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I am about to embark on a home automation project, but I'm having trouble finding Wifi based IoT devices which can be powered from a standard two wire light circuit.

The idea is to replace light switches in each room with smart switches, powered from the light circuit, which also do things like monitor temperature, movement or anything else I can hook up to an Arduino. I could power the low voltage side with batteries, but I don't want the hassle of having to keep replacing them.

Most devices on the market designed to go behind a light switch, such as remotely operated light switches/dimmers expect a three wire lighting circuit (live, neutral and load) at the switch. In the U.K. our circuits are almost exclusively two-wire, i.e. only the live and load wire come two the switch, while only the neutral and load wire goes to the light fitting.

I obviously don't want to have to re-wire my house to make all of the light switches three-wire, but I have only been able to find one device which says it can do what I want without re-wiring (and it doesn't do Wifi).

What I want to know is how this circuit works, so I can work out if I can build something similar myself (given the usual caveats regarding circuits which run on mains voltage). Alternatively, if this circuit has a name, knowing it should help me find a power supply which does what I want.

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  • \$\begingroup\$ This question has already been asked and answered. How do external circuits in light switching units (such as Lightwave RF) use the mains wiring for power? \$\endgroup\$ – Finbarr Sep 26 '17 at 22:18
  • \$\begingroup\$ Sadly @Finbarr the answer on that question neither tells me how the device works (other than in the most generic way, and the device I referenced doesn't "only work with ...resistive... loads") nor what this kind of circuit is called (so I can find a commercial implementation of a power supply which implements that circuit). \$\endgroup\$ – Mark Booth Sep 27 '17 at 10:25
  • \$\begingroup\$ Well, you only have two wires going into the device so it's hard to get power any other way than passing a small current through the load. \$\endgroup\$ – Finbarr Sep 27 '17 at 11:15
  • \$\begingroup\$ Please be aware that hooking up to an arduino means you have the arduino at mains voltage. Thus you have to protect the whole devices from being touched, even accidentally. \$\endgroup\$ – Attila Kinali Sep 27 '17 at 13:29
  • \$\begingroup\$ Thanks @AttilaKinali, this is why I was hoping that I might be able to use answers here to find a stand alone 3.3/5v power supply which worked with a two-wire supply. Unfortunately, it's looking like no-one has actually created one which isn't already integrated into a product. \$\endgroup\$ – Mark Booth Sep 28 '17 at 11:07
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enter image description here

Figure 1. Triac waveform with exaggerated turn-on delay.

I suspect that to power this switch in-line they have to steal power from the line. There are a few possibilities that I can think of:

  1. Use a triac to delay switch-on until the voltage across it has risen far enough to charge a capacitor for power through the rest of the cycle.
  2. Skip a mains cycle periodically and switch off the load momentarily. This will apply full voltage across the switch and would allow internal PSU recharge.
  3. Put back to back diodes or Zeners in series with the load to drop some voltage across the switch. This has the problem of power dissipation in the switch.

I think the second option would be likely to give noticeable flicker so it is unlikely.

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