Scenario: power outlet in the yard for electric vehicle, to avoid running an ugly extension from inside home and leaving an open window; I need to avoid that anyone just comes and uses my power though.

Enter the switch I'm looking for.

This would be the operational flow:

  1. Attach the charging cable of the vehicle to the outdoor outlet (no current will flow since the outlet is not powered - yet)
  2. get inside, arm said switch, current will start flowing as the vehicle begins charging, and the switch will stay on
  3. Either the vehicle stops charging, or I unplug it, or someone else does because they want to use my energy for free: the switch will detect no more current flowing through it, and will disarm, leaving the outdoor outlet inactive

I found two somewhat similar questions:

Need a switch/circuit breaker that stops power supply to equipment when power comes back after outage (not good for me, since it is intended to check for incoming power, not outgoing current flow)

I'm looking for a circuit breaker that interrupts the circuit when current drops to zero (not good, as it is for DC rather than AC)

Is there a name for this peculiar kind of switch, and/or is DIY easily accessible?

  • \$\begingroup\$ Welcome to EE.SE but please note that direct shopping questions will result in fairly prompt closure of your question as per site rules. \$\endgroup\$
    – Transistor
    Jan 8 at 14:38
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    \$\begingroup\$ Anyway you need some current sense device. Current transformer may feed some relay coil. And momentary switch. \$\endgroup\$
    – user263983
    Jan 8 at 14:41
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    \$\begingroup\$ One can buy these current switch modules (not shopping advice, just to illustrate it exists). One could design a relay circuit with a start button to turn on the relay, have a contact from the current switch in parallel to keep the relay on after the start button is released, but when current drops below the setpoint the contact opens and the relay turns off. \$\endgroup\$ Jan 8 at 14:42
  • \$\begingroup\$ Can you think of a scheme which does not involve sending current? If yes, that would be much simpler. I propose a timer. \$\endgroup\$
    – winny
    Jan 8 at 14:55
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    \$\begingroup\$ You can definitely do it using the current switch modules linked to by Unimportant (or something similar). You would plug in, then push a "start" button inside the house and after that the output relay would latch on until current drops to zero (or very low, anyway). At that point it would turn off again, and you would have to press the start button again to activate it. \$\endgroup\$
    – mkeith
    Jan 9 at 2:42

4 Answers 4


One can buy current switch modules, such as these (google "current switch"), basically a current transformer and electronics to operate a switch contact based on the measured current in one module.

A relay circuit can be designed that allows activating the relay using a momentary start button. A normally open contact from the current switch module placed in parallel with the start button would keep the relay enabled after the start button has been released, but when the monitorred current drops too low the contact opens and the relay turns off.

Current switch requirements:

  • Self powered - These "steal" the energy required to operate themselves from the conductor being monitorred. Not requiring a power supply simplifies the circuit.
  • Adjustable setpoint for the current level at which the contact enables, so you can tune for your load.
  • Monitor side must be rated for AC mains and the current your load requires.
  • Contact side must be rated for AC mains and the current the relay coil requires. Normally open contact that closes when the current is above the setpoint.

Relay requirements:

  • DPST (or DPDT) with contacts rated for AC mains and the current your load requires.
  • Coil that runs on AC mains.


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ I went for this solution, thank you! The modules you linked to seem the most "elegant" as in "self powered, self contained, all in one box", but I looked elsewhere as I wasn't sure the included relay that controls the screw terminals could manage the load of a charging EV (possibly up to 10A). So I bought a cheaper, arduino-like module, with mechanic relay and 5V DC power input, that I will in turn use to control a bigger relay, that I can easily swap for another one in case it fails, instead of playing surgeon on the larger module. \$\endgroup\$
    – ephestione
    Jan 11 at 20:25

There are control panels for pool/submersible pumps with dry run protection (switch off when current is below a setup value). Maybe you find some one-phase starter that can fit your needs.


If a fixed power-on time is acceptable, then a lamp timer or bathroom heat lamp/fan timer can be wired in series with the outlet. This would make power available for 4 hours (for example) no matter how dead the vehicle's batteries are. Granted, someone could still come up and steal your power, but not for very long, and not at all after the timed period (like overnight). If nothing else, this is a temporary solution until you get a sensing circuit built.


Install a security camera. What you describe is definitely theft and is prosecutable. The presence of a camera should be enough to dissuade folks anyhow. This isn't like porch pirates who can come and go in a few seconds...someone using your juice would be vulnerable to detection for quite a long time.

  • \$\begingroup\$ heh the kind of people who could be serving themselves are not actually random ones that I can call the police on... that's why I'm more prone to just remove the object from anyone's disposal \$\endgroup\$
    – ephestione
    Jan 8 at 21:54

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