Does anyone have any idea of how to reduce the sensitivity of a PIR detector?
I have a PIR driven plastic owl in my garden to scare the critters out. It surprisingly seems to keep them out, but the PIR sensor is so sensitive, that any critter within 20-30 ft sets it off when I really want a 2-3 ft range. The result is that I am replacing the batteries every two or three days.
you can adjust range by adjust a potentiometer as shown in below image
You can learn more in this Arduino Motion sensor tutorial
That might be useful if he had an HC-SR501 module as used for Arduinox.
But he doesn't.
He has an owl! 
Dunno, I imagine by partly obscuring the sensor. Have you tried taping over it?
BTW a PIR outside will trigger from things like rising hot air caused by the sun and the like.
IoT_hobbyist:
you can adjust range by adjust a potentiometer as shown in below image
You can learn more in this Arduino Motion sensor tutorial
There is no adjustment. The PIR is in a moulded enclosure with no way to access whatever module is inside. Any changes have to be external to the PIR sensor.
"with no way to access whatever module is inside."
You mean there's no ready access panel.
You can alter your software so it requires a number of hits before operating the owl . This doesn’t reduce sensitivity but helps reduce false triggers .
I am intrigued into what the plastic owl does when triggered?
Could a larger, rechargable, battery pack be an answer?
The problem with reducing the range is that light/IR doesn't really have a range - if you make it sensitive
to small critter nearby it will also be sensitive to larger critter further away. And the basic sensor isn't
adjustable, only the circuit responding to the sensor can be adjusted to wait longer before believing
the sensor, or have a higher threshold (which risks missing smaller critters).
Frosted sticky tape.
Or vandalise it with a black "Sharpie". 
runaway_pancake:
"with no way to access whatever module is inside."You mean there's no ready access panel.
Right. It's completely sealed- probably to make it waterproof.
hammy:
You can alter your software so it requires a number of hits before operating the owl . This doesn’t reduce sensitivity but helps reduce false triggers .
That assumes that I know what's inside the plastic owl.
MarkT:
I am intrigued into what the plastic owl does when triggered?
Flashing LED eyes, which I don't think actually fool the chipmunks at all. But the pulsing ultrasonic transducer does seem to be keeping them out of the garden. As long as the batteries last, which is about two days.
Solar panels to charge the batteries? Hampster wheel linked to a generator with something to encourage the critters to run in the wheel?
Maybe you could add "blinders" to limit
the field of vision?
