Random input HIGH detect on GPIO

Question

Trying to setup my door bell using raspberry PI B+, the input from the door bell stays at 0 DC and turns to 2.9v when the switch is pressed, I'm using the following python code to detect this activity

GPIO.setmode(GPIO.BOARD)
GPIO.set(13, GPIO.IN, pull_up_down = GPIO.PUD_DOWN)
try:
    while True:
        sleep(0.1)
        if (GPIO.input(13)==GPIO.HIGH:
            print time.strftime("%H:%M:%S")
except KeyboardInterrupt:
    GPIO.cleanup()
    sys.exit()

the problem i'm getting is that even no one press the doorbell switch, randomly, around once per 10~20seconds, my Pi detects a GPIO.HIGH, thus, my doorbell project failed :-(

Any ideas?

PS: I used my multimeter to make sure my input is correct from doorbell, it read 0~0.01 in a steady 5 mins, while my Pi still gives me random GPIO.HIGH reads every 10~20 seconds in the same period.

Answer 1

Interference? Perhaps the long wires are bringing up noise beyond the ability of the internal pull-down resistor to suppress. Try something like a 5-10k external resistor pull-down on the GPIO.