# How to monitor the health of a relay and it's contacts

**URL:** <https://community.sparkfun.com/t/how-to-monitor-the-health-of-a-relay-and-its-contacts/67610>\
**Category:** Projects\
**Created:** [December 18, 2025, 1:00am UTC](https://community.sparkfun.com/t/how-to-monitor-the-health-of-a-relay-and-its-contacts/67610 "2025-12-18T01:00:18Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![WOGoos](https://avatars.discourse-cdn.com/v4/letter/w/d6d6ee/32.png) [@WOGoos](https://community.sparkfun.com/u/WOGoos)\
**Post date:** [December 18, 2025, 1:00am UTC](https://community.sparkfun.com/t/how-to-monitor-the-health-of-a-relay-and-its-contacts/67610/1 "2025-12-18T01:00:18Z")

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Hi, I have a challenge and hope someone has and idea how to solve it.

I want to monitor the health of a relay by monitoring the state of relay contacts in relation with the state of the coil. I want to see if the contacts are open or closed when the coil is energized or not energized. I want to do this with an Arduino. It must be something like you see in the drawing below.

Can you tell me if there is a component or sensor that can do this. Let me know what idea you have how this can be done.

Thanks

 ![image](https://canada1.discourse-cdn.com/sparkfun/original/2X/b/b6078465e76c364b3576b1edcc458d88b9757cbb.png)

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**Author:** ![TS-Russell](https://yyz1.discourse-cdn.com/sparkfun/user_avatar/community.sparkfun.com/ts-russell/32/6092_2.png) [@TS-Russell](https://community.sparkfun.com/u/TS-Russell)\
**Post date:** [December 18, 2025, 3:50am UTC](https://community.sparkfun.com/t/how-to-monitor-the-health-of-a-relay-and-its-contacts/67610/2 "2025-12-18T03:50:00Z")

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Some companies sell ‘smart’ relays now that have wifi/bt, might that be easier?

Otherwise you could use some stepdown converters to step the voltage to an arduino-friendly level

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**Author:** ![PaulZC](https://yyz1.discourse-cdn.com/sparkfun/user_avatar/community.sparkfun.com/paulzc/32/67_2.png) [@PaulZC](https://community.sparkfun.com/u/PaulZC)\
**Post date:** [December 18, 2025, 8:26am UTC](https://community.sparkfun.com/t/how-to-monitor-the-health-of-a-relay-and-its-contacts/67610/3 "2025-12-18T08:26:20Z")

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Hi @WOGoos ,

If I understand correctly, you want to measure the small AC voltage that is generated across failing relay contacts? If the relay contacts are perfect: when open, your sensor circuit gets very close to the full 120VAC; when closed, your sensor circuit gets zero volts AC (but is still at hazardous Live Line voltage, compared to Earth). When the contact is failing, you will see a small AC voltage across the closed contacts because the contacts have some resistance.

To do it safely, I’d recommend using two safe, opto-isolated AC multimeters: one measuring L1 to N; the second measuring the Load voltage to N. Subtract one from the other and you have the voltage drop across the contacts.

Or, keep it simple and use a single opto-isolated multimeter that can auto-range safely from the full 120VAC to the small AC voltage across the closed contacts.

There are low cost multimeters available which have an opto-isolated USB interface. I have a “BOLYFA 117” meter with a mini-USB interface. I wrote some [Python 2 code](https://github.com/PaulZC/DMM_Data_Logger) to read the interface. (Side note: the interface is weird in that you read data representing what is displayed on the LCD display segments - i.e. if each segment is on or off. You have to convert each 7-segment digit into a number.) You could re-use that code on (e.g.) Raspberry Pi or another single board computer which supports USB.

For a true “Arduino” solution, you’re probably going to want meter(s) which have opto-isolated serial (UART) interfaces. Sounds like a job for Google. (Other search engines are available…!)

I hope this helps. Stay safe!  
Paul

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**Author:** ![WOGoos](https://avatars.discourse-cdn.com/v4/letter/w/d6d6ee/32.png) [@WOGoos](https://community.sparkfun.com/u/WOGoos)\
**Post date:** [January 25, 2026, 4:40am UTC](https://community.sparkfun.com/t/how-to-monitor-the-health-of-a-relay-and-its-contacts/67610/4 "2026-01-25T04:40:25Z")

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Hi Paul, the contact sensor to TTL is in fact and optocoupler. Thanks for your advice.