You can get these mini PIR detectors on eBay for less than $2 each with free postage. I modified some of them to have an open collector output when triggered. This suited my Halloween props better.

Here’s the unmodified unit. The output goes high (3.3V) when triggered.

The mod involves removing the zero Ohm resistor (at blue arrow) as well as adding an NPN transistor in the space provided (at red arrow). I used a BC847 transistor as it’s common and does the job.

I found this nice schematic for the module here (Unusual Electronics in the UK). The 150K resistor on pin 7 of the chip looks to be for timing the output extension. Adding a 47K resistor across it (making ~ 35K) reduced the time to about 4 seconds.

Here’s the PIR mounted into some conduit fittings.