How to use 2 GPIO pins for 3 buttons

Question

I am using almost all the GPIO pins on a project so I had to be resourceful when adressing the pins for the buttons.

But I just can’t get it to work. I don't know if something is wrong with the code or the wiring, but the buttons don’t work like I want them to. Does something like this even work or am I missing something?

import RPi.GPIO as GPIO
import board
from signal import pause

def my_callback(l, r):
    if (l == 14 and not(GPIO.input(r))):
        print("Button 1")
    elif (l == 14 and GPIO.input(r)):
        print("Button 2")
    else:
        print("Button 3")
    
try:
    GPIO.setup((14,15), GPIO.IN, pull_up_down=GPIO.PUD_UP)
    GPIO.add_event_detect(14, GPIO.FALLING, callback=lambda x : my_callback(14, 15), bouncetime=200)
    GPIO.add_event_detect(15, GPIO.FALLING, callback=lambda x : my_callback(15, 14), bouncetime=200)
    pause()
except KeyboardInterrupt:
    GPIO.cleanup()

Wiring:

Answer 1

You could try something like the following - which will at least isolate buttons.

Pushing the centre button will activate both inputs - the others a single input. Strictly no diode is required on the isolated buttons.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 2

Just for reference, there are two well-known approaches for addressing multiple keys with fewer pins: voltage ladder if you have analog pins and key matrix if you have digital pins. The Pi itself has no analog pins, but if you use an ADC, it's possible to use some of its channels for buttons.

Any non-standard wiring schemes are either suboptimal compared to the standard approaches, or only work for specific cases, and since they don't have proper names it will be next to impossible to find code samples for them if you can't write your own code.

For 3 buttons I'd just use 3 pins (plus ground), with trivial logic for reading the state of the buttons, and the possibility to detect simultaneously pressed buttons. If you really want to get creative, you could do with only two pins and no ground:

^{simulate this circuit – Schematic created using CircuitLab}

Pushing SW1 will copy the signal of any level from GPIO1 to GPIO2 and vice versa. Pushing SW2 will copy the LOW level from GPIO1 to GPIO2. Pushing SW3 will copy the LOW level from GPIO2 to GPIO1.

Assuming you have pullups enabled on both pins, reading the buttons is done by driving GPIO1 LOW and reading GPIO2, and then driving GPIO2 low and reading GPIO1:

• if you read LOW both times, it's SW1
• if you read LOW then HIGH, it's SW2
• if you read HIGH then LOW, it's SW3
• if you read HIGH both times, no buttons are pressed