Measure 5-25 Amp AC current very quickly with Photon

I have a pump that I would like to know when it’s on or off. By extension, this will allow me to calculate the duty cycle. The current peaks at 22A when it first comes on, stabilizes around 8A when running, and is effectively 0A when it’s not running. I don’t need to know the exact current; rather, I need to see if the pump is on or off by assuming on if the current is > 5A and off if below.

The NCD current sensors like this one and this one can’t be queried more than once per second or two. The pump cycles for only a few seconds, so this low of a query resolution would make the duty cycle calculation wildly inaccurate.

I saw another means of measuring the AC current using a Hall effect sensor like this one. I don’t see the response time of this sensor though. I saw on this site someone used an Allegro IC and the specs on that were 4µs. Does that mean I could query that fast? Ideally, querying between 10 and 100 ms should be sufficient. I’m currently using a stopwatch to measure the cycle times, so as long as the query time is faster than my reflex (250 ms?) that should work.

Reading the NCD user guide for the first link you posted, it seems that the sensing speed is a limitation of the controller chip on the NCD board. It seems that the 1-channel board only takes 1/2 second to sense.

Perhaps you should just buy a split-core current sense transformer and interface with it directly instead of using an I2C current sense chip as a middle-man. Check out the spark fun product here: Then look at the “Documents” tab and the “Arduino Energy Monitor Project”:


I’m familiar with the I2C stuff with the Particle boards. Would this be readable on some of the other pins? From what I gather, this puts out a DC voltage. Would I read that on the Analog inputs of the Particle boards? I’d like to stay with those boards because of the small footprint.

The EmonLib library is good for this. All you need is a photon and a CT.

I am running it with two CTs at the moment. I monitor my well, furnace, water heater, A/C and Microwave. I can even tell when my wife is doing laundry.
This is my setup:


Do you have a part number for those CTs? And more pics :rofl:

Here is a picture of the CTs. I put the link for the Amazon page as well as the Open Energy Monitor website, which is a good resource for using the emonlib.

Also, depending on the size of cable your primary is, you need to make sure your magnets in the split core CTs are touching. I had to put zip ties around mine. Better yet, you could get bigger CTs.

Let us know what you end up doing.

Good Luck!

Amazon page for CTs

Open Energy Monitor Website


Awesome, thanks!

Yesterday, before your reply, I ended up ordering one of the NCD Hall Effect I²C sensors. These CT sensors may be more practical though since I can use some longer leads, avoiding mounting the Photon/Electron next to the panel.

I did consider adding the ability to sense current on the mains just to see if power goes out, but that requires adding a longer I²C cable from the Photon/Electron. The other idea I had - to check if power is out to mains or that circuit - is adding a nightlight in the bathroom that branches off the monitored circuit. If that’s out, the circuit to the pump motor is off.

I am very new to all of this and have those same CT sensors shown below. Would you be willing to share you diagram (with values)? I have the PZCT-02 (100amp) version and am trying to piece together a solution, thanks in advance!

You may check @TeraBull link, there’s a schematic: Open Energy Monitor Website

By the way, if you merely need to know if there’s current in the AC circuit, you might want to consider a current switch rather than a current transformer. I use Dwyer CCS-221100 in many applications to know if a pump, fan, compressor or whatever is running. They’re relative inexpensive, operate off of parasitic power and connect as a simple digital input. You can also get similar devices that permit setting a current threshold if that’s important.

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