Hi,
Some of the combined PIR quartz halogen outdoor security type lamps have a built in circuit for an alarm or remote warning buzzer, many do not or were installed before this innovation became available.
A number of years ago I installed three of the basic PIR operated units, all wired from one source. I was looking for a method which involved no additional mains wiring, but which would provide an indication when any of the three PIR lamps had been triggered. This meant working out some means of detecting the flow of current in the main feed for all three lamps.
The circuit I settled upon and which works well, is as follows...
I found an old transformer which had a little space between the windings and the iron core, enough room to apply a few turns of the cable feeding the lights. I then wound six turns around the transformer. It doesn't matter whether the live or the neutral is wound around, but obviously it should only be one of the two. I then experimented with a 12v Peizo sounder with built in oscillator, connected across the original windings of the transformer via a bridge rectifier with a capacitor to help smooth out the raw DC a little. On the AC side of the rectifier I temporarily wired in series, a variable resistor as a means to easily adjust the output.
The number of turns of the mains wire around the transformer can be adjusted, along with the actual winding used on the transformer plus the variable resistor to achieve a reasonable level of audio from the sounder without over driving it. I adjusted it so that when all three lights were on, it just managed to develop the 12v DC specified for the sounder. Once this is finalised, the variable resistor can be replaced with a fixed one.
Just one lamp triggered will thus produce less voltage/current output. Thus it is possible to hear changes in the note, as more PIR lamps are triggered.
Once the circuit is finalised, the sounder can be mounted where ever you like. A switch can be added to disable the sounder when it is not required.
The output of the transformer is limited to just a few mA and the voltage developed varies tremendously depending upon how much load is placed upon it. It would be perfectly possible to replace the Piezo sounder with one of those tiny low current relays intended for 12v, if that better suited the application.