Converting an PIR outdoor light circuit for 12v

Being lazy I did not want to turn on and off my circuit tree on my desk at work, so I was just leaving it off.  However, with the Christmas season upon us, people started to bug me about not leaving it on, so I decided to get creative and convert a PIR circuit from an old outdoor motion activated spotlight to run on a safer 12v and control the tree on my desk.

First off, after eagerly prying the circuit from the lamp I quickly realizes that it did not use a transformer like I was expecting.  Most newer lights I looked at did.  So I went out looking for a datasheet on the PIR controller ic, but the one used here is either too old, or is just unlisted. This was not going to be as easy as I was expecting it to be.

This was trouble some as every PIR ic has different pinouts, and some use a first/second amp other do not.  So I went back to the basics and traced out the circuit with my multimeter and carefully sketched out the paths so I could better see what was going on.  The first thing that stood out was the four large diodes which are clearly a rectifier, to convert ac to dc.  After this, it passes through the biggest resister (blue tube) I have ever seen, which drops the voltage down to around 13v for the relay which is switched by a transistor.  From there it is dropped further to 8v and smoothed out by a capacitor for the rest of the circuit. Which is odd as an ic usually takes 5v max, but as it was unlisted I only have the trace readings to go on, so I will assume it was built to handle a larger voltage.

After figuring out this much, it was not hard to see where I would need to apply 12v to have the circuit work exactly the same but at a far safer voltage.  So after removing a small resister to break the original +120v (black) path, I now had an isolated PIR circuit and a separate circuit switched by the relay.  Right now the relay will only deal with 9v, but this leaves it open to once again control higher voltages, for potential use at Halloween.

You can see the final result below.


I am quite happy that my digital/analog circuit skills have not gotten too rusty with all the programing I tend to focus on.