Alarm Relay Contact Detection

[Michael Ingraham]

I recently had an air purification device installed in our home's HVAC system. Since the unit is installed in a remote location (in our case, inside the fan housing which is under the house in the crawl space), it comes with an optional connection for an external indicator. The indicator is a normally closed contact which "trips" (i.e., turns off the indicator light) when there is a problem. According to the doc, the contact is rated for 240VAC/1A. The wiring example shows a 24VAC source being used to illuminate an LED through the contact.

Rather than running wires to turn a rarely noticed light on in some similarly remote location, I would like to install an ESP82xx based setup and detect whether the contact is opened or closed.

I would rather use a typical 3.3VDC-5VDC source and run that into one of the GPIO inputs. Is there a way for me to determine whether this contact is an isolated dry contact?

I am attaching the information from the instruction bulletin. Please review this and see what options I have.

Any guidance is gratefully appreciated!

Mike

[Michael Ingraham]

I have confirmed with the manufacturer that the contact is indeed a dry contact. So, if I apply 3.3VDC on the unit's input, I can connect the output to one of my Wemos D1 Mini GPIO and detect that as a Switch<x> in order to trigger a Tasmota rule to send a notification. Does this sound reasonable?

[Philip Knowles]

I’d actually connect one side to 0V and the other to a GPIO connected to 3V3 via a pullup resistor but, otherwise, yes. As the contact is NC it would be switch1i

If you want it to ‘fail safe’ connect 3V3 to one side of the switch and a pull down resistor between the GPIO and 0V. It would then be switch1.

 

Regards

 

Phil K

 

 

Sent from Mail for Windows 10

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[sfromis]

Yeah, for a dry contact (with both sides connected to nothing else) it sounds pretty reasonable to run 3.3V through it, and have it as a "typical" Tasmota switch/button input. Wires could make a difference with depending on them being long, thin or subjected to electromagnetic noise (which worst case might induce "signals" in a wire). If one side is ground (meaning having to have common ground), the electrical side could get a bit more complex; I'll not claim expertise in how best to manage that.

[Michael Ingraham]

This?

What size resistor?

[Philip Knowles]

4k7 or higher. It’s just to ensure that the GPIO is either high or low and not ‘floating’ – helps prevent nuisance tripping.

 

The NC contact will keep it at 0V. If the contact opens (or you lose the wire) then the GPIO will go high.

 

Regards

 

Phil K

 

Sent from Mail for Windows 10

 

From: Michael Ingraham
Sent: 17 November 2020 16:01
To: TasmotaUsers
Subject: Re: Alarm Relay Contact Detection

 

This?

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[Michael Ingraham]

thx