I am trying to temporarily take control of a temp control actuator.. There are 3 wires going to it. One is ground and one is 12v.. The 3rd wire varies between 0 and 12 volts.. 0 for full cold and 12v for full hot.. What I want is to have normal operation until I have a trigger set to wake up the arduino.. Then I want to have the arduino cut out the varying signal and output it's own.. The only way I know how to is to use a small relay using pins 30 and 87a to intercept when activated, then output my own on pin 87.. I will pretty much only be using full hot or full cold so I just planned on using a small transistor to drive the signal to 12 if needed. I was thinking of trying pwm to try and see if I could adjust it, but there really isn't a need. There is no load on this signal. It is only a reference voltage for the actuator.. What I was wondering is if there is some sort of easy circuit I could build instead of using a relay.. I can't simply pull the line high or low without disconnecting the original signal as I may damage the factory controller.. Thanks for any input..
P.S. If anyone is wondering, I am tired of forgetting to leave my blower on high and temp full hot the night before.. I have a remote start already, but I thought I could add an arduino and make it self control the hvac depending on ambient temp when started..