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COMPONENT VALUES D1: 1N4148 switching diode D2: LED, see Materials list IC1: 555 timer R1 and R2: 1K P1: 100K trimmer R3: 20Ω for green or blue LED, 100Ω for red C1: 0.01μF for operation, 47μF for testing C2: 0.01μF C3: 100μF C4: 0.1μF
Fig. E: In this circuit, a 555 timer can generate pulses of varying length while maintaining a nearconstant frequency. Capacitors C3 and C4 smooth the power supply and need not be duplicated for additional copies of the circuit.
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P1. Since R1 and R2 are fixed (they’re included just to protect the chip when P1 is at either end of its scale), the left side of P1 establishes the charge time and the right side establishes the discharge time. Because the left side and the right side of P1 always add up to the same resistance, the cycle length will be constant while the charge and discharge times vary. You can use this circuit anytime you want to create varying pulses within a fixed frequency. A breadboard layout is shown in Figure F, using a trimmer for P1 that plugs directly into the board. The first time you try it, use a 47μF capacitor for C1. The pulses will be very slow and easily visible as you adjust the potentiometer. When you’re happy that the circuit works, substitute a 0.01μF capacitor for C1. The LED will now flash beyond your persistence of vision, and the potentiometer will function as a dimmer. You’ll need 2 more copies of the circuit to drive the remaining 2 LEDs. Figures G and H show a completed breadboard. I used high-output Lumex LEDs that withstand more current than most, and need smaller series resistors (R3) than you might expect. Note that the green and blue LEDs in this series require different 150
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Fig. F: Breadboard layout of the schematic in Figure E, using the same components. Interior functionality of P1 is shown for clarity. Pale-colored conductors are hidden inside the breadboard. Fig. G: Breadboarded version of the schematic in Figure E, in triplicate to drive 3 LEDs. Fig. H: The color-mixing circuit in action.