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I'm playing around with the idea of adding a capacitive sense switch to an all-metal lamp, and have some questions around safety as I'd, obviously, prefer not to turn the whole thing live.

In the plastic, isolated base, I'll put a 220V->5V SMPS which powers a board with an ATTiny45, opto-isolater/triac, and zero crossing detection circuitry. What sort of trace isolation do I require for the 220VAC? And between the live and isolated side? Anything else that's likely to kill me?

Board top: top

Board bottom: bottom

(edit for improvement)

Is the below better? better

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    \$\begingroup\$ Why do you have such a ridiculous huge bridge rectifier in there just to drive the zero-crossing detector? In fact, if this is just for on-off control, why do you need to detect zero crossings at all? \$\endgroup\$ – Dave Tweed Jan 6 '15 at 15:45
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    \$\begingroup\$ Yeah, that trace spacing for a start. You have LV traces about 1mm from mains traces. The closest they should come is the two rows of pins on each opto, about 6-7mm apart. Bottom opto is easy to fix : rotate it 90 degrees. \$\endgroup\$ – Brian Drummond Jan 6 '15 at 15:45
  • \$\begingroup\$ @DaveTweed I was actually going to ask that, but didn't want to conflate questions. I want the ZC for dimming, but can just do half-wave rectification with a single diode I'd think \$\endgroup\$ – Bryn Divey Jan 6 '15 at 15:52
  • \$\begingroup\$ @BrianDrummond Thanks Brian, exactly the sort of info I was looking for. The opto pin-width sounds like a sensible rule of thumb - I'd used some online calculator but don't think I did it right. \$\endgroup\$ – Bryn Divey Jan 6 '15 at 15:54
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    \$\begingroup\$ Re: your edit -- much better! \$\endgroup\$ – Dave Tweed Jan 6 '15 at 16:44
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The key concept is that you need to be able to draw a line across your board, with all of the mains-connected traces on one side of it, and all of the isolated traces on the other:

enter image description here

The copper-free gap through which this line runs needs to be at least 6mm wide, in order to provide adequate arc-over and creepage clearance between the two sets of traces.

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  • \$\begingroup\$ Thanks Dave - so something like the additional image I've added would be sufficient? \$\endgroup\$ – Bryn Divey Jan 6 '15 at 16:44

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