Trying to learn about the PI and I have acquired a pi Zero W with the GPIO header. I have an external device that signals activity via an LED, which blinks @ 1.8v 2X and then remains on until reset, which then drops to 0v.

I would like to have a small script running to text me 1x when the LED remains on. I have found suitable demos for this, but I was a little disheartened to learn the PI would not consider 1.8V high.

So, I understand I need to probably build some additional circuitry, maybe a opto isolator or just some sort of pull up/down for this. Can anyone point me to the right solution? The device I am monitoring is powered by a simple wall wart @ 9v.

  • 1
    Wrong site for the question as it is not specific to the Pi. However you probably just need a transistor. Look up transistor switching circuits.
    – joan
    Feb 9, 2018 at 16:52

1 Answer 1


Depending on what the device is, and how much you know about whats going on inside it, I would use some sort of isolation, the easiest way to do it would probably be using a photocell (which measures visible light). You would just need to shield the sensor and LED so that ONLY the LED is going to trigger the sensor, not you turning on/ off lights in the room

Here is a thread from the arduino site that talks about using a photocell as a digital input. Remember the pi has 3.3V logic on the GPIO

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