Sunday, June 26, 2011

3 Channel Spectrum Analyzer

This 3 channel 15 LED spectrum analyzer is the perfect addition to any audio amp project. It produces fantastic displays on three LED bars that can be individually adjusted for any particular frequency range. The circuit will take line level output from most any audio source, and operates on 12V DC. This means that it can even be run in a car.

R1                                            (1) 100K 1/4W Resistor
R2                                            (1) 820K 1/4W Resistor
R3, R14, R16, R18                    (4) 2.2 Meg 1/4W Resistor
R4, R5, R6                               (3) 22K Pot
R7,R8,R9,R25,R27 R29            (6)10K 1/4W Resistor
R10, R11, R12                         (3) 680 Ohm 1/4W Resistor
R13. R15, R17                         (3) 580K 1/4W Resistor
R19, R20, R21                         (3) 39K 1/4W Resistor
R22, R23, R24                         (3) 47K 1/4W Resistor
R26, R28, R30                         (3) 33 Ohm 1/4W Resistor
C1, C5, C6                               (3) 0.012uF Polystyrene Capacitor
C2, C9, C10, C11                     (4) 3.3uF Electrolytic Capacitor
C3, C4                                    (2)0.0022uF Polystyrene Capacitor
C7, C8                                    (2)47nF Polystyrene Capacitor
C12, C13, C14                        (3)0.47uF Electrolytic Capacitor
C15, C16, C17                        (3)22uF Electrolytic Capacitor
D1, D2, D3                             (3)1N4002 Silicon Diode
D4, D5, D6, D8, D8                 (5)Green LED
D10,D11,D12,D13,D14            (5)Amber LED
D16,D17,D18,D19,D20            (5)Red LED
U1                                         (1)LM3900 Quad Op Amp
U2, U3, U4                             (3)AN6884 Bar Graph IC
MISC                                     (1)Board, Wires, Sockets For ICs


  1. The circuit expects line level inputs. If you connect it to speaker level, you will have to readjust the circuit every time you change the volume.
  2. After the circuit is connected, apply power and signal. Now adjust the pots until the corresponding group of LEDs reacts.
  3. Thanks to Alterac for correcting some errors on the schematic that originally appeared on this page. Read all about it in the 3 Channel Spectrum Analyzer Project thread in the forum.


Friday, June 24, 2011

Strobe Light

Disco anyone? Actually, this strobe serves a much more useful purpose then making it look cool when you dance in the dark. You can use it to view fast moving objects, look for craks in PC boards (hold the strobe on the trace side of the board and look from the component side), and it is a great attention getter in a store window.

R1            1        10 Meg, 1/4 Watt Resistor
D1, D2      2       1N4003 Silicon Diode or 1N4007
C1, C2      2       10uF 350V Capacitor
C3            1         0.1uF 400 Volt Mylar Capacitor
T1             1        4KV Trigger Transformer (see "Notes")
FT             1       Flash Tube (see "Notes")
L1             1        Neon Bulb
Q1             1        106 SCR
S1             1        115V 1A SPST Switch
MISC         1        Case, Wire, Line Cord

  1. This circuits is NOT isolated from ground. Use caution when operating without a case. A case is required for normal operation. Do not touch any part of the circuit with the case open or not installed.
  2. Most any diodes rated at greater then 250 volts at 1 amp can be used instead of the 1N4003's.

Thursday, June 23, 2011

Infa-Red Remote Control

I have received a number of emails requesting schematics for infa-red remotes. So here is one. This remote transmits a tone using an infa-red LED. This tone is decoded by the receiver. Since the receiver only switches when it "hears" the tone, there are no accidental activations.

Schematic For Transmitter

Schematic For Receiver


R1              1               22K 1/4W Resistor
R2              1               1 Meg 1/4W Resistor
R3              1               1K 1/4W Resistor
R4, R5        2               100K 1/4W Resistor
R6              1               50K Pot
C1, C2        2                0.01uF 16V Ceramic Disk Capacitor
C3              1               100pF 16V Ceramic Disk Capacitor
C4              1               0.047uF 16V Ceramic Disk Capacitor
C5              1               0.1uF 16V Ceramic Disk Capacitor
C6              1               3.3uF 16V Electrolytic Capacitor
C7              1               1.5uF 16V Electrolytic Capacitor
Q1              1               2N2222 NPN Silicon Transistor or 2N3904
Q2              1               2N2907 PNP Silicon Transistor
Q3              1               NPN Phototransistor
D1              1              1N914 Silicon Diode
IC1              1              LM308 Op Amp IC
IC2              1              LM567 Tone Decoder
LED1          1              Infa-Red LED
RELAY       1              6 Volt Relay
S1              1              SPST Push Button Switch
B1              1              3 Volt Battery or Two 1.5V batteries in series
MISC          1              Board, Sockets For ICs, Knob For R6, Battery Holder
RELAY       1              6 Volt Relay

