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ESP8266 and HC-SR501 false positives 
Trying to install a motion detector in my living room to trigger lights I decided that constructing one myself using a ESP8266 module and a cheap HC-SR501 module was an interesting challenge.
Some soldering and an Arduino sketch later it was operational.
During the day my setup works fine but for some reason during the night it kept triggering.
After much searching it seems that the gain stage of the HC-SR501 suffers from interference from the wifi. To eliminate this I placed a 220nF capacitor across pin12 and pin13 of the chip (verify that it actually is a BISS0001).
Unfortunately the package isn't exactly geared to receive any capacitors so some improvising was in order.
After that no more false positives.
Some soldering and an Arduino sketch later it was operational.
During the day my setup works fine but for some reason during the night it kept triggering.
After much searching it seems that the gain stage of the HC-SR501 suffers from interference from the wifi. To eliminate this I placed a 220nF capacitor across pin12 and pin13 of the chip (verify that it actually is a BISS0001).
Unfortunately the package isn't exactly geared to receive any capacitors so some improvising was in order.
After that no more false positives.