Long term 110vac monitoring? What sensor?


Hello cayenne gurus of the world.

I’m new here, and would appreciate the community’s feedback.

I have a remote site with internet acces and need to monitor two 110-120vac lines, for voltage and interruptions.

I thought this would be a good project for cayenne and a raspberry Pi.

The question is, which would be the appropriate sensor, I would like a compatible approach. I’m better at wiring and soldering that writing code.

Please advice on the sensor to buy, and any feedback you consider important, as I don’t want to fry my raspberry.

All pointer are appreciated.


Hi Alejandro,

You could use any VAC sensor, check this link:

Then you just need to add an analog converter like MCP3004 and that`s it

I hope I have helped you


50:1 step down transformer for 120VAC, or a 100:1 step-down transformer for 240VAC (teensy current needed by the MCP3004, you only need, like, <5mA for the MCP3004 input)- with a diode full wave bridge and a 47mF filter capacitor, plus a 1k ohm bleeder resistor.
The MCP3004 in this application needs a single-ended DC input, -not AC. This is basically an “unregulated” 2.4Vdc power supply. The created DC output voltage will be directly proportional to the AC input voltage.

total cost- about $5

Do some programming math on the MCP3004 digital output to calibrate- and you’ll get your RMS calibrated readings. :wink:


$200 for a voltage sensor- NOT good!


Will the 120vac lines always have a load on them…say .5amp or more? You could use a cheap current switch to indicate such… I like these for testing purposes…Dwyer Current Switch

Set your threshold low enough to keep your switch activated…if you loose power, the load will not draw…and the switch will open. Just an idea.


Hi Alejandro,

Make any headway on this?

I was thinking on this a bit, and you know what I would do? I would hack a cheap 5V wall wart (AC/DC adapter), remove the bridge rectifier and regulator, then connect the transformer output to the existing wires. Glue it back together, and put a label on it so you know it’s the hacked one. Now you have a voltage you can safely monitor with some signal conditioning and your ADC.

For current, you could get one of these babies. https://www.sparkfun.com/products/11005 The output is in mV, so you will want to use an ADC with some built in gain or again, an instrumentation amp to condition the signal for the full range. Note that both signals will be steering around ground so will need a DC bias added at half your VCC for measurement.

You will also want to take a look at this Arduino library for processing the ADC waveforms into useful numbers. https://github.com/openenergymonitor/EmonLib

This should keep you busy this weekend :stuck_out_tongue:

Maybe our hardware engineer @HighTech, will build it for you :wink:




OUR hardware engineer?

I like the idea of a wall wart. Yeah. I’d suggest using something like a 2.5Vdc- 3Vdc unit. As small and cheap as possible. “No regulator” is the only requirement there. Just a filter capacitor. Excellent.

As far as “current monitoring” goes-
well, you’d need either:

-a “current measuring shunt resistor” https://www.arrow.com/en/products/shd1-100c075de/ohmite . The shunt has to meet max current draw from the load, and has to be a value that corresponds to the applied voltage, and the shunt causes a teensy (but measurable) voltage drop…

-or a current transformer https://www.sparkfun.com/products/11005 (thanks, kreggly)

-and an Op-Amp to amplify and calibrate your signal- then feed it into an MCP3004 A/D converter. Probably easier to buy one ready-built. Complicated…but do-able.


I apareciate the replies.
I unfortunatelly have had not a chance. Lots of work.

The idea is to monitor the input and output voltage of a APC UPS that is connected in an industrial environment where I borrow roof space for a radio point to point link.
And the APC has been misbehaving lately.

Having the pi work on les
Voltage i can monitor what the hell is happening.

I like the wall wart idea
I will read and bother you guys with questions if they arise. Thanks!!!