Ok, so I have a basic understanding of things and I hit a roadblock on a project. I need a buzzer for a project at work. We were traveling and decided I could do this out of simple things we had on hand. The switch was easy. The buzzer, I thought would be easy too, we have all these old smoke alarms for some reason. So, the challenge. Use only componets off a smoke detector to make the buzzer. Soldering is available.

 

Where's the old buzzer in your photos? Did it have its own oscillator or does that chip in the black circle provide that function? What's the number on that chip?

 

Yes it has it's own oscillator. I can't find any numbers on the chip

 

That chip does not look like a buzzer. Albeit some new buzzer chips are quite small, the buzzers I have seen in older alarms are round and have a hole in them to let the sound out.

 

So, the challenge. Use only componets off a smoke detector to make the buzzer.

Apart from the unknown IC there doesn't seem to be any active device on that board, so that pretty well kills any chance of making a buzzer.

 

A buzzer is one type of thing, and a piezo beeper is a very different thing. Do you know which device you have?

Image of the underside of the board?

Is the detection chamber the large round three-legged part?

ak

 

Excellent point, my bad, it has a 3 wire piezo disc.

 

I removed the detection chamber to see the chip underneath

 

And as for photos of underneath, I just packet it up to head to the next hotel. As soon as I get there, I will dig it to take a photo

 

I really just can't figure out where the transistor is. I wierd up a piezo buzzer before, a couple resisters and a bc568 transistor I think it was and done. I see nothing comparable here, I plan to remove the resistors i need, but... that's about as far as I figured out

 

Is the beeper a bare piezo element or a packaged component? If packaged, it could have the 1-transistor circuit inside.

ak

 

I am pretty sure they leads are just soldered to the back of the disk but again, it will be a minute before I can check

 

Pics of disc and back of board

 

OK, that looks like the buzzer. Now you need to trace the three wires back to the PCB. In all likelihood, the white wire is the signal.

If it is connected to an IC [edit] or transistor to an IC, try to get the numbers. Spit and oblique light help. Some of the markings are laser etched and very hard to see. However, it is not unheard of for manufacturers to use unmarked IC's.

 

Hello,

The buzzer needs a transistor to produce sound:
https://electronics.stackexchange.com/questions/263187/piezo-buzzer-coil-or-resistor-oscillator

Bertus

 

So how does it work in the smoke detector?

 

It has nothing to do with detection of smoke per se. It is just the signal that smoke has been detected.

 

Sorry, I didn't word that correctly. I understand how the smoke detector works, where is the transistor that lets the buzzer work?

 

A 3-terminal piezo disc needs an inverting amplifier with gain to oscillate and make sound. The piezo element is both an electro-mechanical resonant device (crystal) and the speaker. The amplifier can be a single transistor (lowest cost) or an inverter (CD40106, etc.). The chip is a part custom designed as a smoke detector controller. 99% chance that the amplifier for the piezo beeper is on the chip.

Here are examples of both circuits.

ak