Electronic – Different voltages with Ferrite Beads

The PupDAC schematics are terrible, and it is hard to know anything from them. The R's and C's are not even correctly labeled (having to reference a BOM is not acceptable). I honestly doubt that the people who designed the PupDAC did any detailed analysis of the beads.

Most of the beads are used on the output of a linear regulator. I highly doubt that the beads are actually doing anything useful in that situation, and could actually be adding noise to the system. I would remove L3, L4, L5, and L6.

As for L1 and L2, either the Vishay or Bournes will work. I would probably go with the Bournes because of the higher current rating, but it has the disadvantage of the higher DC resistance. In this application, the higher resistance isn't a huge disadvantage.

If you choose to not remove L3-6, either bead will work. The Vishay would be slightly better because total current isn't as important but DC resistance is (for noise performance) But you'd be better off with just a piece of wire instead of a bead.

Two possibilities - either or neither may be correct :-).

• They wish to meet emission standards to some extent for unknown reasons. They may have found that the device caused problems for other equipment it was used with.

• More likely - just the opposite. You note that the design and construction suggest a very low construction cost product. They may have cut corners in areas that they have little or no understanding of. A PIR detector can be quite a sensitive device. They may have experienced marginal operation and noise problems and found that eliminating noise via the external connections improved operation reliability.

Larger version of your photo here
The added-afterwards power supply decoupling electrolytic (probably) suggests desperation, and the added to its leads after-afterwards 0.01 uF suggests that desperation was not enough to make it work well enough. That is hardly a level of production level assembly that you'd expect.

Other:

The 4 8's at top middle do not seem to have been auspicious enough to remedy the problems.

The hot-melt glue at left will fall off within a year and probably less.Regardless, it is not doing much of a job of providing strain relief. The blue wire looks to be in mortal peril if external movement can move it, and others are not much better.

Added:

The caps across the IC are between VCC and the sense input from the sensor. I'm not 100% sure of function, they aren't there on normal BIS0001 boards.

That adds weight (but not certainty) to the theory of noise input being a problem. If they were sensor to ground it would be more likely, but unless the Vcc line is noisy and they are providing feedback (!) then they should reduce noise (and signal) variations at the input. The beads would help keep noise out of the system overall. They do however seem to be part of the original design, although how they are connected electrically is not certain from the photo.