Electronic – can I use a board that has resistor ladder buttons on raspberry pi

gpioraspberry piresistor-ladder

I have a clock that has a circuit board with a resistor ladder setup for the buttons. I want to (disconnect the clock and) put a pi inside and use the buttons to control the pi. In normal use the board is supplied with voltage and the buttons connect to ground. The clock measures the voltage somehow (I dont think its from the ground side because If I connect the ground side of the board direct to ground it still works as expected.) If I connect the gpio pins to the buttons after the resistors but before the buttons the voltage gets backtracked through the resistor to the next resistor also. The result is that the pi detects more than one button being pushed at a time. This happens because the high side of the voltage ladder resistors are tied together. How can I wire this up to the raspberry pi without removing the resistors?
schematic

Best Answer

I'd suggest that the schematic you show uses a 'time to discharge' algorithm to decide which switch is pressed (or even if multiple switches are pressed).

On the R'Pi you could connect 'key' line to a GPIO pin using a 1k Ohm resistor (needed because of SW7) then do something like this:

  1. Set the GPIO to output a "1", this will charge up the capacitor (there might be switches pressed so the high voltage depends on which one(s) are pressed)
  2. Setup an interrupt on the GPIO going low
  3. Read the system timer (like this)
  4. Set the GPIO to input with no internal pullup
  5. When you get an interrupt read the system timer again and store it (the difference value is related to the discharge time of the RC network to the GPIO low level). Then you would execute a RET
  6. Back in your mainline code you have both the initial and second system timer value ….subtract the two to get the difference.

It's not very accurate, but good enough to find out which switch is pressed. It's going to be fun to catalog all the variations if multiple switches are pressed, but should be doable.

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