Hawes Mechanical Television Archive by James T. Hawes, AA9DT
Mechanical Television Camera Experiments, Part 2

Build Stage-by-Stage

Here's an easy way to beef up that amplifier: "Tinkertoy" stages that you just snap together. Perfboard construction is fine for this circuit. Notice that NPN and PNP stages alternate. For the PNP device, use a 2N3906, 2N2907 or compatible device. Again, Hfe (beta) must be 200 or better, and the device must be silicon.

You can add one, two, or several stages this way. Just follow each NPN transistor with a PNP stage. Then follow each PNP stage with an NPN stage. You'll need an LED driver to test the circuit with. Here's the LED driver circuit.

Easy, snap-together stages
give us the gain that we need.


Here's the design procedure...

  1. Add one transistor stage at a time.

  2. Then test the circuit revision with an LED driver. The only reliable test is a scanner test. Scan the LED and watch for some sort of image. The image doesn't have to be perfect. It must have contrasting areas, though. Reflected light falling on the phototransistor must cause the LED to flicker.

  3. As necessary, add stages one by one.

  4. After adding each stage, retest.

  5. When you get enough gain, stop adding stages.

  6. If you don't succeed after adding five stages, you have a wiring error. Quit and troubleshoot the circuit.

Construction Pointers

How do you know when you have enough gain? Easy. Plug the last stage into the input of the LED driver. Do the LEDs twinkle when they pick up reflected (not direct) light? Do the LEDs twinkle this way when you scan them? Good. Then you probably have the gain you need. Otherwise, repeat the design procedure.

Be sure to wire the stages correctly. Before switching on the power, check everything. A visual check takes far less time and costs less money than a smoke test! Make sure that you haven't reversed the PNP and NPN stages. You can damage some transistor types by reversing the collector and emitter connections.

Picture polarity. Some camera circuits produce positive-polarity signals. Our second schematic (page 1) is an example of this type circuit. Other circuits invert the polarity and produce negative signals. For example, see our first schematic (page 1).

Polarity effects. For our purposes, signal polarity is only important in the resulting picture. If we reproduce negative signals on the TV screen, the light and dark shades appear reversed. (In color TV, the colors might also reverse. That is, red appears as cyan, green becomes magenta, and blue becomes yellow.) Obviously the effect is totally unnatural.

Restoring positive polarity. We can easily restore a negative picture to its original polarity values. The trick is to run the signal through an inverting amplifier. At the output, a negative signal again becomes positive.

The US television system NTSC transmits negative-polarity TV signals. The advantage of this transmission method is that noise bursts appear in black. To the viewer, blacked-out noise bursts seem less distracting than white bursts. You can read more about picture polarity at Troubleshoot Negative Mechanical TV Pictures.

Go to Page:   1   2   3   Next


Main mechanical television page





Build Stage-by-Stage


Construction Pointers


Use Preamp Designer

Google Search
Web www.hawestv.com

Watch the latest videos on YouTube.com

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, 2003, 2004, 2006 by James T. Hawes. All rights reserved.

•URL: http://www.hawestv.com/mtv_cam/mtv_cam.htmWebmaster: James T. Hawes
•Revision—November, 2013  •Page design tools: HTML & Notepad •Also Word, Explorer & FrontPage