Value of GPIO in a slider buttton changing without doing anything

Question

I want to use a slider button of 3 pins.

Actually I'm using 2 of it, like the schematic below:

^{simulate this circuit – Schematic created using CircuitLab}

My code is:

import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BCM)
green_led = 21
GPIO.setup(green_led, GPIO.IN)

while True:
    green_led_state = GPIO.input(green_led)
    print('#### - ',green_led_state, ' - ####')

In my opinion whith this code, if don't touch my slider always have to print the same value (1 or 0) depending of its position. And if I change the position of the slider, print the other value all the time.

But now, without touching nothing is printing 0 and 1 randomly (I think randomly). Why this? Why some times has current and sometimes not?

Thanks

Answer 1

A GPIO set as an input will float (read randomly low/high) until a voltage is applied.

The simplest thing to do is set the internal resistor pulls to the opposite of the value set by the switch. If I read your circuit correctly use

GPIO.setup(green_led, GPIO.IN, pull_up_down=GPIO.PUD_DOWN)