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Author Topic: Using a devices LED AS a trigger. Suggestions please.  (Read 418 times)
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If I wanted to use another devices LED as a trigger for the Arduino, what would be the options?

I know I can use a photo resistor to point at an LED and use its input on the Arduino as a trigger. But is there another way?

Can I clip on to the LED power and ground leads and get a reading from that on the Arduino? Or must the Arduino be in the circuit for that to work?

Thanks for any help.
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It depends upon how that LED is wired up. If you're lucky enough that the LED has its cathode wired to ground and you can share a common ground with that circuit, you can just measure the voltage at the LED anode with an analog A/D input. It'll be 0 when the LED is off and quite a bit not 0 when the LED is on (probably in the 2-3V range, but could be as high as 4V or so for white LED's).

If you don't want to connect the circuits together, you can use an optodarlington to draw a bit of current from the LED (like an LTV-815) and switch an Arduino input to ground when the LED turns on (use the Arduino pull-up resistor to keep it high otherwise). You'll need a resistor in series with the optodarlington input to prevent too much current from going to it and making the LED dim. I'd start with 10k and go down as necessary.

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