I'd like to check if this circuit I'm making is correct -- I'm concerned I'll mess up my Arduino. Essentially, I would like to determine if an LED on another device is on or not by measuring the current etc running through it. I know that device uses a CR2032 watch battery, with 3V and 220 mAH. I'd rather not do this using a photosensor since there will usually be a lot of ambient light.

I looked at this answer, and tried to adapt its circuit, which looks like this: example circuit

The solution I came up with would look like this on a breadboard: Breadboard

Not pictured is the existing connections to the two leads of the LED of the device, which send current through it.

I was hoping that with this setup I could measure the current hitting the analogue input of my Arduino when the LED is turned on by the device, and in doing so figure out when the device LED is on.

Does this circuit seem accurate, or is there something I need to change? Thank you!


Would this simpler circuit work, given that the device only has 3Vs?

enter image description here

  • 2
    Since the device is only 3v you could connect the "power" directly to a digital pin and read HIGH when the led is on and LOW when it is off.
    – Gerben
    Sep 19 '16 at 19:59
  • 1
    Would the edited connection diagram I made above reflect what you mean by this? Basically just connecting straight to the Arduino. Sep 19 '16 at 20:38
  • 1
    Depending on the led type you might need to connect to the resistor, but that's basically it.
    – Gerben
    Sep 19 '16 at 21:16

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