Interfacing Arduino Uno with my Furnace Alarm output (30V AC, 2amps)

I have a Honeywell R7184U Oil burner controller. It features an alarm pinout that outputs 30vac 2amps continuous. Yes, that's AC not DC voltage. As seen in the PDF inked below.

I'd like to interface this with my Arduino UNO. I imagine that I need a relay board, but the relay boards that I currently have expect DC input not AC. I realize that I could build a bridging rectifier to normalize the AC into DC current, but wondering if there's a "Better" way?

Can someone please advise as to how to make this interface without blowing up my UNO board?

The gritty details: I've written a prototype smartthings application that will send me a notification when a certain event is triggered. The Arduino Uno will be paired with a smartthings shield. I envision that I'd write an arduino program that will trigger that smartthings [furnace is on the fritz] event when a voltage (or relay closure) is detected as given by the furnace (via some interface that I hope you can help me with).

I hope I've asked the question clearly. I'm not an expert by any means. Strictly amateur.

Thanks in advance, Mike

I could be wrong but it sounds like the question you want to ask is:

"how do I use the 30vac/ (current not important) output as a sensor to tell my arduino the furnace alarm has gone off ?"

Why not just use a single diode and resistor, and an opto coupler ?

opto sensor to sense AC voltage

Ask yourself this: If the alarm is OFF (no problem) , What signal/voltage will you read on the output ? If you read the opposite of that , even ONCE, (even for 8 mS) what would that have to mean ?

Looks like this could work well: AC Line Monitor Logic-Out Device In stock at Digi-Key: MID400S

Be sure to look at page 6 for the connection schematic.

Thanks guys. Sorry I’m late with the accolade. We’ve been digging out of snow for what seems like the last week.

I had to order the MID400. In the meantime, I did build a small test circuit as seen [in example #2].:
It’s not optimal in the sense of isolation, but for the sake of progress it gets the job done until the opto coupler arrives and I can construct that circuit.

So now, the circuit outputs 5vdc so now I can close a relay without any smoke! :slight_smile:

At this point, I believe that the next step is to teach the UNO board to detect the closure. I imagine that I’d use the analog inputs A0 & A1, but not sure how to code that up.

Here are some more relevant questions that I think the forum can answer.

  1. Can the UNO detect a relay closure? (or some other physical “event”). If so, then how?
    Update: yes, UNO can detect a relay contact closure as depicted in this example but uses a button instead.

  2. Can the UNO in turn send an event to smartthings in this case? (and if so, how)…
    Update: Yes, after integrating the smartthings library #import “<SmartThings.h>” into my source code, calling it’s constructor, then I am able to do smartthing.send(String) which sends the string message to the ST cloud.

In the meantime, I’ll be poking around the forum and looking at examples and (hopefully) find the answer to my own questions in which case I’ll post my own found answer here.

Thanks in advance,

You could always use an AC relay that takes 30v to close a contact too...

DrAzzy, Thank you for your reply. I was wondering why the suggestion to use an AC relay was not one of the first replies. Likely because of the way I had framed my original question. No disrespect to any of the previous for sure! However true, I've certainly had fun revisiting the electronics notes I took in college (early 80s) and smoking some pots (linear of course) along the way as I tried out a few voltage regulator circuits out. A few times over the past few days have I smoked some components! LOL

Anyway, thanks again for taking the time to reply. Mike


The alarm terminals on the oil burner control should be dry contacts. The 30VAC 2Amp is the rating of the contacts. It means that you can switch up to 2A @ 30VAC with it. No additional relay should be needed.

You can verify this by using a meter and checking for contact closure whenever the unit goes into alarm.

As a reminder, oil burners need yearly maintenance at a minimum. This would include replacing the nozzle and filter, removing any soot from the furnace, inspecting the flue and performing a combustion calibration among other things.

YMMV, If in doubt please have a qualified technician make the terminations to the controller.