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I have installed and wired low voltage exterior lights. They required a transformer to bring the AC to 24 volts. I want them controlled by a motion sensor so before the transformer I installed a motion sensor. The lights work, but only with the switch. The motion sensor does not engage.

Is it possible to have the motion sensor control the lights, and if so, how do I do it?

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You will have to install the motion sensor (if it is rated 110v) in the feed to the low voltage system. that is, hot going to switch, then motion sensor, then low voltage transformer then low voltage lights.

that is, sensor powered by 110v, feeding transformer

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You will need a motion sensor that is rated for a transformer load -- many of the in-wall types are rated for incandescent loads only. – TomG Mar 8 '13 at 13:52

It is surely possible, just wire the motion sensor in series with the power switch.

The sensor should have three wires. on hot, one neutral and one to the lamps.

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