How to build howling WWII-type Siren - voltage regulation

[hobbybuilder]

Hi everybody, I have bought a wonderful mechanical siren. This gives that lovely wailing sound. It draws 10 Amps at 12 volts. Now what I want to achieve, is I want the siren to start howling more slowly when the motor starts running. Then, on switching the siren off, I want it to keep howling down slowly. At present, switching the siren on or off happens within a second. So there is little actual howling. The howling sound is what I'd like to hear.

For clarification, the mechanical siren is driven by an electric motor. So I need to slowly raise the voltage for the motor from 0V to 12V at a current draw of 10 Amps. By slowly, I mean over the course of say 15 seconds. On switching off, I need the current to wind down over 15 seconds from 12V to 0V, but delivering 10 Amps all the way.

Being a novice, I bought a 47000 uF electrolytic capacitor and tried that. I now realise that I need far more current than a cap can ever deliver to drive down a 10Amp 12V motor.

For starters, I am looking to find a voltage regulator circuit, but one that can deliver 10 Amps from 12V down to 0V. Anyone got links where I can best start my search?

Then I need some sort of way to control the voltage descent. I suppose a variable resistor could do it, starting at 0 Ohms going up to a high resistance value, but where do I get a 120W (12V*10A) variable resistor? Also, I want the siren to howl by itself, i.e. not anyone turning a knob. I imagine an thyristor with a capacitor that slowly discharges could somehow create the variable resistance I require - but I have no idea regarding types or values.

So what simple circuit could I use to achieve this?

[hgmjr]

Pulse-width modulation should do the trick.

The information in the article will aquaint you with the concept. The drive in the circuit is not sufficient to power a 10 amp load but a mosfet could be found that would handle the load.

With such a solution, you would control the rate at which the siren's frequency output would ramp up or down by changing the duty-cycle of the PWM signal used to switch the driver.

hgmjr

[03GTS]

do u have a sound clip of it? im interested in hearing it. where did you get it?

[mrmeval]

You could find a three terminal adjustable regulator and couple that with a high power transister then design a control circuit that would allow it to work at the rated voltage but when you signal that it needs to shutdown or start up there is a gradual decrease or increase in voltage.

[hobbybuilder]

Originally Posted by 03GTS

do u have a sound clip of it? im interested in hearing it. where did you get it?

Thanks for your interest and sorry for taking so long to reply. I have yet to record my siren. I got it from a local supplier here, they probably sourced it from China but Ebay has many sorts of sirens. You wanted to hear a sound. There is lots on youtube, here an example sound: http://www.youtube.com/watch?v=bdQ8BxDZ_20

[hobbybuilder]

Originally Posted by hgmjr

Pulse-width modulation should do the trick.

The information in the article will aquaint you with the concept. The drive in the circuit is not sufficient to power a 10 amp load but a mosfet could be found that would handle the load.

With such a solution, you would control the rate at which the siren's frequency output would ramp up or down by changing the duty-cycle of the PWM signal used to switch the driver.

hgmjr

Thank you for your excellent 'Pulse Width Modulation' answer and sorry for getting back to you only now. I want to try the 555 circuit you referred me to. I'd like the circuit to make my siren sound something like this:

http://www.youtube.com/watch?v=bdQ8BxDZ_20

Perhaps I can put 4 circuits using the TIP31C in parallel, so to achieve 4*3A = 12A to drive my 12V 10A siren. I will then still need a method to drive the pot from minium to maximum. Sorry again for getting back on this thread after so many months, but I will now make time to get this thing working.

Thanks again for the excellent reference.

[hobbybuilder]

Originally Posted by mrmeval

You could find a three terminal adjustable regulator and couple that with a high power transister then design a control circuit that would allow it to work at the rated voltage but when you signal that it needs to shutdown or start up there is a gradual decrease or increase in voltage.

Thanks for your reply. Sorry for my long absence. Your reply is along the lines of what I was originally thinking. I am less concerned about getting the wailing effect for now, if I can just get the power transistor part to work I would be very happy. I will have a play with a 555 circuit and TIP31C to see how far I get, my siren draws 10A however. I now also see a Darlington pair speed control circuit which I will be looking into, which is what I think you may have meant.

Thanks again for your answer.

[hgmjr]

Originally Posted by hobbybuilder

Thank you for your excellent 'Pulse Width Modulation' answer and sorry for getting back to you only now. ......

You are certainly welcome.

Once you have a first cut at your circuit, post it here and our members will be more than happy to answer any question you may have.

hgmjr

[beenthere]

A real low-tech solution would be to add some extra mass to the rotating shaft. Don't know if there is a place to attach something like a barbell weight, but the added inertia would delay the build-up of the howl, and prolong the howl after power is removed.

Then all you might need is a relay controller that puts power on for X seconds and drops it for Y to get the up-and-down WWII howl.