Function of an electronic circuit

Thread Starter

johnbaker1

Joined Jan 31, 2021
2
Hello, I need to understand the exact working of an electronic circuit (see picture) but I am very bad at electronic. This is the electronic circuit of a personal alarm that triggers an alarm when you pull out a metal part (like a grenade). All I know is that when you remove the metal part (that looks like a piston) from , the metal disk on the left (which is a piezoelectric disc) produces a loud sound. I would like to know what exactly happens. What is the part that transforms the electricity from the battery to an "alternative" current that makes the piezoelectric vibrate ? Thanks a lot to anyone that could help me.
thumbnail_IMG_0427.jpg
 

wayneh

Joined Sep 9, 2010
17,152
What is the part that transforms the electricity from the battery to an "alternative" current that makes the piezoelectric vibrate ? Thanks a lot to anyone that could help
That would be an oscillator. There are many ways to accomplish it but they all provide the same basic result, a voltage that alternates hi to lo over and over again. In this case it oscillates at an audible frequency, probably 500-1,000Hz or so, where human hearing is very sensitive.

From the picture alone it's hard to tell where on the board the signal is coming from. My guess would be the 8-pin IC near where the piston goes. Just a guess from a quick look.
 

Thread Starter

johnbaker1

Joined Jan 31, 2021
2
That would be an oscillator. There are many ways to accomplish it but they all provide the same basic result, a voltage that alternates hi to lo over and over again. In this case it oscillates at an audible frequency, probably 500-1,000Hz or so, where human hearing is very sensitive.

From the picture alone it's hard to tell where on the board the signal is coming from. My guess would be the 8-pin IC near where the piston goes. Just a guess from a quick look.
Ok , thank you very much. Do you know the fonction of the black cylinder at the left of the piston.
 

GeBJT

Joined Nov 3, 2020
19
Do you know the fonction of the black cylinder at the left of the piston.
My guess is a transformer. The slight ridges at top and bottom of the black cylinder indicate a tiny, dumbell-shaped ferrite bobbin. There look to be 3 terminals (bright wires) at the bottom, so the transformer schematic might be like this:

3-terminals.png

Note the positions of the phase dots, suitable for a simple transistor driven blocking oscillator. I can see Q1 and Q3 on the pcb, but not a Q2. Perhaps one of those transistors powers the DC to AC conversion you asked about in post#1.

In a similar component I salvaged from a broken personal alarm, the winding wire is so fine that I can see it only with a magnifying glass. However, that further suggests that the windings are the high inductance needed for an audio frequency blocking oscillator.

In order to get maximum intensity of ear-splitting output, three aspects of the design have to be aligned: the piezo's resonant frequency; the oscillator frequency; the acoustics of the piezo's mechanical mounting.
 

Needhelp1232

Joined Jan 14, 2021
12
My guess is a transformer. The slight ridges at top and bottom of the black cylinder indicate a tiny, dumbell-shaped ferrite bobbin. There look to be 3 terminals (bright wires) at the bottom, so the transformer schematic might be like this:

View attachment 229245

Note the positions of the phase dots, suitable for a simple transistor driven blocking oscillator. I can see Q1 and Q3 on the pcb, but not a Q2. Perhaps one of those transistors powers the DC to AC conversion you asked about in post#1.

In a similar component I salvaged from a broken personal alarm, the winding wire is so fine that I can see it only with a magnifying glass. However, that further suggests that the windings are the high inductance needed for an audio frequency blocking oscillator.

In order to get maximum intensity of ear-splitting output, three aspects of the design have to be aligned: the piezo's resonant frequency; the oscillator frequency; the acoustics of the piezo's mechanical mounting.
Ok, thank you very much for the help !
 
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