Google has failed me so I'm asking here.

I am looking for an automatic switch, which will change state depending on the number of Watts/Volts/Amps flowing through the circuit.

As an example: When the amount of Watts flowing through the switch is less than 40, the switch remains in the default position. When the amount of Watts is greater than 40, it changes position and opens a different circuit.

Does this type of device even exist? (Forgive my ignorance, I am a beginner)


The problem I have: I have the following circuit, where the sensor is generating enough Watts in its off state, that the LEDs turn on even then.

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I am thinking of a workaround like this:

enter image description here

Where an automatic switch would workaround the issue and only connect the LEDs when the sensor has 'sensed' something.

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    \$\begingroup\$ Welcome to EE.SE! Do you have fixed voltage in your system? \$\endgroup\$ – winny Jan 8 '20 at 9:46
  • \$\begingroup\$ @winny I have 230V, but it's AC - does that answer your question? :) \$\endgroup\$ – Illidanek Jan 8 '20 at 9:48
  • \$\begingroup\$ Yes! You can take a look at one of these. It only turns on one circuit at a given load so if you need to turn off another you need to use a relay to invert that signal. theenergychoices.blogspot.com/2017/09/… \$\endgroup\$ – winny Jan 8 '20 at 9:50
  • \$\begingroup\$ @winny Thanks, but I don't think that will work for me. Mostly because I'm looking for something quite small, a little device I can wire in and hide. And it needs to be programmable to turn on at a given amount of power \$\endgroup\$ – Illidanek Jan 8 '20 at 9:56
  • \$\begingroup\$ it changes position and opens a different circuit - what does this mean? Sounds like a recipe for a power oscillator to me. \$\endgroup\$ – Andy aka Jan 8 '20 at 10:05

The crux of the problem

This is a bog-standard problem seen over and over in the DIY stack, when obsolete/cheap dimmers, motion sensors or other smart devices are used with efficient, driver-based LED lighting.

Older switch loops do not have a neutral wire. To solve this problem, older/cheaper, obsolete dimmers, motion sensors and other smart devices use a characteristic of incandescent lights - their extremely low impedance when off. They leak their operating current through the "off" incandescent filament. This doesn't work on LEDs; they are so efficient they turn all current into light.

The proper way to solve this problem is bring neutral to the switch, and have the smart switch power itself between supply hot and neutral. Like a normal load.

The direct solution

The problem with your idea is you're not allowed to horkle-dorkle "hacks" like this where mains wiring is concerned. You must follow your local Electrical Code, which says every device must be approved for use in mains wiring, and be installed according to its instructions.

Here's what you can do: a relay that is approved, such as an Aube or RiB. You wire the hunk-a-junk dimmer in series with a relay coil, with the coil returning to neutral. Separately (it can even be off a different circuit), you take supply hot to one relay contact and connect the other relay contact to the lamp(s).

Now, the relay coil may be too high impedance for your obsolete sensor. If so, you can add an approved "dummy load" in parallel with the coil, such as the Lutron LUT-MLC, which is approved for this purpose.

It acts like a resistor. Actually, it's a capacitor tuned for your mains frequency.

Or, just fit the dummy load directly across your lights

Like I say, dummy loads like the Lutron LUT-MLC are designed and approved for the purpose of shunting across LED lights to provide an alternate current path to get obsolete dimmers to work. So you could just fit this directly to your lights. Problem solved.

This method will work no matter what, even if you have an old-school "switch loop" which has no neutral at the switch. Never misuse safety ground for neutral, it defeats the purpose of having a ground wire.

Indeed, one of the diagnostics we give people who report this problem is "Replace one of the bulbs with incandescent, does the problem go away?" The incandescent acts like the dummy load, or to be more precise, the reverse.

Or get a modern motion sensor

Get a motion sensor that has a neutral wire. It powers itself via the supply hot and the neutral. It places no demand on the bulbs at all, and will in fact work properly with no bulbs connected.

  • \$\begingroup\$ Thanks a lot! I will evaluate all these options. Regarding the last one, I have been trying like crazy to find a motion+sound sensor that turns on when it's dark to absolutely no avail :/ The only one I found is this old-school one. \$\endgroup\$ – Illidanek Jan 10 '20 at 8:07
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    \$\begingroup\$ @Illidanek Maybe two sensors. A motion sensor that works in the dark should be easy: that's the entire point of a PIR, when it is pitch black, turn on the light when a human sized infrared source comes into view. You can have more than one sensor if it has a neutral or uses a relay. \$\endgroup\$ – Harper - Reinstate Monica Jan 10 '20 at 8:10

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