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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
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The motion sensor needs to be hot 24x7.

Motion sensors actually calibrate for the environment, and if you constantly sever power to them, they constantly are reset and have to start calibrating all over again. The sensor won't tell you this, the only symptom will be poor performance, which you will misattribute to a lousy or defective sensor.

If you also need a switch, put it downline of the motion sensor. So that the motion sensor always gets always-hot and neutral.

Switch it to DC. Low voltage DC motion sensors are cheaper.

About half the inherent cost in an AC motion sensor is the parts to make low voltage out of mains power, and to switch mains power safely. Therefore low voltage DC motion sensors are much cheaper.

AC low voltage lights don't care if they're DC. But low voltage DC motion sensors do. So do LEDs and it's time to switch to those, now that choices are available that give good quality light.

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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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Sure you can, mostly just like everyone has been telling you. I put in a system that uses all 120v flood lights due to the amp load. Once I hooked all those up to power, I used a 24v relay and when a motion detector trips, all the lights come on at once, leaving a prowler nowhere to hide. With this system, all you have to do to expand the system is add motion detectors where you want them, pull low voltage wire to them, and hook it into the relay activation circuit.

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