Magnetic switch on PCB - how do they do it?

Question

A recent round of scrounging for a Hall effect semiconductor led me to pry open a cheap magnetic burglar alarm. Instead of a semiconductor the alarm used a reed (?) switch mounted on the PCB.

(Is an open-air switch still considered a reed switch?)

formatting link

shows a picture of an old style magnetic switch on top with a picture of the cheap alarm below. The old style switch moves a good 1/8" between making and breaking. The new style switch shows no visible movement whatsoever to the naked eye.

How do they do that? Is automated parts placement accurate enough to manufacture a microscopic gap in the PCB reeds?

TIA.

-- Don Kuenz

Phil Allison · Accepted Answer

"Don Kuenz"**  The term "reed switch" implies a glass encased one.**  Looks like a simple wire link ( near the magnet) in that PCB to me. The thing is the TO92 pack has to be identified  -  it may well be a Hall switch IC. The black blob looks like a ferrite cored inductor.....   Phil

Don Kuenz · Answer

It's actually a switch. A VOM confirms that slight finger pressure breaks the connection.

AFAIK the rest of the circuit, including the S9014 small signal NPN, and the transformer (which looks like a coil in the picture) form a high amplitude oscillator to function as an alarm. (Two of the red leads in the lower picture connect to a piezoelectric device.)

-- Don Kuenz

Phil Allison · Answer

"Don Kuenz"** Wild. WFT not use a real, glass encased reed switch  -  their reliability is legendary. If you have a problem with that device, then make the change yourself....   Phil

Greegor · Answer

Glass reed switches are standard for this use.Did some yahoo think they were too expensive to use?

Magnetic switch on PCB - how do they do it?

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