Hawes Mechanical Television Archive by James T. Hawes, AA9DT

Animation: Mechanical TV receiver with magnifier

Build a Mechanical TV Monitor, Part 3


  1. When you build the circuit, wire it up except for the resistor between the transistor Q1 base and Vcc (10K on the schematic).

  2. A 10K resistor works fine in my circuit, but may not work with all transistors.

  3. Clip in a 10K resistor and check the circuit performance with the CD audio.

  4. If the contrast range is good, then solder in the 10K resistor.

  5. If the LEDs tend to remain dim or bright, but don't vary much, try a resistor that's close in value to 10K.

  6. Some other resistors that might work are 4.7K, 5.6K, 6.8K, 7.5K, 12K, 15K or 22K. Try each of these resistors in sequence and retest after you clip each one into the circuit.

  7. One value should work much better than the others do.

  8. Also, during the test, be sure to run the CD volume control through its range. I expect the best resistor value to perform optimally with the volume control between its midpoint and all the way down.

  1. When you're ready to use the LED driver, make sure that the left channel of your CD player outputs inverted video. If the CD player outputs positive video, either flip the driver's input leads or use an inverter. (The inverter goes between the CD output and the LED driver.) If you don't know which type video your CD outputs, skip this step. When you begin watching pictures, you'll either see a positive or a negative image. If you see a negative image, you have a CD player with positive video. For a positive (normal-looking) picture, flip the input leads.

  2. Scan the display with a Nipkow disc or other means.

  3. If the CD player features an isolated phone output: Connect the display circuit directly to the CD player.

  4. If you don't know what type output your CD player has: Use a 1uF isolating capacitor. Remember that any capacitor adds visible phase shift artifacts to your images. A capacitor causes average gray values to vary in brightness. Depending on the scene, background objects tend to dim or brighten. (This problem is the reason for a DC corrector circuit.) An electrolytic capacitor introduces leakage that changes the stage bias and affects all gray values. In high-impedance amplifiers, the leakage can be great enough to block the signal. Avoid electrolytic coupling capacitors!

  5. Set the CD volume control. You should now have usable video.

  6. Sync up the picture by your own means.

                    schematic: Inverting preamp.

Conventional inverting preamp with gain. (Parts values for reference only.)

                    schematic: Phase splitter produces inverted or non-inverted output

No-gain preamp with phase splitter. Switch allows you to choose inverted or non-inverted video. (Parts values for reference only.)

                    schematic: High-Z, inverting preamp.

High-impedance inverting preamp with gain. (Parts values for reference only.)


Right: Front view of the Radio Shack 276-150 PCB. Note the small, PNP voltage preamp below the Darlington device. The ground connection is on the top of the PCB. Vcc connects to the rings on the bottom of the board. Note: A plain perfboard would have probably worked better than this board did, but the size is about right. If you want to add a phase inverter, you'll need another half inch or so of PCB.

PCB photo; front view

Right: Back view of 276-150 PCB. LED matrix wiring is on the top. The driver electronics are on the bottom. Between the display and drive electronics, I cut the board's ground and Vcc buses. I then substituted my own ground bus across the top of the PCB. The Vcc bus runs across the bottom of the PCB.

PCB photo; back view

• Questions about this amplifier? Troubles with the circuit? See our LED Driver FAQ page. New for 2007!

• Also see Simplest TV circuit.

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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 © 2002 by James T. Hawes. All rights reserved.

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