PIR occupancy sensor

I'm helping my son replace a simple three terminal, (two way or "upstairs/downstairs") switch with a motion control occupancy sensor unit. The unit, which 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 occupancy 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 simple 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 given me a headache, and for the life of me I cannot see how this unit has a return 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 it 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:

https://www.google.com/search?q=occupanc+y+sensor+wiring+diagram&client=firefox-a&hs=GL&rls=org.mozilla:en-USfficial&channel=sb&tbm=isch&imgil=mfUXOOULFD1--M%253A%253Bhttps%253A%252F%252Fencrypted-tbn3.gstatic.com%252Fimages%253Fq%253Dtbn%253AANd9GcS4H4_1mgd5XghUHt9MxHpWn0iqNzQjB8dCjSEZf8AdLpeCpgx2%253B532%253B299%253B1LJpxSZgvPcm4M%253Bhttp%25253A%25252F%25252Fwww.ask-the-electrician.com%25252Finstalling-a-occupancy-sensor-switch-for-a-bath-exhaust-fan%25252F&source=iu&usg=__umFAzAOKHhBj-N1Swi0y0-Qqi1E%3D&sa=X&ei=n9CTU-uoF6_JsQSI04KABg&ved=0CCgQ9QEwAg&biw=1024&bih=615#facrc=_&imgdii=mfUXOOULFD1--M%3A%3BrpNwQKvBBhVyBM%3BmfUXOOULFD1--M%3A&imgrc=mfUXOOULFD1--M%253A%3B1LJpxSZgvPcm4M%3Bhttp%253A%252F%252Fwww.ask-the-electrician.com%252Fpics-diagram-electrical-wiring%252Fwiring-daigram-motion.JPG%3Bhttp%253A%252F%252Fwww.ask-the-electrician.com%252Finstalling-a-occupancy-sensor-switch-for-a-bath-exhaust-fan%252F%3B532%3B299

 

[Pat]

On Sat, 7 Jun 2014 20:49:22 -0700 (PDT), captainvideo462009@gmail.com
wrote:

I'm helping my son replace a simple three terminal, (two way or "upstairs/downstairs") switch with a motion control occupancy sensor unit. The unit, which 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 occupancy 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 simple 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 given me a headache, and for the life of me I cannot see how this unit has a return 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 it 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:

https://www.google.com/search?q=occupanc+y+sensor+wiring+diagram&client=firefox-a&hs=GL&rls=org.mozilla:en-USfficial&channel=sb&tbm=isch&imgil=mfUXOOULFD1--M%253A%253Bhttps%253A%252F%252Fencrypted-tbn3.gstatic.com%252Fimages%253Fq%253Dtbn%253AANd9GcS4H4_1mgd5XghUHt9MxHpWn0iqNzQjB8dCjSEZf8AdLpeCpgx2%253B532%253B299%253B1LJpxSZgvPcm4M%253Bhttp%25253A%25252F%25252Fwww.ask-the-electrician.com%25252Finstalling-a-occupancy-sensor-switch-for-a-bath-exhaust-fan%25252F&source=iu&usg=__umFAzAOKHhBj-N1Swi0y0-Qqi1E%3D&sa=X&ei=n9CTU-uoF6_JsQSI04KABg&ved=0CCgQ9QEwAg&biw=1024&bih=615#facrc=_&imgdii=mfUXOOULFD1--M%3A%3BrpNwQKvBBhVyBM%3BmfUXOOULFD1--M%3A&imgrc=mfUXOOULFD1--M%253A%3B1LJpxSZgvPcm4M%3Bhttp%253A%252F%252Fwww.ask-the-electrician.com%252Fpics-diagram-electrical-wiring%252Fwiring-daigram-motion.JPG%3Bhttp%253A%252F%252Fwww.ask-the-electrician.com%252Finstalling-a-occupancy-sensor-switch-for-a-bath-exhaust-fan%252F%3B532%3B299

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.

 

[c4urs11]

On Sun, 08 Jun 2014 08:32:05 -0400, Pat wrote:

On Sat, 7 Jun 2014 20:49:22 -0700 (PDT), captainvideo462009@gmail.com
wrote:

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

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

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

Cheers!