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Activating a relay off an LED circuit

Posted by garymunson (My Page) on
Sun, Jul 21, 13 at 10:09

Anyone had luck activating a small PCB relay off an LED circuit? What I would like to do is add a relay to get a make/break circuit that follows an LED. I'm assuming I'd just wire the relay in parallel with the LED but just 'upstream' from the LED's current limit resistor. Any other ideas? I do worry that whatever drives the LED won't have enough reserve capacity to power the relay coil. Wondering if I should actually cut the LED out of the circuit and use a set of relay contacts to light it from a different power source. Should an LED driver circuit be enough to run both the LED and relay coil or only one of them?

Follow-Up Postings:

RE: Activating a relay off an LED circuit

I don't yet have a relay to meter the coil resistance. I'm assuming that I'll be using a 5 volt coil one...

RE: Activating a relay off an LED circuit

Another thought I had was to fabricate a photo-electric circuit of some type that 'looks at' the LED and follows the operation that way with the photo circuit driving the relay. This might be outrunning my ability to design something reliable...

RE: Activating a relay off an LED circuit

Assuming the LED has a typical max 20ma rating, you're not going to be able to parallel a relay in there. Without knowing what's driving the LED before the current limiting resistor, it's impossible to say if it would work.

If you have access to the circuit's 5VDC to power the relay coil, maybe your could use an NPN transistor to switch the relay circuit to ground. Just run the transistor's base, through an appropriate resistor - maybe around 1K - to the LED source, collector to the negative side of the relay coil, and emitter to ground.

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