Phantom events on Pull-Down Button

Question

I’m the author of this project, an automated feeding device controlled by a custom 4-button keypad. The device is in use 24/7, since over a year. In rare cases I‘ve noticed fake button events. To be more precise, whenever this happened it was precisely the moment the cat touched the casing with his nose. So I am wondering if it is possible that the cat acts as a capacitor, giving a slight shock to the GPIOs?

I suspect that the problem is the PULL-DOWN configuration. Currently I use a 230 Ohm resistor. I‘ve read in a post about button wiring that this could be too low, but I don‘t know if that is credible or if my problem is related.

EDIT: This is the schematic for the tactile buttons:

^{simulate this circuit – Schematic created using CircuitLab}

(7) provides a 3.3V steady supply from the Raspi Zero. (8), (9), (11), (25) serve as inputs to listen to the individual button clicks.

I use this configuration (lines 52-65) for the GPIOs:

# KEYPAD PINS
# PIN layout
keypadPowerPin = 7
button1 = 11
button2 = 8
button3 = 25
button4 = 9
GPIO.setup(keypadPowerPin, GPIO.OUT)
GPIO.setup(button1, GPIO.IN, GPIO.PUD_DOWN)
GPIO.setup(button2, GPIO.IN, GPIO.PUD_DOWN)
GPIO.setup(button3, GPIO.IN, GPIO.PUD_DOWN)
GPIO.setup(button4, GPIO.IN, GPIO.PUD_DOWN)
# Power up 3.3V input pin
GPIO.output(keypadPowerPin, 1)

Edit: The listeners to actually handle input events are further down in the referenced listing (lines 344-348):

# Add keypad button handlers
GPIO.add_event_detect(button1, GPIO.RISING, callback=button_pressed_callback, bouncetime=300)
GPIO.add_event_detect(button2, GPIO.RISING, callback=button_pressed_callback, bouncetime=300)
GPIO.add_event_detect(button3, GPIO.RISING, callback=button_pressed_callback, bouncetime=300)
GPIO.add_event_detect(button4, GPIO.RISING, callback=button_pressed_callback, bouncetime=300)

Thanks!

Answer 1

What you have is NOT a pull down. The resistor actually does absolutely nothing (except draw current from GPIO which is close to the maximum a GPIO can safely supply).

It is normal engineering practice to use pullup and wire switches to Gnd.

See https://elinux.org/RPi_GPIO_Interface_Circuits#Buttons_and_switches

The internal pull resistors are high impedance 50k and only suitable for very short wires.

Answer 2

A bit belated, but I wanted to post the pull down schematic that eventually solved my issue:

Essentially, this circuit overcomes the "Floating" GPIOs, by adding a resistor + common GND for each GPIO. It is safe to use the same voltage supply for all buttons and use a single GND. The trick was to use separate resistors, so one button click does not impact the others.

Also, notice that as long as no button is pressed there is no closed circuit / no current.

Hope it helps someone, sometime.