I have just opened up a small, low-cost motion-activated (by PIR) LED light as I am planning on just using the casing for another project.
The PCB inside is clearly designed to be very cheap. It uses a BIS0001 PIR IC found in many other PIR products. There is heavy flux residue, the PCB is thin, and there are bodge capacitors straddling the IC. It's cheap.
For this reason, I am surprised that there are 4 ferrite beads on small wire jumpers, as shown in this image. The wires are the battery connections and the SPDT mode switch.
This is fairly common on mains powered equipment, switch-mode power supplies, networked equipment and anything involving high frequencies.
The device is battery powered with no option for mains power and is totally standalone.
The pyroelectic sensor and phototransistor are mounted on a separate PCB. These are connected by hookup wire that is twisted, potentially indicating there are other noise issues on the board.
But why are there 4 of them on this dirt-cheap PIR?