Hawes Mechanical Television Archive by James T. Hawes, AA9DT
Simplest TV Circuit

NOBODY SAID THAT LIFE HAD TO BE HARD. Besides, you came here to take it easy. I won't disappoint you. How about a hobby that's as easy as watching TV? In fact, in a way, it is watching TV: A TV that you can build yourself. How much more satisfying than just watching!

MECHANICAL TV is what you get when you apply Occam's Razor to regular TV: All the kludges shear off. What you have left just works. Better still, you can see how it works. It's malleable, so you can change it, too. To your heart's desire. Since you're here, I assume that you already know most of the other details about mechanical TV.

YES! ONE-TRANSISTOR TV. The circuit at right is a one-transistor TV monitor. It's one of the simplest TV circuits you can build. I've seen a no-transistor version, but it might not work as reliably. Anyway, my circuit reduces a TV monitor down to the scanning disc and an LED driver. Where does the TV signal come from? It comes from a CD. You can buy that CD from the Narrow Bandwidth Television Association. While you're surfing over there, join our organization. You'll learn a lot more about television and how to make better sets.

THE TRANSISTOR accepts the TV signal from the CD phone jack. The incoming signal is a variable current. The transistor amplifies this signal enough to drive the LED. The LED requires about 20 milliamperes at 1.5 to 2 volts or so. This circuit can drive a red, orange or yellow LED. Some green LEDs might work at reduced brightness. I don't recommend blue LEDs. At minimum, they require a change in the collector resistor value. The actual value depends on the LED voltage drop. If your LED needs 3 volts to light, then this circuit's power supply voltage is probably too low. You need an entirely new circuit. Click here to design your own.

HOW TO FORM THE PICTURE. Design a shadow box for the LED. This is a small, rectangular box. I use a metal, Bud® chassis box from Radio Shack®. You'll mount the LED a few inches back from the end of the box. At the end opposite the LED location, cut a window the size of your video frame. Cover this window with diffusion material such as tracing paper, ground glass or milk bottle plastic. Use a 32-hole, NBTVA-standard Nipkow disc to scan the LED. While mounting the LED, test it. The light from the LED must completely fill the diffusion window. The window should be as evenly illuminated as you can get it. Rotating the LED helps. (Due to LED element mounting, LED light isn't even or uniform.) You might see some rings on the diffusion material. Try to minimize them. If they aren't extreme, you probably won't notice them. Adding LEDs helps to reduce the rings. Of course, we aren't adding anything! This is simple TV!

MECHANICAL ASSEMBLY. Position the shadow box squarely behind the disc. The diffusion window must face the back of the disc. You must be able to see the LED light through the disc holes. Switch on the disc. Place the CD in the player, and turn on the player. You should also darken the room light. For the sake of simplicity, this set doesn't put out much light.

LISTEN to the CD. When you hear the scanning sound, connect the LED driver to the CD player. Now adjust the disc motor speed until you achieve sync. You should see pictures. Are they too bright? Too dim? Adjust the CD volume control for best brightness. Do you see negative pictures? Then reverse the input wires at capacitor C1 and ground. You should now see positives. That's it! I told you it'd be simple!

VERY BRIGHT DISPLAY WITH ONE TRANSISTOR. Mouser sells a staggeringly bright red LED. This LED is fairly economical. I adapted my "simplest LED driver" to power three of these LEDs. I have the LEDs on order, and will report my findings. Meanwhile, on this page, I've added a new driver for this super-bright LED. Since the top schematic is an inverting driver, the new driver (bottom-right) doesn't invert. Pick the circuit that works best for you.

Please be sure to stick to the parts I've shown. The collector resistor is a half-watt part. The other resistors can be quarter-watt or half-watt parts. The Mouser LEDs have a very narrow viewing angle. Keep them a few inches back from the diffuser. Doing that should spread the light out.

January, 2009. I've now tested a circuit with three of the Mouser LEDs. They are very bright, as advertised. Still, these LEDs have a fairly narrow dispersion angle. The light must spread up to the screen size. For that reason, these LEDs aren't as bright as their millicandela rating suggests. Anyway, my circuit produces a much brighter picture than I achieved from one LED. Also, the multiple LEDs cancel the "light ring" effect that one LED produces. (This is a very slight effect, and it didn't bother me.) The LED color is actually more orange than red. This orange color is ideal for producing realistic flesh tones.

The circuit that I used differs from the above. I connected the LEDs to the emitter of a Darlington driver. I'm not sure that I have as many gray tones as I noticed before. I'll have to run more experiments.

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Schematic: Simplest mechanical TV monitor circuit: 3V, 1 LED

Simplest mechanical TV monitor circuit! (Hint: It's just a super LED driver) Build it!

LED: RS 276-086
Transistor: RS 276-2009
Capacitor: RS 272-1013
Perfboard: RS 276-1394
Project box: RS 270-238
R1 & R2 are quarter-watt parts.
R3 is a half-watt part.

Schematic: Non-inverting version of simplest mechanical TV monitor circuit

Non-inverting driver operates on 12 volts; drives 3 LEDs.

LED (3): 78-TLCTR5800
Color: Red
MCd: 20,000 max
Manufacturer: Vishay
Nominal volts: 2.1
Nominal current: 20 mA
Peak current: 50 mA
Order from: Mouser

WARNING. This is your project. Your achievement is entirely yours. I assume no responsibility for your success in using methods on these pages. If you fail, the same is true. I neither make nor imply any warranty. I don't guarantee the accuracy or effectiveness of these methods. Parts, skill and assembly methods vary. So will your results. Proceed at your own risk.

WARNING. Electronic projects can pose hazards. Soldering irons can burn you. Chassis paint and solder are poisons. Even with battery projects, wiring mistakes can start fires. If the schematic and description on this page baffle you, this project is too advanced. Try something else. Again, damages, injuries and errors are your responsibility. — The Webmaster

Copyright © 2006 by James T. Hawes. All rights reserved.

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