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?