Volume of recycled burglar alarm piezo sirens

Thread Starter

Russell

Joined Sep 20, 2006
9
I've been trying to use some piezo sounders salvaged from a couple of burglar alarm exterior sounders to make a horn that uses less power than a car horn for starting sailing races. I have salvaged one sounder with associated auto transformer and darlington transistor from one alarm and two sounders with one auto transformer and a mosfet from the other.

I have built a rectangular wave 555 circuit with variable frequency in the audio range and duty cycle variable from about 30 to 70 per cent. I'm running the 555 on 12V. I've got this set up on a breadboard using the single piezo sounder and its associated auto transformer and I can set the frequency and duty cycle to an optimum. However I am still disappointed with the volume. I would expect it to be uncomfortably loud when it's sitting on the bench in front of me and it's not.

I've been wondering whether the answer is just to increase the voltage - I know burglar alarms normally work on 12V - could they be converting to 12V AC which would be equivalent to a 24V square wave from the 555? Should I use all three piezo sounders in parallel? In which case would I need the second auto transformer?

I'd be grateful for any thoughts or advice.

Russell
 

MisterBill2

Joined Jan 23, 2018
6,008
A sinifigant portion of the volume is provided by the enclosure providing better coupling to the air. So that may be part of the problem. In addition, the sounders are very resonant and so they must be driven at their resonant frequency. And the duty cycle may not be right.
 

Dodgydave

Joined Jun 22, 2012
9,153
Most burglar alarm bell boxes use a Bridge Output for the sounder, where the speaker is across two outputs in anti phase to each other, this gives nearly twice the sound volume, also they are tuned around 3.5Khz sweeping frequency.
 

MisterBill2

Joined Jan 23, 2018
6,008
Most burglar alarm bell boxes use a Bridge Output for the sounder, where the speaker is across two outputs in anti phase to each other, this gives nearly twice the sound volume, also they are tuned around 3.5Khz sweeping frequency.
Since there were transformers in the original application it makes sense to understand the original circuit as a source for the new circuit. Quite likely the drive signal was not a square wave, and quite possibly the frequency may be a bit different.
And it may well be that the breadboard arrangement has resistances that cause unintended voltage drops. It might also be that the transformer connections are different. Piezo sounders are very much voltage driven devices and so the drive voltage matters.
 

Audioguru again

Joined Oct 21, 2019
1,458
A piezo sounder is loud only at a couple of frequencies. Vary the frequency to select the loudest sound.
Are you old with a normal high frequency hearing loss like me? I can hear a piezo beeper only if my hearing aids are turned on.
Maybe you need a real car or truck horn.
 

Thread Starter

Russell

Joined Sep 20, 2006
9
Thank you for the suggestions. You have given me some food for thought. Resonance is one of the issues I was thinking about which is why I tried varying the frequency and duty cycle.

The suggestion that burglar alarms are normally around 3.5kHz is interesting. I think that's above the range I was trying but I definitely had a peak for both frequency and duty cycle - but I'm now wondering about harmonics.

I was wondering a bit about a bridge configuration - but I don't think that the original alarm boxes couldn't have been configured like that as I think they would have needed more than one transistor to drive them. I couldn't see anything that looked as though it was increasing the voltage either.

My hearing may be starting to degenerate - but eyesight is a far worse problem - especially trying to read component values.

I apologise for the lack of a schematic - I have one somewhere - a scrappy bit of paper with scrawled component values that I can't find right now.

I appreciate the suggestions for other sounders. I didn't want to make my original question very long but I'll explain a little more. At present (when we're not in lockdown) the starter system has a car air horn which is very loud but draws about 10A. The starting sequence is controlled automatically from a PIC so the horn has to be controlled electrically. The system is powered from a car battery which has a solar panel and only needs occasional charging. In theory a computer power supply would provide enough current but the surge makes it cut out.

I would like to dispense with the car battery and thought it worth experimenting with the piezo sounders. Maybe I should be looking for a better power supply.

Russell
 

MisterBill2

Joined Jan 23, 2018
6,008
Thank you for the suggestions. You have given me some food for thought. Resonance is one of the issues I was thinking about which is why I tried varying the frequency and duty cycle.

The suggestion that burglar alarms are normally around 3.5kHz is interesting. I think that's above the range I was trying but I definitely had a peak for both frequency and duty cycle - but I'm now wondering about harmonics.

I was wondering a bit about a bridge configuration - but I don't think that the original alarm boxes couldn't have been configured like that as I think they would have needed more than one transistor to drive them. I couldn't see anything that looked as though it was increasing the voltage either.

My hearing may be starting to degenerate - but eyesight is a far worse problem - especially trying to read component values.

I apologise for the lack of a schematic - I have one somewhere - a scrappy bit of paper with scrawled component values that I can't find right now.

I appreciate the suggestions for other sounders. I didn't want to make my original question very long but I'll explain a little more. At present (when we're not in lockdown) the starter system has a car air horn which is very loud but draws about 10A. The starting sequence is controlled automatically from a PIC so the horn has to be controlled electrically. The system is powered from a car battery which has a solar panel and only needs occasional charging. In theory a computer power supply would provide enough current but the surge makes it cut out.

I would like to dispense with the car battery and thought it worth experimenting with the piezo sounders. Maybe I should be looking for a better power supply.

Russell
One battery option is one of those 12 volt 7 amp-hour gell cells. One project that I inherited used a string of them to provide 100 amps for almost a minute.So ten amps for a minute could easily be provided, although 10 amps for 15 seconds should be adequate. And if the compressor could pump up a pressure tankthat used an adequate solenoid valve to sound the air horn then probably even louder noise could be obtained. Depending on how many operations were required even a 2-gallon air tank could deliver a whole lot of noise, since it can be charged to a much higer pressure that the pump can provide. Some of those horns can work with 100PSI or more.
 
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