or perf board will do. Keep lead length short.

FM Transmitter

Here is the schematic, PC board pattern, and parts placement for a low powered FM transmitter. The range of the transmitter when running at 9V is about 300 feet. Running it from 12V increases the range to about 400 feet. This transmitter should not be used as a room or telephone bug.

Schematic

PC Board Layout and Parts Placement

Parts

Part Total Qty. Description Substitutions C1 1 0.001uf Disc Capacitor C2 1 5.6pf Disc Capacitor C3,C4 2 10uf Electrolytic Capacitor C5 1 3-18pf Adjustable Cap R1 1 270 Ohm 1/8W Resistor 270 Ohm 1/4W Resistor R2,R5,R6 3 4.7k 1/8W Resistor 4.7K 1/4W Resistor R3 1 10k 1/8W Resistor 10K 1/4W Resistor R4 1 100k 1/8W Resistor 100K 1/4W Resistor Q1, Q2 2 2N2222A NPN Transistor 2N3904, NTE123A L1, L2 2 5 Turn Air Core Coil MIC 1 Electret Microphone MISC 1 9V Battery Snap, PC Board, Wire For Antenna

Notes

1. L1 and L2 are 5 turns of 28 AWG enamel coated magnet wire wound with a inside diameter of about 4mm. The inside of a ballpoint pen works well (the plastic tube that holds the ink). Remove the form after winding then install the coil on the circuit board, being careful not to bend it.
2. C5 is used for tuning. This transmitter operates on the normal broadcast frequencies (88-108MHz).
3. Q1 and Q2 can also be 2N3904 or something similar.
4. You can use 1/4 W resistors mounted vertically instead of 1/8 W resistors.
5. You may want to bypass the battery with a .01uf capacitor.
6. An antenna may not be required for operation.
7. A thanks goes out to Mike_Springer@iname.com for cleaning up the original image once displayed on the page. It looks a lot better now!