# How to build howling WWII-type Siren - voltage regulation

#### hobbybuilder

Joined Mar 24, 2007
16
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

Joined Jan 28, 2005
9,029
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

Joined Jan 5, 2007
12
do u have a sound clip of it? im interested in hearing it. where did you get it?

#### mrmeval

Joined Jun 30, 2006
833
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

Joined Mar 24, 2007
16
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

Joined Mar 24, 2007
16
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:

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

Joined Mar 24, 2007
16
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.

#### hgmjr

Joined Jan 28, 2005
9,029
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

Joined Apr 20, 2004
15,819
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.