CR Magnetics CR9321-NPN Current Switch Product Review

CR Magnetics CR9321-NPN

Home automation and monitoring is a hobby of Recently, we wanted to add monitoring to a replacement sump pump in our basement. Our Arduino based system monitors digital inputs on a real time basis every second, so I needed a simple on/off detection system for when the pump motor was drawing current.

The CR Magnetics CR9321-NPN Current Switch looked perfect for the job.

Ordering and delivery from DigiKey was as good as it has always been for us. There isnt much to the current switch. A donut shaped core which you thread through the 120 VAC hot wire. And a twisted pair of 22 gauge wire with the open collector connections. The wires are brought to our Arduino, where we tie the black wire to ground, and the red wire to a 10K ohm resistor tied to +5V, and feeding a digital input pin. Thats it!

The spec sheet indicates the switch needs a minimum of 0.35 Amp to turn on solid. Our sump pump draws just over 3.5 A when it runs, so no concerns there. The product works as advertised, and our arduino reports the run time and other cumulative stats based on the device.


We also appreciate the full isolation of the device in terms of ground path.


We will be keeping this device in mind for future applications!