Slot Car Police Siren

Thread Starter

Mark 42

Joined Jan 22, 2013
I'm planning to add a siren to a 1/32 slot car.
It runs on up to 14 volts (depending on throttle level)

I was thinking if the siren could just rise and fall as the voltage to the car
fluctuates, that would be fairly simple.

I found a circuit to use:

And I'm wondering if it will work to just short S1 (the push button switch),
or do I need a transistor circuit that senses when voltage is applied and emulaties a finger pushing the momentary switch?

If so, how would the additional schematic elements look?

I do intend to just breadboard the circuit and see if shorting S1 and fluctuating the supply voltage give a result close to what I am looking for... but my breadboard is in storage at our old house, and we won't be going there for a week or two (until the current tenant moves out and we are there to prepare for the new renter).

So, I figured I'd ask, and see what I might expect &/or how to build a "pushbutton emulator".

I'd like to keep it small and light - the car has limited space in it.



Joined Nov 23, 2012
You will not be able to get everything you need onto a pcb that will fit into a 1/32 car. You would be better off putting an LED on the car and put the siren circuit next to the track. Some LEDs are available with a flashing circuit built in so it will take up almost no space.

You can use one circuit as shown in your link (it simply makes one tone when button is not pressed and another tone when pressed. You can google "555 timer" to make a self-toggling switch. Google "555 timer calculator" to find the right resistor and capacitor sizes to make the switch speed correct for the tone changes you want.

Thread Starter

Mark 42

Joined Jan 22, 2013
I bought some LEDs on eBay that flash red/blue.

I put 4 of them on the roof of a 1/24 NASCAR slot car... they flash at random,
which was fine for what I wanted (they came on only when the car was getting
power, so it was very random indeed).

Everyone at the track really was impressed with it.

Now I'm making my wife a Womp Womp / Thumper / Legends car.
It's 1/32 scale, but there is more space than most 1/32 cars
It's the '34 Ford 4-door body, so it's got more volume than most.

I'm sure I can fit the circuit at the link in the first post in it (I'll
make the PCB small by scaling it down to barely fit the components in)

I had considered putting the siren at the controller, but that would
not be very impressive (the guys at the track are not easily impressed)

Here's a picture of what the body is shaped like:

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Thread Starter

Mark 42

Joined Jan 22, 2013
I was thinking I could use a transistor somehow to emulate a relay
to use as a substitute for the push button switch. Something so that
the switch would close at about 5 volts, stay closed after that
(up to about 14 volts), and then go open when it drops below 5 volts.

But the 5V threshold may need to be adjusted to get the right effect.


Joined Feb 19, 2009
You may find a piezo buzzer with the "siren effect" already programmed into it, it's just a matter of hooking it up.

I'm not sure how large that one is, but they get small, but not down to PCB piezo size (<1cm³).

A low current one could have a 5V or 12V Zener diode in the power path so it would only turn on at > 5/12V, assuming track is DC and not AC.