PIR occupancy sensor

Jun 08, 2014 2 Replies
C

I'm helping my son replace a simple three terminal, (two way or "upstairs/d ownstairs") switch with a motion control occupancy sensor unit. The unit, w hich takes the place of one of the switches is a PIR 180, (as shown in the diagram at the link I've included below).



The schematic, which illustrates one three terminal witch and a PIR occupan cy sensor doesn't seem to make much sense to me. For one thing it has you tying one of the travelers of the remaining three way switch to the common terminal of that same switch. Why not simply replace the switch with a simp le SPST, (on/off) switch? And I have another concern as well.



I've looked at this simple circuit until, (I'm embarrassed to say) it's giv en me a headache, and for the life of me I cannot see how this unit has a r eturn to neutral. We set this up on the bench first, (mainly because I had my doubts as to whether or not it would even work). To my surprise though i t did work, and quite well too. The unit loses power however if the load is removed! What???? And although I expected it, disconnecting ground had no effect.



So what's going on here? How is this a complete circuit? Lenny



Link to diagram:



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P

The device gets its power through the load rather than a neutral connection. (The power needed to operate the sensor is very low).

If you replace the other switch with a SPST, the new device would be completely disconnected when the SPST was in the off position. Plus, why make a non-expert home owner replace the second switch when the existing one still works?

The remaining switch just acts as a signalling device to the new sensor. You bypass it to make sure the new sensor gets power, but still use the other pole of the switch to signal the new sensor you want something to happen.

Finally, don't be surprised if you change the load to a compact florescent or LED and wierd things start happening like the light flashes. That very low current through the load isn't nearly enough to make an incandescent bulb glow, but it is sometimes enough to build up a charge in the capacitors of a compact florescent and make it flash occassionally. I think new designs are less apt to do that, but a lot of older ones did.

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C

I think this is US patent US7791282. The description page on google has quite a bit of technical details, even when skipping the legalese:

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Cheers!

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