12v sensor light

I wanted to add a security/sensor light near the main doorway that would run of 12 volts. You can buy 12 volt PIR sensors, but these are very expensive at over $50 each and designed for security systems. However you can pick up a 240v sensor light for less than $20 from any hardw
12v sensor light - schematic

are store, so I wondered if one of these could be converted to 12v operation. I already had this sensor lamp, so it was the guinea pig. Once I opened the sensor case ( after I of course disconnected the lamp from the mains power ), and accidentally broke off the small adjustment knobs, I found the sensor consisted of two circuit boards. Shown in the picture below (remember you can click on most pictures to see a larger image ), the left circuit board is the sensor, and the right circuit is power and relay driver. There is also a LDR ( Light Dependent Resistor ) there, this is used to stop the lamp turning on during the day. When you test your sensor light, remember to cover this with black tape to block out the light. Another thing to remember when you want to test you sensor, is the PIR will only work with the frenzel lens in place. This is a generic circuit for these sensor lamps, the component values may vary from model to model. R1 is a fusible resistor, so it acts as a fuse as well as a dropping resistor. C1 and R2 are the main voltage dropping components. Diodes D1 to D4 make a bridge rectifier, fed into C2, our main filtering cap, and then a 5 volt regulator, to supply 5 volts to the sensor circuit. In some models I`ve see zener diodes used in the bridge rectifier, or across the output, to keep the voltage below 30 volts. This protects the 5v regulator from excessive input voltage. Transistor Q1 receives it drive...

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