Hawes Mechanical Television Archive by James T. Hawes, AA9DT
Colorize Your Mechanical TV System, Part 2

Farbfernseher: TV displaying color bars

The Monitor

  1. If you use red, orange, blue, green or cyan LEDs, go to Step 3. Otherwise, go to Step 2.

  2. Change your monochrome circuit to use red, orange, blue, green or cyan LEDs.

  3. Count the LEDs that you use.

  4. Buy LEDs. Base the number of LEDs on two facts...
    • The LED brightness specification

    • The color sensitivity of the human eye. (For example: If you have red LEDs, buy twice as many green LEDs and one-third as many blue LEDs.)

  1. For best color balance...

    • 59% of your brightness should come from green LEDs.

    • 30% of your brightness should come from red LEDs.

    • 11% of your brightness should come from blue LEDs.

    • For example, with LEDs of the same brightness, and 10 LEDs total, use 6--GRN, 3--RED and 1--BLU.

    • With fewer LEDs, the circuit is simpler, but the color balance tends to be less accurate.

    • You can balance inaccuracies by adding potentiometers in series with the LEDs.

    • Compensate for brightness differences in the different colors. For example, your red LEDs might be twice as bright as your green LEDs. You can compensate by reducing the number of red LEDs.

    • Use LEDs that have about the same viewing (dispersion) angle.

    • Assure full screen coverage by mounting LEDs at a sufficient distance from the viewing window.

    • Use an adequate diffuser. Make sure that your diffuser doesn't block too much light.

Schematic: Example: 18-volt, three-color driver circuit 
     (Farbfernsehen, gruen, rot, blau)

  1. Duplicate your LED driver circuit (power amplifier) once for two-color TV. For three-color TV, make two copies of your LED driver.

  2. Connect the LEDs to the new LED drivers.

  3. Test the set.

Option: Derived Color (D/C)

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.

    1. 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.

    2. 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.

    3. 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.

    1. Connect a potentiometer between the red and blue transistor collectors (or op amp outputs).

    2. 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.

    3. 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.

    Schematic: Green signal is inverted mixture 
    of red & blue signals.

  • 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.

Schematic: Inverting mixer circuits. Produce missing color
    from other two color inputs. For example, feed in red and blue. Circuit
    produces green.

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.

Option: Pseudocolor (P/C)

No camera. Pseudocolor (P/C) requires no modifications to the camera or video source. Instead, pseudocolor is an automatic process that interpolates color from a monochrome source. The output color will likely not be naturalistic, but it might be useful in some applications. Although most contemporary pseudocolor systems are digital, very acceptable analog systems exist. Either type system can work in real time.

Quantizing. You can derive pseudocolors from monochrome in many ways. The most sophisticated systems rely on digital quantizing of gray levels. A computer assigns a color to at least the medium levels. Typically, the peak levels remain black and white.

Schematic: Green signal is inverted mixture 
    of red & blue signals.

The simplest pseudocolor system outputs two colors. This method requires no quantizing and no digital circuits. A two-ended analog amplifier will serve excellently. You can achieve very effective pseudocolors by applying the monochrome signal to the inputs of a difference amplifier. The inverted signal then drives one color output. The non-inverted signal drives the other color output.

In the nearby schematic, the collector of transistor Q1 provides the inverted or orange signal. The cyan, non-inverted signal appears on the collector of transistor Q2. The two collector signals are 180 degrees out of phase with one another.

The output resistors may be fixed or variable. Using variable resistors and a maximum signal, you can adjust the white balance. When the output appears white, you've achieved balance.

If you desire a third color, it could be the midrange or difference signal between the two output signals. (For example, the midpoint of a 50K pot VRZ that connects between the Q1 and Q2 collectors.) The third signal is 90 degrees out of phase with each of the other two signals. If VRZ is smaller than 33K, it introduces crosstalk between the Q1 and Q2 outputs. The crosstalk alters the phase relationship of the three signals.(This is how a matrix in a color TV works.)

Amplifier. In most cases, the third color requires its own amplifier after VRZ.

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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 ar8:39 PM 12/4/2011e 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.

CAUTION. Values in the figures above are for reference only. The circuits are theoretical. No one has built or tested them. Your application will likely differ from the examples. If you use circuits like those on this page, test and modify as necessary. Safety first!
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