Cannot identify part - Door alarm

Hi guys,

I bought a few cheap [$1!] battery operated door alarms from the dollar store. I thought there might be some good parts inside.

I took it apart and there is a flat piezo element, a transistor, a resistor and a capacitor, all through hole components. There's an expoy blob, which I think is some sort of IC for oscillation since the alarm "warbles" at 110 decibles. The cool part was a little glass cylinder with tiny metal elements that snap shut when the magnet is nearby. A Reed switch I believe?

Anyway the one thing I cannot identify is a little thing that looks like an electrolytic capacitor, except it has 3 prongs. It was wrapped in black plastic. I cut a bit off with an razor blade and saw that it basically some sort of inductor, it was a tightly wrapped coil of copper wire. My main question is what is this thing called and what is it used for? My guess is this is some sort of inductor that is used to step up voltage/current [I forget which an inductor deals with] to get a louder volume from the piezo element?

I thought it might be a neat project to expand on, say wireless so I can have an alarm in my bedroom etc.

I myself have one of those door alarms, i was also intrigued by that part, but never found out what it was.

if you want a wirless door alarm, and have some electronics knowhow, just combine a wireless doorbell (dollar shop for under 5$) and that alarm, instead of pressing the button by hand, replace the button with the reed switch, and you get the point

good luck

fuper said:

The cool part was a little glass cylinder with tiny metal elements that snap shut when the magnet is nearby. A Reed switch I believe?

Click to expand...

Yes, that would be a reed switch.

My main question is what is this thing called and what is it used for? My guess is this is some sort of inductor that is used to step up voltage/current [I forget which an inductor deals with] to get a louder volume from the piezo element?

Click to expand...

That sounds about right. A tapped inductor to give a larger voltage swing for the piezo. If the piezo is connected across two of the pins, that would confirm it.