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Topic: Getting power from behind the wall. (Read 77 times) previous topic - next topic

tarrinr

I have researched this too much, and I had given up until I saw the new wemo switch...
I understand the risks, but is there a way to power an arduino or microcontroller behind a light switch? Could I put it in parallel with the light, and only take a few volts from the light itself?
Thanks in advance.

polymorph

Power requirements? Only on when the light is on, or all the time?

If you have to ask, I fear that you -don't- understand the risks.
Steve Greenfield AE7HD
CET Consumer Electronics and Computer
Please don't read your attitudes into my messages

LarryD

#2
Oct 22, 2014, 10:58 pm Last Edit: Today at 12:35 am by LarryD
What is your background?

Edit:
Hope your insurance won't be voided.
The way you have it in your schematic isn't the same as how you have it wired up!

DVDdoug

Sometimes you can get power (120VAC here in the U.S.) at the switch, but sometimes there is only a constant-hot wire going to the switch and a switched-hot coming back, with no neutral.   Without the neutral, you can't get current through a load...  You need a "complete circuit".

With the switch off, there is always voltage across the switch (unless the bulb is burned-out).   That's how the neon light-up switches work...  The kind of switch that lights-up when the switch is turned-off.     You can "steal" a little current that way, but if you steal too much, the voltage will divide between the bulb and whatever you've connected...  You won't get full voltage to whatever you have connected and the bulb will glow dimly.

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Could I put it in parallel with the light, and only take a few volts from the light itself?
Again, a parallel connection may not be available at the switch.   And of course if you connect in parallel you'll only get power when the light is on.    And, you can't "take a few volts".   You'll get the full voltage that's across the bulb.

tarrinr

Thanks! That makes perfect sense.

So do you know how those Belkin WiFi switches work? Is it only possible when you've got that third neutral wire?

DVDdoug

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So do you know how those Belkin WiFi switches work? Is it only possible when you've got that third neutral wire?
I don't know...  Do you have a link to the specs?    I know some X-10 and Insteon switches require the neutral and some don't.
  
You can use some "tricks" when the device stealing the power is the switch because the switch doesn't have to completely "short out" the voltage like a normal mechanical on-off switch.

I don't know exactly what these tricks are, but it would be possible to steal a few volts for a millisecond or so, and charge-up a capacitor without any noticeable drop in light-output.

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