Pimoroni Automation HAT: understanding ADC


#1

I’m looking to drive my furnace with a raspberry pi. Pimoroni’s automation HAT looks ideal due to it’s low profile and the fact it has the exact number of relays I need. I’m a little confused by what ADC is though and how it differs from straight alternating current. The C wire on my thermostat is a straight 24V AC, my hope is that I could connect that into the hat and control my fan, RH, and RC wires through the relays. Will the ADC input support 24V AC? Also, I see that the hat outputs 5V DC, is it actually possible to power a pi off the hat?

I’m having a lot of trouble finding good documentation or examples of the HAT online and am having issues determining if it’s suited for my application.


#2

As far as I understand it, the “C-wire” on thermostats is for connecting a 24VAC power supply (for modern thermostats that require power for displays, etc) and wiring this into the ADC on Automation HAT will have no effect. The ADC is designed to read a DC voltage between 0 and 24V and wont supply power.

Automation HAT itself does not supply any power at all. The 5V DC output you refer to is routed from the Raspberry Pi itself. If you need a 24VAC supply for your system, you would have to source it from elsewhere. The Pi must be powered via a 5V mains adaptor as normal.


#3

ADC is Analog to Digital Converter. You feed a DC voltage (analog signal) in and it gives you a digital representation of it.
Ones and Zero’s, 00000000 for Zero volts and 11111111 for 24 volts. Not exactly like that, I think its actually 12 bits, but you get the idea.


#4

I would be very careful with the inputs. I toasted an Explorer HAT by applying a negative (with respect to ground) voltage to the inputs of the HAT. The 24V AC used by most HVAC equipment is of course NEGATIVE half of the time. I’m not quite sure what you are trying to sense but you could maybe drive a relay to switch a positive voltage to the desired input or perhaps you could use a rectifier with a capacitor filter and a few resistors to create the necessary positive voltage from the 24V AC signal.