This is my first question on here. I am good at programming but am a little stuck when it comes to electronics and connecting two circuits together. I would like to interface a spa controller with 3 basic elements - increase temperature button, decrease temperature button and “currently heating” LED. The MCU I am using runs on 3.3V and the physical controller runs on low voltage (12v I think).
I can access the back of the PCB on the physical controller and can see the 2 push buttons pads behind each button. I also have access to the + - controlling the red LED. The voltage reading is 0.45v when the LED is turned on and -0.29v when the LED is turned off. Logically to me there is a voltage change detected on my multimeter so there must be a way to interface this to an input of 1 or 0 on the micro. Also not sure if it helps but I got a reading of 135mA across the two pads while the LED was off.
The button pads are joined when I hit the key on the front of the controller unit. I would like the ability to simulate a button press from the micro (so I can change the temp up and down from the micro). I would also like the ability to sense if someone has pressed the physical button so I can update the current temp stored in the micro to keep them in sync. I realize this would require 4 pins (2 for input to the micro and 2 for output from the micro).
I have researched other questions here and on the net and am having trouble finding anything to do with interfacing with external circuits. I have found some information on 4N35 optoisolators and also reed relays.