How to change the output sound to like "Police car whistle"?

Thread Starter


Joined May 31, 2018
Here is the circuit diagram
when is use a lighter to burn the thermistor, it will generate a sound.
But it is not like the police car whistle
how to change to the high low sound?
Is it to change R5, R6, C2 to change its frequency?
below is the formula




Joined Sep 17, 2013
R5 and/or C2 could be changed to alter the generated tone. Increasing their value will lower the tone frequency and vice versa.
You would need a second 555-based circuit to control the present one if you wanted an automatic alternating 2-tone sound.


Joined Mar 14, 2008
As Alec stated, you could use another 555 astable operating at a low frequency to switch an additional capacitor in parallel with C2.
The added 555's operating frequency would be how rapidly you want to switch between frequencies (perhaps about 1Hz).

The circuit below will provide for switching between two capacitance values.
R6 and C2 are the present 555 resistor and capacitor.
C4 and M1 are added.
When M1's input is high, C4 is connected in parallel with C2, giving a lower siren output frequency.

M1 could also be an NPN BJT with an added series base resistor (10kΩ-100kΩ)

Last edited:

Daniel Sala

Joined May 28, 2015

The circuit/schematic you posted is a monotone beeeeeep fire alarm that is triggered by the thermistor, it's not a nee-naw circuit.

I seem to recollect having breadboarded this a few years ago and it worked: 555 police siren.


Joined Dec 20, 2007
The sound of a police siren depends on which continent it is on. In North America it is not simply two frequencies alternating, instead it is a varying frequency slowly up then slowly down and over and over. We laugh when we hear foreign bee, boo, bee, boo, bee, boo, etc in a movie.


Joined Mar 19, 2019
Yep feeding the 1st 555 into pin5 (variable control voltage) on the 2nd 555 and using potentiometers on the timing inputs lets you tweak the "NASTINESS" of the godawful output.

Oops.. And feed it into an LM386 amplifier chip to really get mean with it.

Daniel Sala

Joined May 28, 2015
Hi christw16,

Had a quick look for (American) wailing siren 555 circuits. You might want to take a peek at:

‘555’ ASTABLE CIRCUITS - figure 21 is an American wailing siren and figure 20 is a Birtish wailing siren. Nuts and Volts is a reputable online electronics magazine that's been going for years so you can certainly trust the circuits and articles there.

Police siren using NE555 - this one appears a lot so I assume it works, whatever siren sound it outputs...

HEE HAW SIREN Circuit - Another one from the 555-timer-circuits website.

The font has gone crazy and refuses to be non-bold arial, as you can see, oh well...



Joined Dec 20, 2007
In the 555 Astable Circuits, the British siren does not produce wailing like the American one, instead it warbles between two frequencies.
Circuits is in India so their Police Siren warbles the two frequency bee, boo, bee, boo sound.
The Hee Haw Siren also makes the bee, boo, bee, boo sound.


Joined Jul 29, 2018
Daniel Sala said:
The circuit/schematic you posted is a monotone beeeeeep fire alarm that is triggered by the thermistor, it's not a nee-naw circuit.
:) :) :) :)
That's the funniest thing I've read all day
It reminded me of the Minions........nee naw, nee naw, nee naw