SensingRebuild shelfHand tools, works as-is

Piezo horn

Also called piezo siren, smoke alarm horn, self-drive piezo.

The ear-splitting disc in a photoelectric smoke alarm. Often three pins: drive, feedback, and ground, because the transistor that makes it oscillate is outside the horn, on the board. Two-pin types are self-contained and just want DC.

If you are new to this category

A sensor reports a condition such as weight, light, temperature, or position. Many sensors do not switch fully on or off. A kitchen scale bar, for example, changes by thousandths of a volt. Keep the small board that read the sensor if you still have it.

Keep the connector. Write down the wire colors before you cut them. If you do not know which wire is which, the sensor is hard to reuse.

Four wires on a scale sensor

Red and black feed the sensor a steady few volts. Green and white are the tiny signal, measured between them, not from either one to ground. Pressing the bar changes that signal by millivolts. A normal meter on DC volts can show that it responds. Turning it into a number on a screen wants a small amplifier board, often an HX711.

Americium in ionization smoke alarms. Ionization alarms contain a small Am-241 foil. It is not a parts project. Opening the chamber contaminates your tools.

Specification. The right-hand column is what is typical. The label on the part in your hand wins if they disagree.
What to look for	Usual figure
Voltage	A 9 V alarm horn is happy around 9–12 V
Current	Low. Volume is not a sign of a large current.
Pins	If it has three, you cannot just apply DC and expect the tone. Keep the driver transistor or buy a two-pin horn.

Volts are electrical pressure, amps are how much current is flowing, and ohms are how hard the part resists that flow. A higher voltage than the part was built for is how salvage parts die on the bench. If a cell in the table says “read the stamp”, that stamp is the whole specification.

What people actually use it for

An alarm that does not need a speaker amplifier
A continuity tester you can hear across a room

Leave this one when

It came out of an ionization alarm you should not have opened. Photoelectric only.

Buying a new one is the better idea when

A two-pin 12 V piezo with a known sound pressure, if the three-pin wiring is not worth it.

How to tell if this one still works

Rated DC on a two-pin horn. On a three-pin, trace the one transistor that used to drive it and keep that corner of the board.

Tools you will want: Meter, 9 V.

Where people actually find it

2 notes, most useful first. “Leave it” means the part is in that machine and you should not take it: it is dangerous, worn out, or only useful if you already know that exact repair.

Harvest at a glance
From	Call	How many	How often	Where it sits
Photoelectric smoke alarm	Prize	1	always	The board.
Uninterruptible power supply	Bonus	1	usually	The beeper.

Prize — take this firstPhotoelectric smoke alarm1 · always
In this particular machine
Often three-pin, very loud on 9–12 V.
Where to look
The board.
How to get it out
Keep the driver transistor with it if it will not sound on DC alone.
The condition to expect
Good.
Bonus, if you are already in thereUninterruptible power supply1 · usually
In this particular machine
The beep you hated.
Where to look
The beeper.
How to get it out
Small.
The condition to expect
Good.