I want to monitor a 240VAC line from an Raspberry Pi, my thought was to hook the 240VAC line to an appropriate relay then just sample the contacts from the Pi.

I have a relay which I think would work but I don't know how to be sure the coil is rated for 240VAC. It's about 1 inch in each dimension, the coil resistance is 670k Ohms.

I've attached a photo, I did google around for specs on this but I can't find any clear info.

Relay Photo

  • \$\begingroup\$ I meant to say the coil resistance is 670 Kilo Ohms. \$\endgroup\$
    – Matt Noel
    Apr 2, 2018 at 21:56
  • \$\begingroup\$ A schematic would be appropriate, use the tool embedded in the question toolbar and edit your question. \$\endgroup\$
    – Voltage Spike
    Apr 2, 2018 at 22:31
  • 1
    \$\begingroup\$ @MattNoel The coil resistance cannot be 670k unless the relay is broken, as no relay on the datasheet goes that high. However, 670 ohms lines right up with the 24D version, which has a nominal resistance of 660 ohms +/-10%. \$\endgroup\$
    – C_Elegans
    Apr 2, 2018 at 23:42

3 Answers 3


Your solution is unlikely to work, as the relay is going to buzz a lot, and probably start a fire. The coil voltage should be shown somewhere on the relay, though I don't see it on yours. In any case, \$640\Omega\$ is way too low for 240V, as it will be dissipating 90W, so instead of a relay, you'd have a lightbulb and a house fire. After looking at the data sheet (see TonyM's answer), and looking in the chart on page 2 for 'AZ-2110-1A-24D', we see that the coil voltage is 24V, and the winding resistance is ~\$660\Omega\$, which matches up nicely.

As for solving your problem, I'd recommend using an optocoupler. Something like this:


simulate this circuit – Schematic created using CircuitLab

Basically, it's a full bridge rectifier feeding the optocoupler (the led and transistor, the schematic editor didn't have a real optocoupler) with a current of ~5.5mA. The two 22k resistors will need to be 1W or larger to remain within temperature tolerances, and need to be rated for greater than 120V.

  • \$\begingroup\$ The resistors R1 and R2 should be rated for more than 120 V AC. Three or four resistors in series may be used to improve security. \$\endgroup\$
    – Uwe
    Apr 3, 2018 at 8:47
  • \$\begingroup\$ @uwe it has been fixed \$\endgroup\$
    – C_Elegans
    Apr 3, 2018 at 12:47

I also confirmed it is a 24Vdc relay SPST


  • 24 as in voltage for coil

3/4 Hp rated

As @C_Elegans posted,

With 240Vrms or +/-340Vp +/-??% the IR LED current will be 7.7mA Peak is to have a logic level pulse for each half cycle for instant detection by IRQ or sustain the output by some routine that samples the input, more than 1 cycle intervals then change the output from 1K pullup to 100K with RC= 100ms ( 10 cycles) , using C = 1uF using hysteresis or a Schmitt trigger on the input.


According to this, its a Zettler AZ2110, contacts 1 Form A, coil 24D.

The specs on that webpage list the above variant as having a 24 VDC coil.

enter image description here

  • \$\begingroup\$ The one pictured is the 24D variant, with a coil voltage of 24V. \$\endgroup\$
    – C_Elegans
    Apr 2, 2018 at 22:00

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.