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I'm working on a home project where a microcontroller records time when outdoor lights are switched on/off. Outdoor lights are 220V activated by built in PIR sensor. They're located outdoors 20-30m from the controller board. I was thinking to put a reed switch next to power lines so that when lights are switched on reed switch is shorted and microcontroller is interrupted.

I've checked other topics where people use current transformers but that seems over complicated to me as I don't need exact current value but rather just on/off state. My aim is to keep it simple and robust. Now questions:

  1. What is minimum distance between different reed switch/power line combos?
  2. There are some relays on the same board that control loads. Would relay coil affect reed switch? How should I protect reed from unintentional activation?
  3. What would happen to reed switch if it's activated by AC generated magnetic field?
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  • \$\begingroup\$ Would you please post any references to any examples where this has worked. My first reaction is the field around a mains cable is too weak to activate a reed switch. That assumption partly explains the popularity of more expensive, alternative technologies. \$\endgroup\$ – gbulmer Aug 18 '14 at 12:31
  • \$\begingroup\$ Well seems like reeds are only good for DC magnetic field. Under AC magnetic field it's going to fail. \$\endgroup\$ – Farid83 Aug 18 '14 at 20:04
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1) That depends on the sensitivity of the reed switch and its proximity to the power line.

2a) Yes.

2b) Either locate the reed switch away from the relay coil or align its axis 90 degrees away from the relay coil's axis, or both.

3) It will chatter.

If you're looking for something simple and robust, you may want to consider a current switch; something like:

enter image description here

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  • \$\begingroup\$ Thanks EM Fields! Unfortunately getting specialized components is not easy here. Also I've opted out from reed option because it's not intended for AC induced magnetic field. Looks like only solution is stick to current transformer. CHeers! \$\endgroup\$ – Farid83 Aug 18 '14 at 20:05

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