James Bryant of Analog Devices and his dirty dozen ways for circuits to fail.
The PIR circuit I recently installed has been challenging me. Just two days after it was installed it began to at times fail to switch the load on even if the PIR sensor was detecting motion (I know because the LED had turned on, ergo, PIR output was high). The fault was infrequent and usually occurred the first time the pantry was accessed in the morning--circuit must be too groggy to start the day. After days of checking and monitoring supply voltages (and finding nothing wrong with them), I decided to reprogram the MCU. The problem disappeared. Could've been an intermittent contact problem--which was licked when I removed the MCU and reinserted it--or it could've been a bad burn of the firmware.
Another problem which was noticed along with the above has yet to be resolved. When the dining room lights are turned off, the pantry light sometimes switches on. This occurs probably once every couple of dozen switchings. Rather obviously transients in the power line from switching off four conventionally ballasted plug-in type 11-watt compact fluorescent lamps are falsely triggering the circuit. I've already monitored MCU VDD using the Min-Max with Peak detect feature of the Fluke 87V for some 24hours and found the voltage to be from 4.930V to 5.048V, so no problem there. On the other hand, given the cable length to the PIR module of some 40cm, the problem may lie there. That's No.5 in Bryant's list. I've been mullling where in the sensor circuit to install additional bypass/decoupling caps. May have to put one at the VDD of the module and another at the MCU input pin (GP3); 0.1 to 1uF for the former and 10 to 100nF for the latter.