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RPi Disconnecting HDMI impacts GPIO

Former Member

I am helping my son with a school diorama with buttons and LEDs. Pretty simple Python script that works perfects until we unplug the HDMI video. The GPIO pins start changing state without us touching any buttons. I read one article that talked about the HDMI providing a ground but didn't make sense.  Any ideas what we could try. We want to use the raspberry pi with no keyboard, mouse or monitor...just the buttons and LEDs. We are also using the speaker output to some battery powered speakers through the Audio jack. Any ideas?

  • That's a strange one.  Could you post a schematic of how you hooked up the buttons?  Maybe they're picking up noise somewhere, and they're more sensitive if the monitor isn't hooked up.  (No, I don't know why this would be the case but with noise pick-up anything can happen.)  Usually buttons have a pull-up or pull-down resistor to set a solid logic 1 or 0 when the button isn't pressed.  This could be an external resistor -- say, 10K Ohms -- or you can configure RasPi GPIO inputs to connect an internal pull-up or pull-down resistor.  If you don't have a pull-up or pull-down, it takes very little noise to cause an input to flip even if not pressing the button.  This is just a hypothesis based on no knowledge of your circuit.

     

    If you don't have a schematic editor handy, you can sketch something on paper and upload a digital photo of it.

  • in reply to johnbeetem

    You answer reminded me to try a few things and thankfully one of them worked.  I went back and specified the GPIO pull down on the Python code:

     

    GPIO.setup(channel, GPIO.IN, pull_up_down=GPIO.PUD_DOWN

     

    The documentation had said it was optional and I had a resisitor on the breadboard with the pull down layout so I figured it would be OK.  I was trying to keep it simple so my son could explain every line of code in the script if asked.  Anyway, I want back and added the argument to set the internal resisitor on the pin set and works perfect.  Thanks for the feedback to get me close.