Read input from the GPIO

Question

i was printing the input from a RF433 MHZ receiver.

I'm using this simple code, just to see what was coming in:

#!/usr/bin/python
# Hello world python
print "Gestart"
import RPi.GPIO as GPIO

GPIO.setmode(GPIO.BOARD);
GPIO.setup(18, GPIO.IN)
while True:
    input_value = GPIO.input(18)
    print "--" + str(input_value)

But then i got a very high number of True and False values in the command prompt.

When i decoupled the receiver from the cable, i still got these values printer out to the command prompt.

Only when i completely removed the cable from the device, i got only False values.

Is it 'normal' that when you attach a connector cable

you already receive True values?

Answer 1

If you program a pin as an input, and just connect it to an open cable, it's reasonable that you get a mix of high and low. This is because an input pin is somewhat like an open circuit - a little tiny capacitor, actually - and when connected to an open wire, it will just float up and down according to whatever RF field the wire is receiving. You could add a pullup resistor to avoid this - even a weak one like 100k-470k would prevent this.

You could try this: disconnect the cable, then connect the pin to ground using your fingers (one to ground and one to the pin - touch the ground first to avoid zapping the pin). You should see False, and it will stay like that even when to remove your finger. Then connect the pin to +3.3 v using your fingers. You'll see a True, which may also stay after you remove your hand. What you are seeing is that the tiny capacitor is acting as a memory cell and just retaining the most recent voltage.

(I've not actually tried this with the Rpi, but I've seen this happen several times, and quite repeatably, with digital input pins. It's a great way to demonstrate what happens when you leave digital inputs floating).

Some of the rPi pins have pullups on the circuit board (GPIO0 and GPIO1, since they can be used as I2C that requires pullups).