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.

hc-sr505_origHere’s the unmodified unit.

The output goes high (3.3V) when triggered.




hc-sr505_oc_modThe 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.

The PIR output now pulls to 0V when triggered.


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.