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- Duplicate your LED driver circuit (power amplifier) once for two-color TV. For three-color TV, make two copies of your LED driver.
- Connect the LEDs to the new LED drivers.
- Test the set.
Option: Derived Color
Following the instructions above, build a two-color camera and a three-color monitor. You can build your camera to pick up any two of the additive primary colors. Just install the proper color filters over the phototransistors or photodiodes. With a matrix circuit and an inverter, you will derive the third color. The trick? You might not have the third color, but you can create its complement. A bit of electronics turns the complement back into the "missing" primary color.
- Derive Blue... You want a blue signal, but your camera only picks up green and red.
- At the monitor, connect a potentiometer to the red and green LED driver outputs. This potentiometer is your resistor matrix. Some point on the pot's resistance element corresponds to yellow, the complement of blue.
- With a transistor, invert the yellow signal, and you have your blue. The result is as accurate as if it came from a blue phototransistor.
- You can derive this third color signal at the camera or at the monitor. For maximum savings in circuitry, derive the signal at the monitor (LED driver).
- Derive Green... Suppose that you only have red and blue, but want green.
- Connect a potentiometer between the red and blue transistor collectors (or op amp outputs).
- The red signal is at one end of the potentiometer. The blue signal is at the other end. With the wiper at the center position, you get a mix of red and blue, or magenta. This magenta signal is what you want. Center the wiper.
- Run the wiper of the pot to the base of a new transistor (or an op amp input). With the transistor or op amp, invert the magenta signal. The inverted signal is your green drive signal.
- Derive Red... Now you want red, but only have green and blue. Cyan is between green and blue. With a transistor, invert the cyan signal and you have your red.
Another way to derive a third color: An inverting mixer.
Inverting mixer. An inverting mixer is an easy way to derive a third color. (See the schematics above.) For experimental use, use the circuits as-is. The phasing is only approximate. For better phasing, add level-adjustment pots to the transistor inputs.
- Red. To derive the red signal, use the left circuit.
- Green. To derive the green signal, use the middle circuit.
- Blue. To derive the blue signal, use the right circuit.
Theory. Each transistor is an inverting phase-splitter. After a 180-degree phase shift, the input signals mix at the common collector resistor. Neither amplifier has any voltage gain. Yet each amplifier offers an 11.8-times power gain. You can take the in-phase signal off the emitter of each transistor. The emitter signal includes the power gain.
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