Tuesday, June 21, 2011

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.
R1,R3            2               10k pot
R2,R4            2               100k
R5                 1               6.8k
C!,C2,C3        3              0.1uF capacitor
Q1                1               2N3819 junction fet
MISC            1               wire,shielded(metal)case,phono or other plug for ouput


  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.

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.
C1                1           0.1uF ceramic disc capacitor
U1                1           DS1669 digital pot ic
S!,S2           2           momentary push button switch
MICS           1           board,wire,socket for u1


  1. S1 turns the volume up, S2 turns it down.
  2. The input signal should not fall below -0.2 volts.
  3. 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.

Saturday, June 18, 2011

LASER Transmitter/Receiver

This set of two circuits from the basis for a very simple light wave transmitter. A LASER beam is modulated and then aimed at a receiver that demodulates the signal and then presents the information (voice, data, etc..). The whole thing is very easy to build and requires no specialized parts execpt for the LASER itself. LASERs are available from MWK Industries.

Schematic Of Transmitter

Schematic Of Receiver

C!,C2               2             0.1uF ceramic disc capacitor
C3                    1             100uF 25v electrolytic capacitor
R1                    1             100k ohm 1/4w resistor
R2                    1             1M ohm 1/4 resistor
R3                    1             10k pot
Q1                    1              NPN phototransistor
U1                    1              741 op amp
U2                    1              LM386 audio amp
SPKR1            1              8ohm speaker
T1                     1             8ohm:2k audio transformer
MISC                1             wire,board,knob for r3,laser tube and power supply


  1. In the transmitter schematic, no ballast resistor is shown because most small LASER power supplies already have one built in. Yours may differ, and a resistor may be needed.
  2. The receiver should be kept away from bright lights. You may want to put a piece of wax paper in front of Q1 to keep the LASER from swamping it.
  3. In order to get any decent amount of modulation, you may need to drive T1 with more then a watt.
  4. The circuit can be made to transmit computer data with the use of two modem chips.

Friday, June 17, 2011

Coin tosser

R1                     47k
R2                     10k
R3                     330ohm
R4                     330ohm
C1                     0.01uF
IC1                   555
IC2                   7473
leds                   2(red and green)
S1                     switch

9v will also run this circuit.


Thursday, June 16, 2011

Car Battery Charger

  1. This charger will quickly and easily charge most any lead acid battery. The charger delivers full current until the current drawn by the battery falls to 150 mA. At this time, a lower voltage is applied to finish off and keep from over charging. When the battery is fully charged, the circuit switches off and lights a LED, telling you that the cycle has finished.

R1                         500ohm
R2                         3k
R3                         1k
R4                         15ohm
R5                         230ohm
R6                         15k
R7                         0.2ohm
C1                         0.1uF 25v ceramic capacitor
C2                         1uF 25v electrolytic capacitor
C3                         1000pF 25v ceramic capacitor
D1                         1N457 diode
Q1                          2N2905 pnp transistor
U1                          LM350 regulator
U2                          LM301A opamp
S1                          normally open push button switch
MICS                     wire,board,heatsink for u1,case,binding posts or alligator clips for output


  1. The circuit was meant to be powered by a power supply, which is why there is no transformer, rectifier, or filter capacitors on the schematic. There is no reason why you cannot add these.
  2. A heatsink will be needed for U1.
  3. To use the circuit, hook it up to a power supply/plug it in. Then, connect the battery to be charged to the output terminals. All you have to do now is push S1 (the "Start" switch), and wait for the circuit to finish.
  4. If you want to use the charger without having to provide an external power supply, use the following circuit.
C1             6800uF 25v electrolytic capacitor
T1              3A 15v transformer
BR1           5A 50v bridge rectifier
S1              5A SPST switch
F1              4A 250v fuse

The first time you use the circuit, you should check up on it every once and a while to make sure that it is working properly and the battery is not being over charged. 

