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Figure 2 - Only the motion sensor part of the security light is needed for this project
Figure 2 - Only the motion sensor part of the security light is needed for this project

Most motion sensing security lights will have an AC cover plate that includes one or more light sockets and a plastic sensor unit that can be removed from the base plate. Since you will not need any of the AC electric parts, tear down the sensor light until you have isolated only the motion sensor box as shown in Figure 2. This part of the system is a standalone device that uses 120 volts AC to switch an AC load through a mechanical relay. You could actually run this system from the AC line by isolating the relay contacts to switch your control electronics, but to play it safe, it is best to convert the motion sensor to run from low voltage DC so you can power it from a battery pack.



Figure 3 - Motion sensors are also available that run directly from DC current
Figure 3 - Motion sensors are also available that run directly from DC current

If you are not interested in hacking up an AC appliance for battery operation, you could just purchase a ready to use motion sensor board or kit from an electronics supplier. The two motion sensors shown in Figure 3 are from SparkFun.com and are ready to run directly from 5 volts DC. If you do plan to order a PIR sensor kit from an electronics supplier, ensure that you also receive the small Fresnel lens needed to spread out the sensing area over a larger area. The PIR sensor by itself is only capable of sensing movement a few inches away from the tiny window on the device, so the Fresnel lens is necessary. This lens will look like a small bubble or plastic sheet with tiny grooves on its surface.

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