GPIO Input fluctuating between high and low when no input is received

Question

I posted this on the Raspberry Pi StackExchange but I feel as though it might be better suited for here, as the SE is pretty inactive anyway.

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:

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?

Answer 1

Add a pull-up or pull-down resistor. Try 10k. If that doesn't work, try 1k. My guess is that your long wire is picking up some EMI and the very high impedance GPIO pin is converting that to a high enough voltage to change the pin state. A pull-up or pull-down resistor will make this more difficult.

Answer 2

Based on the schematic you've supplied, your motion detector is either connected as a short, leaving the GPIO floating or it applies VCC to the GPIO. It doesn't show that there is a common ground reference. Perhaps the circuit is incorrect.

First, make sure that the circuits at both ends of your wire share a common reference (GND). Voltage must be measured relative to a reference. If each side of the wire has its own uncommon reference, then the sensor is essentially always floating on the GPIO input.

If there is indeed a common ground on both ends of your circuit, you should pull the input either high or low with a resistor to ensure that when left open, it will not be left floating. The connected device may already have a pull up/down, but one isn't shown in the schematic. Even if you measure ~0V DC, there is likely some AC EMI picked up on the floating input, especially with a 30ft wire attached.

With such a long wire attached, even if the input is not floating, you may be picking up enough EMI to overwhelm the weak drive strength of your sensor.