I’m trying to make a cheap manual service thermostat that I can wire inline on a thermostat cable (right inside a rooftop air handler for example) for servicing and testing HVAC equipment. Essentially it’s just going to be a big dip switch where toggling one of the switches closes a circuit between the thermostat R wire and one of the other thermostat outputs (G, W1, Y1, etc.) That part is simple though. The part I’m struggling with is that I also want a switch that can disconnect the actual remote thermostat outputs while I’m using this manual service thermostat. I could just use another large dip switch and manually toggle off all of the outputs one at a time but it would be nice to instead have a single switch that toggles them all off at once. The problem is if I only use a single switch then that means I need to open 3-9 different circuits (G, Y1-Y4, W1-W4) with one switch. I haven’t seen any 9P1T switches available anywhere. I know there’s all sorts of things I could do fancier circuitry to achieve this but I want this to be as simple, cheap, and robust of a device as possible.
Is there a simple and ideally entirely mechanical way to open several low power circuits with a single switch or am I just stuck using multiple switches?
If I understand what you’re describing, I’d probably use a relay board. They’re not expensive:
You could wire all of the control circuits to one wire, and with the flip of one switch on the control circuit turn on/off all 10 relays on their load circuits.
Using the NC side of the relays would be the best way too, but I’m not sure how to find one of these with name brand relays that id actually trust in a commercial setting.
I suppose that comes down to “what is the worst case scenario assuming the board fails”. If you simply can’t have it fail the design should have redundancies built in. If you’re looking for build quality/established MTBF ratings then piggyback on top of someone elses regulatory approvals where those tolerances are built in. I just did a quick google search for “relay board FAA approved” not even knowing what the regulatory rules are of aircraft. Apparently those are FAA-PMA and TSO. Plugging that in with “relay board” and you get something like this:
Sandia Aerospace 12 Pole Switching Relay
This level of build quality and certification comes at a much higher price of $524 for one board though. Another choice I might chase would be “medical”.