Category Archives: Video

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

General Purpose 2 x 2Watts Power Amplifier

A general purpose audio power amplifier is a must have for the electronics amateur. It’s not a good thing to use your HiFi set for an experiment, when there’s a risk of blowing it’s transistor out. Amplifier for your experiments should be simple in construction, durable, and easy to repair. Also a portable, low power consumpting, battery operateable one will be suitable.

Taking the considerations above, I gave you the PCB design for the TBA820M based amplifier. It is rated for 2Watts of RMS power output (16W PMPO) but gives even two times more if you cool it well by some tricks. I’ve been using this circuit for over ten years and personally still surprised by it’s durability, thinking of how many short circuits and overdrives it is subjected.

I did not know if TBA820M is still manufactured (my IC’s are made by SGS-Thomson), so it may be obsolete in your country and hard to find. But if you find it in a local electronics shop, purchase a dozen, they will worth it. A simplified, scaled down version of this circuit may be used anywhere, where a power amplifier is needed.

[code:1:c08b8d4725] Technical datas:

Supply voltage : 3 to 16 Volts
Load impedance : 4 TO 16 Ohms
Power output : 2 Watts RMS
Gain : 40 dB (x100)[/code:1:c08b8d4725]

Here is the PCB design for two channels on it. I tried to make it as small as possible but volume control potentiometer took a lot of space. In a special design, area it’s occupied may be omitted. Also usage of high quality small sized capacitors may help you to reduce the circuit size further. You may download this PCB design as a TIFF file which is prepared to be printed at 300DPI. You may refer to my PCB Design Page ([url][/url])for more details on how to create your own PCB’s.

After the etching process of the PCB, solder all of the components on. Please use IC sockets in order to be able to change TBA820M’s if you blow them out somehow. Here is a photo of the completed circuit, ready to serve for years.

Completed amplifier

Copyright 2006 [url=]Burak[/url]

Decibel Meter

The circuit below responds to sound pressure levels from about 60 to 70 dB. The sound is picked up by an 8 ohm speaker, amplified by a transistor stage and one LM324 op-amp section. You can also use a dynamic microphone but I found the speaker was more sensitive. The remaining 3 sections of the LM324 quad op-amp are used as voltage comparators and drive 3 indicator LEDs or incandescents which are spaced about 3dB apart. An additional transistor is needed for incandescent lights as shown with the lower lamp. I used 12 volt, 50mA lamps. Each light represents about a 3dB change in sound level so that when all 3 lights are on, the sound level is about 4 times greater than the level needed to light one lamp. The sensitivity can be adjusted with the 500K pot so that one lamp comes on with a reference sound level. The other two lamps will then indicate about a 2X and 4X increase in volume.

In operation, with no input, the DC voltage at pins 1,2 and 3 of the op-amp will be about 4 volts, and the voltage on the (+) inputs to the 3 comparators (pins 5,10,12) will be about a half volt less due to the 1N914 diode drop. The voltage on the (-) comparator inputs will be around 5.1 and 6.5 which is set by the 560 and 750 ohm resistors.

When an audio signal is present, the 10uF capacitor connected to the diode will charge toward the peak audio level at the op-amp output at pin 1. As the volume increases, the DC voltage on the capacitor and also (+) comparator inputs will increase and the lamp will turn on when the (+) input goes above the (-) input. As the volume decreases, the capacitor discharges through the parallel 100K resistor and the lamps go out. You can change the response time with a larger or smaller capacitor.

This circuit requires a well filtered power source, it will respond to very small changes in supply voltage, so you probably will need a large filter capacitor connected directly to the 330 ohm resistor. I managed to get it to work with an unregulated wall transformer power source, but I had to use 4700uF. It worked well on a regulated supply with only 1000uF.

Copyright 2006: [url=]Bill Bowden[/url]

Video Clock Superimposer

As a followup to my VCR Pong project, here is a gadget that is actually useful in the Real World! It superimposes the time of day, in “HH MM SS” format, in the bottom right-hand corner of an existing video signal. My friend Scott uses it with his home security system.

In keeping with the tradition of my previous hacks, I use few parts and lots of tricks. All timing signals, including the timebase for the time of day and the clock for the microprocessor, are derived from the incoming video signal. That means if you lose power, or lose video, it loses the time… but in this home security application, that didn’t matter. Lost power would mean the VCR stops recording and starts flashing 12:00 (1:00 during daylight savings).

[b:9d01d86fcf]How does it work[/b:9d01d86fcf]
The LM1881 separates the composite sync signal from the incoming video. The 74HCT4538A one-shot eliminates the vertical sync and equalizing pulses, to provide a nice clean horizontal sync. (Yes, Elantec makes some chips that do this in one package.) A 16.1 MHz signal from the VCO of a 74HCT4046 PLL is phase locked to the horizontal sync via the 74HCT4020 configured to divide by 1024. That 16.1 MHz signal then serves as a clock for the PIC. The PIC in turn operates the 74HCT4051 multiplexer at just the right moments to put the clock pixels up in place of normal video. Having the PIC syncronous with the video signal allows crisp overlaying with no jaggies.

The code uses the Parallax instruction syntax, so you’ll either need to use Parallax’s SPASM.EXE (available for free on [url=]Parallax’s web site[/url]) or Tech-Tools’s CVASM16.EXE (available for evaluation on [url=]Tech-Tools’s web site[/url]). You can also download the preassembled object file below, in INXH8M format, for use with any device programmer.

Copyright 1998 [url=]David B. Thomas[/url]Parts:C1 .1 uF
C2 470 pF
C3-4 .1 uF
C5 .01 uF (shown as 10nF)
C6 .1 uF
C7 47 pF
C8-13 .1 uF
Q1 2N4124, 2N2222, 2N3904 or equiv. NPN
R1 680 ohms
R2 681K
R3 4.7K
R4-7 10K
R8 1K
R9 2.2K
R10 75 ohms
R11-12 10K
S1-2 normally open momentary pushbutton
U1 LM1881 sync separator
U2 74HCT4538A one-shot
U3 74HCT4046 pll
U4 74HCT4020 divider
U5 74HCT4051 multiplexer
U6 PIC16C711 microcontroller (Microchip)

Simple VGA/Video adapter

Several months ago I tried to connect a microcontroller system to a VGA monitor to output data in the form of text. I was surprised to find little on this subject on the internet, to assist me in achieving this goal. Certainly nothing simple a beginner could find useful. There are examples out there that utilise standards such as PC-104 or complex FGPA implementations found at Other solutions include graphic controllers from Fujitsu or even one local Russian person who was offering for sale a project for $5000 on ACEX. These are fine but are little help to most hobbyists etc out there who wish to display text on a VGA or similar screen.

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Do you want to listen to MP3 on your radio? But yout radio doesn’t have a AUX IN or any other Input? Then audioFM can help you. audioFM takes audio signals from a headphone plug and converts it into radio UKW FM signals.

The heart of this small FM Modulator is a tuneable voltage controlled oscillator (VCO) from Maxim-IC called MAX2606. This VCO operates from 70MHz to 150MHz and has a up to -8dBm differential output amplifier.

[color=red:61ebf9f576]Note: in many countries it’s not allowed to broadcast in the UKW (normal Radio) frequency bands!! So use this at your own risk![/color:61ebf9f576]

The audioFM project is based on schematics found in the Internet and from Elektor Magazine!

The EAGLE CAD files will be published at [url][/url] for download after testing. Maybe a filter for pre-emphasis should be added to get a better audio quality.

Copyright Tobias Floery [url][/url]