trying to temporarily intercept variable signal..

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..

Use a relay. You'll be glad. HVAC stuff is a sewer and you do not want that filth in your electronics.

I'd even suggest using (a) relay(s) to cut out any interference by the control and then using the relays on the arduino.

Or just wire up a temperature probe, learn enough about how to interface to that system the right way and program up an arduino with an LCD, temp sensor and bank of relays and make up a barebones board, nice box and have full control of your HVAC.

Then stick a temp probe and servo shielded arduino and a servo controlled vent in every room that can close the vent when needed......

;)

Depending on the type of actuator and HVAC control head you have, you may run into more problems or even possibly damage the actuators. The control head may be connected to end stop sensors that tell it when to stop driving the actuator or it may be using pulse count or feedback to determine the position. You could burn up the actuators if you continue to drive them once the end stop is reached. Also, your control head may be confused if the actuators are not in the same position it thinks they are in (some actuators don't provide constant feedback). You may be better off intercepting and replacing the value the control is reading from the temp pot behind your temp knob.

For the blower control, you may have high current contacts in the control head or blower control module that receives a signal from the control head. If it is the latter I would recommend doing the same thing and sub in your own resistance value for full blower.

Out of curiosity, what is your vehicle make/model/year?

My truck is a 2004 s10 with manual hvac.. The temp pot is simply a manual pot.. It is not pwm controlled and there are no other soft stop switches for end of travel.. My plan was to simply get the circuit I need to intercept the signal and while I'm wiring it, I can simply measure the min and max voltage signal and either pwm control it to match the limits, or use some resistors to adjust the min/max of my circuit so I do not overdrive the servo.. Gm themselves are not too particular on the voltages so honestly I do not think I could overdrive the servo anyways as long as I stay in their specifications.. The temp servo has 3 wires, power,ground and signal. There is no feedback signal coming out, the control and amplifier are internal to the actuator.. All I want to do is interrupt the signal going to the actuator and put in my own... For the blower I was simply going to trip the high blower relay by putting 12 volts on circuit 52 using a transistor that can provide the 80 ma it needs.. I have already got a temp sensor working so I can go full cold or full hot, or nothing depending on temp inside the vehicle when it's started..

By the way, If anyone needs a particular schematic for any gm vehicle, let me know I can search powertrain from 96 and newer and 98 and up everything...