Wednesday, June 15, 2011

Car Alarm Arming Horn Beep Canceller

It's a great convenience that most modern cars come with a built in alarm, however it is nothing but noise pollution that the horn sounds when the alarm is armed. Disconnecting the alarm system from the horn relay will eliminate this, but prevent the horn from sounding in the even of an actual alarm. This circuit serves to silence the arming beep yet maintain the alarm by introducing a small delay into the signal. It sits between the alarm and horn relay. The alarm must provide a constant horn signal for at least 3 seconds before the horn relay is activated. That way the quick "beep" will never activate the horn relay, while the constant alarm signal will.
C1               0.01uF ceramic disc capacitor
C2               100uF  35v electrolytic capacitor
R1                1k
R2                10k
R3                15k
R4                470ohm
D1,3,4         3(1N4004)rectifier diode
D2                red led
U1                555 timer ic
K1                spst 12v automotive relay

Running Message Display

Light emitting diodes are advan- tageous due to their smaller size, low current consumption and catchy colours they emit. Here is a running message display circuit wherein the letters formed by LED arrangement light up progressively. Once all the letters of the message have been lit up, the circuit gets reset. The circuit is built around Johnson decade counter CD4017BC (IC2). One of the IC CD4017BE�s features is its provision of ten fully decoded outputs, making the IC ideal for use in a whole range of sequencing operations. In the circuit only one of the outputs remains high and the other outputs switch to high state successively on the arrival of each clock pulse. The timer NE555 (IC1) is wired as a 1Hz astable multivibrator which clocks the IC2 for sequencing operations. On reset, output pin 3 goes high and drives transistor T7 to �on� state. The output of transistor T7 is connected to letter �W� of the LED word array (all LEDs of letter array are connected in parallel) and thus letter �W� is illuminated. On arrival of first clock pulse, pin 3 goes low and pin 2 goes high. Transistor T6 conducts and letter �E� lights up. The preceding letter �W� also remains lighted because of forward biasing of transistor T7 via diode D21. In a similar fashion, on the arrival of each successive pulse, the other letters of the display are also illuminated and finally the complete word becomes visible. On the following clock pulse, pin 6 goes to logic 1 and resets the circuit, and the sequence repeats itself. The frequency of sequencing operations is controlled with the help of potmeter VR1.
The display can be fixed on a veroboard of suitable size and connected to ground of a common supply (of 6V to 9V) while the anodes of LEDs are to be connected to emitters of transistors T1 through T7 as shown in the circuit. The above circuit is very versatile and can be wired with a large number of LEDs to make an LED fashion jewellery of any design. With two circuits connected in a similar fashion, multiplexing of LEDs can be done to give a moving display effect


Tuesday, June 14, 2011

Simple Lie Detector

R1             33k
R2             5k pot
R3             1.5k
C1             1uF   16v electrolytic capacitor
Q1             2N3565 NPN transistor
M1             0-1mA analog meter
mics            wire,electrodes


  1. The electrodes can be alligator clips (although they can be painful), electrode pads (like the type they use in the hospital), or just wires and tape.
  2. To use the circuit, attach the electrodes to the back of the subjects hand, about 1 inch apart. Then, adjust the meter for a reading of 0. Ask the questions. You know the subject is lying when the meter changes.


LED Mood Light

LED Mood Light


This circuit makes a nice lamp that consumes little power, runs cool, and has an incredibly long lifetime. The lamp puts out a warm yellow shade of light, the color may be adjusted by changing the number of red or green LED strings.


Operating Voltage: 12V DC
Operating Current: 80ma approx.


The current flows and the LEDs light. There are 4 series strings of LEDs in this circuit, the resistors limit the current through the LEDs and prevent them from burning up. The resistors were adjusted to get approximately 20ma through each string. Different LED colors will have different voltage drops and resulting current flows. These resistor values will work fine at 12V.


The circuit was mounted in a plastic 35MM photographic slide box with a translucent plastic top (not shown). A piece of perforated board was cut to fit inside of the slide box, the LEDs and resistors were inserted through the holes in the board and the wires were soldered together on the back side of the board. It is recommended that you use a heat sink on the LED leads while you solder them, it is very easy to damage an LED with a soldering iron. A length of speaker wire was used to connect the lamp to the power source. The circuit board was connected to the plastic box with one 6-32 screw and several 6-32 nuts were used as spacers.


None required


Connect this circuit to a 12V battery or power supply, it puts out a nice glow that won't affect your night vision too much. You can read by this light if you use it at close range.


