Interesting project.
I have some suggestions.
Before you go too far, can you test the compressor?
AT least pump it down and leak test the system?
Maybe try to test for plugged capillary?
As for the electronics, I'd
disconnect the juice to the compressor.
replace it with a light bulb.
put an indicator light on the power to the 4-way valve.
Now, you can pulse the test switch to reverse engineer
the operational parameters without having to run the compressor.
If it looks good, parallel the switch with an opto-isolator and
start banging it with the arduino.
For a single pin, the serial or parallel port of a laptop will let you
experiment with the signals. I bet you know someone who has
a pile of old laptops ;-)
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