SDG #281 A festive auto colour change RGB LED hack you've never seen before!

Cheap auto colour change RGB LEDs are appealing for decorative projects because each package contains its own controller chip, so no data lines, no addressing and, as demonstrated here with a bench supply, no external current limiting resistors are needed. The catch is that they always restart their fade sequence from red on power-up, with a brief white flash first, which makes any kind of blinking or animation look broken.

The workaround shown here is genuinely clever and worth borrowing: by sweeping the supply voltage down, it becomes clear the internal controller keeps running well below the forward voltage of the LED dice, roughly 1.6 V. Holding the LEDs at that standby level keeps the colour cycle advancing while producing no visible light, so switching between standby and full rail blinks the LEDs mid-sequence rather than resetting them. Better still, a PWM waveform superimposed on top of the standby level gives smooth brightness control that these parts were never designed to offer.

The build itself is a large two layer, 2 mm PCB with over 300 LEDs, assembled hot plate first for the high thermal mass parts then hot air for the LED side. Two practical lessons emerge from the debug session. First, a thermal camera catching a hot transistor led to the discovery that a PNP was fitted where a P channel MOSFET should have been; the pinouts are compatible enough that the circuit half worked while destroying the part. A component analyser confirmed the swap. Second, the LM317 standby regulator was fed from the wrong rail, leaving marginal headroom, and standby voltage set too low caused random twinkling as chip-to-chip thresholds varied. Nudging it up stabilised everything.