Simulating button-presses is a common problem, but it doesn't have a common solution - it very much depends on how the buttons are connected up in the existing unit. You can't rely on them doing something simple (e.g. grounding a signal that is pulled up to 3.3V), they might be wired to the 24V supply, in which case if they are directly connected to your Pi, it will be burned out instantaneously.
So I'd start by doing some measurements of the button voltages, and since you mention a 'transformer' supply, check for both AC and DC voltages.
Once you know what you are dealing with, I'd connect the buttons to opto-couplers, and do some simple tests with a resistor and 3.3V power supply feeding the opto LED to see if it is effectively simulating the button-press. I'd start with a 1K resistor, going down to 220 ohms if necessary; if that doesn't work, I'd suspect something is amiss (e.g. check the polarity of the opt-coupler output wires).
Once that works, you can drive the opto LEDs from a Pi; you'd probably get away with a 330 ohm current-limiting series resistor; if you need more current, you can add a simple BJT driver.
So I'd start by doing some measurements of the button voltages, and since you mention a 'transformer' supply, check for both AC and DC voltages.
Once you know what you are dealing with, I'd connect the buttons to opto-couplers, and do some simple tests with a resistor and 3.3V power supply feeding the opto LED to see if it is effectively simulating the button-press. I'd start with a 1K resistor, going down to 220 ohms if necessary; if that doesn't work, I'd suspect something is amiss (e.g. check the polarity of the opt-coupler output wires).
Once that works, you can drive the opto LEDs from a Pi; you'd probably get away with a 330 ohm current-limiting series resistor; if you need more current, you can add a simple BJT driver.
Statistics: Posted by jayben — Tue Mar 18, 2025 9:44 am