Measuring On/Off from relay

I'm currently trying to use an Arduino to measure every time a sensor is triggered. Currently the sensor is connected to a 120VAC relay that resets a clock. My idea had been to measure in between the relay and the clock to determine when the sensor was activated or not. I had a couple of ideas about how to do this.

My first idea had been to step down the voltage and connect it to an input pin. Easiest way I can see to do this would be one of these

My other idea had been to try to measure the current running across the wire to determine if the sensor was triggered. My thought had been to try using this

Would either of these methods be suitable or is there an easier way that I'm missing?

Can you just use a contact from that relay connected to a digital input with suitable pull up resistor ?

Use a real time clock and when the relay operates , read the current time

The 3.3V power supply will work. But, I’d look for a 5V supply. You’re using a regular 5V Arduino. right?

The current sensor should work if it’s sensitive enough. The relay is probably only drawing a couple of milliamps. And, current sensing/measurement just tends to be a bit of a pain.

Both of your ideas provide electrical isolation from the power line (very important!).

The easiest way is with a relay that has a 120VAC coil. They are very common. Relay contacts are isolated from the coil, so it’s also safe (if you wire it correctly). You just need one pair of relay contacts. Enable the Arduino’s internal pull-up resistor, and wire the relay so the input gets pulled to ground when the relay is switched-on. Since it’s low-voltage low-current the contact ratings on the relay don’t matter and you only need one pair of contacts so any relay configuration from SPST and “up” will work.

…I’m not telling you what to do, but I’d recommend you buy your parts from a reliable distributor. I wouldn’t buy the cheapest thing you can find on eBay.

What voltage is the existing relay coil? If its low enough to be safe wire to the ardiuno through an optoisolator, wired across the coil.
Could you not wire another relay in parallel with this one to work the Arduino?

Or another relay as above poster said.


My first idea had been to step down the voltage and connect it to an input pin. Easiest way I can see to do this would be one of these

These devices take time to produce voltage output after applying power and dropping output after removal of power.
How precise do you need the time keeping to be?

Thanks.. Tom.. :slight_smile:

Upon digging in further it looks like the current relay is 2-pole double throw and only one of them is currently used. Would it be an issue trying to connect the arduino to the relay if the other circuit has 100V running through it? If there’s any risk I think I’ll just wire in another relay.