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I have a WS2812B LED strip that is controlled by an Arduino. Normally, both the Arduino and LED strip are powered by the same 5V power supply. The 5V and GND pins of the Arduino and LED strip are connected together. If I want to reprogram the Arduino, I have to disconnect the LED strip or else it might want to try and draw power from my computer's USB.

I have a bunch of (assorted) transistors, (schottky, zener) diodes and resistors. Is there something I can build that will allow the power supply to power both the Arduino and LED strip, while the Arduino's USB can only power the Arduino itself?

I've thought about using a diode between the 5V of the Arduino and 5V of the power supply and using a resistor to pull the 5V terminal of the power supply pin low (when the power supply is not connected) but I'm not sure how stable that will be: once the diode starts conducting the resistor will have no chance to pull the pin low again and I'm not sure if the Arduino will like the voltage drop across the diode.

Obviously the external power would be disconnected before plugging the Arduino into USB. Since the LED strip runs on 5V I can't use the VIN pin.

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I'd try powering the led strip for the power-supply directly, and the arduino board via a (schottky) diode.

That way the led strip will have no power when only usb is plugged in.

The Arduino won't mind that it receive a bit less that 5v because of the voltage drop of the diode.

The only thing to look out for it sending a 5v signal to a WS2812B chip that has no power. According to the datasheet this is not allowed.

So you either need to buffer the output of the arduino going to the input of the WS2812B (e.g. using some transistors, or a simple pull-up with diode). Or, since you are not hot-plugging the usb, have the 5v from the power supply go (via a resistor) to an input on the arduino, so you can detect this in your setup routine, and have it block the rest of the code, or something.

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