Reverse Engineering an Alarm Siren Module

RE: Reverse Engineering an Alarm Siren Module

kaan3553 — Sun, 25 May 2025 17:36:02 GMT

Merhaba arkadaşlar.. Başarılı çalışmanız için tebrikler.. Ben de bunu yapmak istiyorum ama d2 diyot elementinin değeri veya adı nedir? Şemalarda belirtilmemiş.. Bana yardımcı olabilirseniz çok mutlu olurum.. Teşekkür ederim.

RE: Reverse Engineering an Alarm Siren Module

D_Hersey — Sat, 24 Jan 2015 01:06:04 GMT

The reason they are not using the 555 is because they seek sinuate waveforms. There are two oscillators here. The first one seems to be a lower frequency (bigger timing components) intended to warble the second. The last two Qs are a PA. The four Qs to the left are in common-emmiter mode.

Thus they are inverters and introduce a 180-degree phase-shift. The gain of the Q's and the selectivity of the timing components allow us to meet the Barkhausen criteria at a rather specific frequency thus giving us a rather sinuate output.

Barkhausen stability criterion - Wikipedia, the free encyclopedia

I will sim this this weekend so we can look at waveforms. John, you can learn LTSpice in a coupla days and have great fun while doing so. I know I can be a scold, but this would be a wonderful conflux of your interests and abilities. Please give it a try, its easy.

My father was an elementary school science teacher at the beginning of his career. He told me the hardest thing to convey to a person is that they can do it.

I don't know that I would call these circuits vibrators. Vibrators have something like a comparator which cause a state change in the controller. These are feedback oscillators, a different devil. Their characteristic output form is sinuate. A vibrator will have an exponential ramp for a waveform, typically. One controller is a continuous fn, the other discontinuous.

f(x) - f''(x) = 0. For all oscillators.

Feedback oscillators, relaxation oscillators:

Electronic oscillator - Wikipedia, the free encyclopedia

RE: Reverse Engineering an Alarm Siren Module

shabaz — Sun, 18 Jan 2015 16:45:26 GMT

Hi John,

Thanks for the great blog post : )

This is nice, and also probably goes to show that when cost-saving, transistors may end up cheaper than a 555 circuit by a few pence/cents, and hence make more economical sense for a mass-produced device like this.

I'm guessing the siren makes a changing tone? From your excellent very clear diagram it looks like two BJT based astable multivibrators, where the first one repeatedly influences the second one which produces a tone. Very cool,

to see how they coupled the two multivibrators - I'm going to bookmark this : )

RE: Reverse Engineering an Alarm Siren Module

amgalbu — Sun, 18 Jan 2015 06:31:11 GMT

I am not an expert in reverse engineering so forgive me if this is a stupid question. How do you measure resistors and capacitors while they are mounted on the pcb? Aren't all connected components going to interfere with the measure?

RE: Reverse Engineering an Alarm Siren Module

DAB — Sat, 17 Jan 2015 19:48:54 GMT

Hi John,

Nice post.

I have done more of these reverse engineering tasks than I care to remember.

I agree that it is an excellent practice to learn. There are a lot of secrets hidden in some circuits and you are always rewarded by figuring them out.

One of my dream projects is to have and system that you can just take front and back pictures of the board in question and have a circuit diagram drawn automatically.

It would be a great task for a collaborative project.

DAB

RE: Reverse Engineering an Alarm Siren Module

clem57 — Sat, 17 Jan 2015 17:31:27 GMT

@John Wiltrout

Did you know what this board does? This does not seem too obvious to me

Cheers,

Clem

RE: Reverse Engineering an Alarm Siren Module

D_Hersey — Sat, 17 Jan 2015 17:00:46 GMT

Also, I am a little confused by your documentation of the transformer. Are you describing the turns ratio, or the square of the turns ratio, the impedance ratio?

And, BTW, could you model the electrical characteristics of the load as well?

RE: Reverse Engineering an Alarm Siren Module

D_Hersey — Sat, 17 Jan 2015 16:49:41 GMT

You should model the circuit in LTSpice, then we could have more insight into the oscillator. Good work, John!

RE: Reverse Engineering an Alarm Siren Module

mcb1 — Sat, 17 Jan 2015 10:28:34 GMT

Done plenty of those.

Sometimes it's just the area of the board I need to fix, and sometimes it is the whole thing ... because I can/want/should.

Often the three legged devices are a mystery, so an educated guess or a circle symbol is enough, until you can identify it.

Thanks for sharing.

Mark