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I've got a fairly simple circuit that I've made where a motion detector is connected to a Raspberry Pi B+ GPIO by way of a fairly long ~30ft stretch of wire. The detector is being powered by a 9V battery and the schematic is as follows:

PIL Schematic

I'm running a python script to pick up the motion and it is as follows:

import RPi.GPIO as gpio
import time
gpio.setmode(gpio.BCM)
gpio.setup(17, gpio.IN)
while True:
  print ("LOW","HIGH")[gpio.input(17)]
  time.sleep(.5)

I was getting an output that was consistently 9-11 HIGHs, then 9-11 LOWs then repeating, and I was wondering what was up so I disconnected the PIR motion detector, leaving the wire not connected to anything, and I got the same result: a consistent fluctuation between low and high.

From my research, LOW for GPIO is roughly <0.54V while HIGH is >2V. Measuring the difference in voltage between the wire and the Raspberry Pi GND pin (PIN 39) gives me 0.32V consistently, with no fluctuation. Why is this happening, and how can I fix it?

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  1. based on your schema, looks like RPi can get 9V when high? Or am I readint it wrong?
  2. Your setup has one flaw: GPIO not connected to HIGH or LOW is neither of them. So it is oscillating. And your device puts HIGH or transient state. So, GPIO value oscillates

Read about pull-up resistors: https://learn.sparkfun.com/tutorials/pull-up-resistors/what-is-a-pull-up-resistor

  • Although in this case if a +ve voltage indicates switch closed the OP should use a pull-down to 0V. @OP check your sensor outputs 3.3V rather than 9V. All the Pi's gpios are 3.3V. – joan Apr 11 '15 at 7:46
  • It outputs 3.3V but takes 5-12V in. – globby Apr 11 '15 at 18:36

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