10x green LEDs, T1-3/4 size
9x red LEDs, T1-3/4 size
3x yellow LEDs, T1-3/4 size
2x 100 ohm 1/2 W resistors
1x 56 ohm 1/2 W resistor
1x 62 ohm 1/2 W resistor (can also be 56 ohm, it's what I had on hand)
1x 1/2 Amp 28VDC fuse
1x Plastic box with diffused (blurry) top, I used a 35mm slide box.
1x 6-32 3/8" screw
2x 6/32 nuts


Simple Microphone Preamplifier

This simple microphone preamplifier circuit can use between your microphone and stereo amplifier. This amplifier microphone suitable for use with normal home stereo amplifier line/CD/aux/tape inputs. This microphone preamplifier can take both dynamic and electret microphone inputs (preamplifier provides power foe electret microphone elements).

This circuit idea is to keep the design as simple as possible to be easy to build. That was my goal when I needed a simple external microphone preamplifier for my mixer. The performance of the circuit is nothing superior but can be used with many not so serious projects. 

C1,C2,C3,C4         10uF
C5                          100uF
R1                          4.7k
R2                          220k
R3                          2.2k
R4                          120ohm
led                           1 red
Q1                           BC547B 
dynamic mic,input pin,output pin            

Microphone Preamplifier Circuit Features
  • Brief description of operation: Simple microphone preamplifier
  • Circuit protection: No special protection circuits used
  • Circuit complexity: Very simple one transistor circuit
  • Circuit performance: Amplification 35 dB, flat frequency response from 20 Hz to 20 kHz, quite poor distortion performance figures, a little bit noisy
  • Availability of components: Uses common and easily available components
  • Design testing: I have built few microphone preamplifiers based on this circuit and theu have worked without problems.
  • Applications: Interface dynamic or electret microphone to a line level audio input in HIFI amplifier or computer soundcard.
  • Power supply: 9V battery, takes less than 10 mA current
  • Estimated component cost: Electronics components than $10
  • Safety considerations: No special electrical safety considerations.

The circuit is a simple one transistor amplifier with amplification of about 30-40 dB (depends on transitor, temperature and voltage). The dynamic mic input is just a simple one transistor amplifier circuit with nothing special in it. LED D1 is in the circuitto show that the circuit operates. The voltage drop caused by LED (around 1.8V for RED led) has been taten in account when designing the amplifier circuit built around Q1. Resistor R4 and capacitor C5 make a filter to filter out possible noise from battery or other power source which is used to feed this circuit. Capacitors C1, C2 and C3 are used to block the DC bias on Q1 base to flow out of microphone input to microphone (the polarity of all capactors is straigh line = + and curved line = -).

Electret microphone input has a resistor R1 fo feeding current through electret microphone capsule when it is connected to the electret microphone input. Electret microphone needs some current (about 1 mA) flowing through it to operate, because there is a small amplifier circuit inside the microphone capsule. This circuit is suitable for all typical cheap electret capsules which available from any electronic component shop. Because electret microphones have higher signal level output, it is quite easy to overdrive the amplifier when you shout to electret microphone.

The circuit is bet to build to a small metal box like in the picture above. Put the 9V battery inside the case too. Battery power and metal box keep external noise and interference sources away. I used standard 6.3 mm jack for dynamic microphone and 3.5 mm mono jack for electret micrphone both installed to from, panel of the metal box. The LED and power switches are also installed to front panel.


Portable CD Player Adapter For Car

Whenever I'm in the car listening to my favourite CD, it always happens; my batteries go dead. To solve that problem, I built this extremely simple regulator circuit. It steps down the 12V from the lighter socket to 9V which is used by the CD player. Different CD players (I have a Sony Discman) may require different voltages, so just use the correct regulator. All the 78xx series regulators have the same pin out, so the circuit is universal.
Electrolytic capacitor
C1     1000uF 25 v
C2     10uF     25 v
C3     1uF       15 v
C4      0.1uF   15 v
U1     7809
Cigarette lighter plug,plug for cd player

Monday, June 13, 2011

Black Light

This circuit is a simple ultraviolate light that can be powered by a 6 volt battery or power supply that is capable of supplying 1 or more amps.

C110.0047uf Mono Capacitor
C210.1uf Disc Capacitor
D1, D221N4007 Diode
FTB1Filtered Blacklight Tube
IC11555 Timer IC
P1110k Trim Pot
Q11TIP30 PNP Power Transistor
R11470 Ohm Resistor
R21270 Ohm Resistor
T11Medium Yellow Inverter Transformer
MISC1IC Socket, Heat Sink For Q1, Screw, Nut, Wire and PC Board


1. P1 changes brightness of the black light tube.