Automation HAT and 24VDC


Just about to dive in to my first Pi project and I have obviously picked something a little difficult to to really test my programming and wiring skills.

I am wanting to control an Light tower, which will pull information from our Network Monitoring server.

As the light tower we have available is 24VDC, I have purchased the Automation Hat. But looking at the board and the documentation I am complete lost on what I connect what to where, I havent touched a relay in about 20 years!

This is mainly in terms of the following:

  1. Where do i connect the 24V input to - I take it that this will be the Buffered Input and the GND (-ve) section, but I am a bit confused on the Buffered part means?
  2. I take that I would then connect the light tower to the relevant relay connections (1 per light segment) and the GND section.

Please let me know if I have that correct or if I am about to blow something up (and maybe set light to the office!!)




Automation HAT does not supply 24VDC, you’re going to need a power supply to do that for you.

I suspect- but am not sure- that the current consumption of the light tower is low enough, and that the light tower is “Common Anode” (all the lights are powered by a single power input and need to have their connection to ground toggled) that you could use the outputs on Automation HAT instead of the relays.

Automation HATs outputs are powered by a Darlington Array. This is an array of transistor pairs - two per channel - that take an input HIGH signal and open a signal path to Ground.

It’s easier to picture the outputs as a button- one side is permanently connected to Ground and the other side is where you connect your device. When you push the button, the circuit is completed, and your device turns on. Similarly when you turn an output On/HIGH on Automation HAT it acts much like this button, completing the circuit’s path to Ground.


HI Gadgetoid,

Thanks for the quick reply.

I was aware that the HAT didnt supply 24V, but it was more where do I connect the 24v supply to on the HAT. I dont want to blow the unit.

I think you are right about the light tower all the lights have a common ground, which means I can use the HAT outputs rather than the relays!




It depends how you use it. If you’re using the outputs then you need to connect the Ground from the 24v supply to the Gnd connections on the HAT, and the +24v to the lights. And the lights need to share a common supply.

If the lights have common Ground and require multiple separate power inputs to light them, you would have to connect each light wire to the NO (Normally Open) side of a relay, and split your 24v power supply across the COM connections on each relay. In this case, the Ground from your 24v supply would be connected directly to the common Ground of the lights.


I have a similar use case but using a 12V supply and a tower lamp (I got the 4 colour one but don’t think I can use all 5 outputs with the automation pHAT). Correct me if I’m wrong but would the following work:

Lamp has common brown 12V line which would go to the + on the power supply.
Power supply - would go back to the GND pad on the pHAT.

The 3 sinking 24V outputs could then be used for the red, amber and green leads of the lamp (and activated in code as required).

As an aside, would it be possible to use the relay at the same time as the 3 outputs? e.g. have Normally Open connected to the blue lamp and Normally Closed connected to the buzzer (for an either/or configuration)?


Yes, you could probably quite easily strap the centre “common” terminal of the relay with a short jump wire over to another available Ground connection on the pHAT.