PWM RGB Controller
This section describes the design of a Pulse-Width Modulated Red-Green-Blue controller.
Multiple strings of six LEDs with a 24V DC source make for an efficient design. A total current of 2 Amps will supply 600 LEDs per color, sufficient for a sizable light.
The circuit shown below makes use of a remarkable device, a power MOSFET (Metal Oxide Silicon Field Effect Transistor). You can think of a MOSFET as an electronic relay that can switch from OFF to ON in a very short time and requires a miniscule control signal. The type used here can switch up to 10 A DC in a few microseconds.
IC1a and IC1b form an oscillator where only the triangular waveform is used. IC1c generates a reference voltage of 6V DC. The triangular waveform is applied to three comparators (IC1b, IC2a, IC2c). A variable voltage is applied to the inverting input of the regulators. Thus, the output of the comparators is a pulse with a variable pulse width. This drives the MOSFET switch. IC3 is used simply to step down the raw 24V DC voltage to 12V DC for the operational amplifiers.
The diodes D1x suppress reverse voltages that could result from long wiring. The capacitors C3x take the “edge” off the output pulses to minimize EMF interference.
The oscilloscope figure shows the pulsed output at about 60% of maximum as it relates to the triangular waveform of the oscillator. The values of the resistors at either side of the potentiometer (R6x and R7x) were trimmed such that the pulses are absent at one end of the travel (light is off). At the other end of the travel, the pulse width exceeds the pulse period causing the light to be fully on.