Police Siren

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Check out the Rumbler/Howler Add-on here.

I finally got around to building a 100W Class-D amplifier. Check it out here.

The last couple of weeks I have been going through my old Elektor magazines. Feeling a little nostalgic I stumbled upon some old 555 police siren circuits. As far as I could remember none of these siren circuits sounded like a real police siren. That got me started looking for some real police siren circuit designs. In this area some of the big companies are Whelen Engineering and Federal Signal Corp.

On YouTube I found some videos that showed some of the products and how they sound...and on Federal Signals page I found manuals for their older products which included schematics. So I spend some time studying the professional design approach and found that they did in fact use the 555 timer but also a 4046 CMOS VCO. I decided to focus on the Model PA300 Electronic Siren from Federal Signal Corp.

Model PA300 Electronic Siren - Installation, Service, and Operating Instructions.

I only built a small part of the complete schematic. I was only interested in finding out how they made their siren. So I made the VCO section and the Rate Oscillator and just for fun the Air Horn circuit.

Refering to the picture below:

1: Air Horn circuit
2: Rate Oscillator circuit
3: Coast-down cutoff comparator circuit
4: Voltage Controlled Oscillator circuit

1: Air Horn circuit

This LM555 (or LM556 which is just a dual 555 chip) generates the frequency that is used as an air horn. The voltage from this circuit is taken directly from the Trigger/Threshold point in the circuit. The voltage is sent to the 4046 VCO which then produces the audio output. First picture is the Air Horn circuit and the other is a plot of the Air Horn frequency.

2: Rate Oscillator circuit

The rate at which the VCO oscillates is controlled by this part of the circuit. This is essentially the part the makes the Wail, Yelp, Phaser (Piercer) sound or the European Hi/Lo sound. By switching in and out different resistor values the rate is changed and thereby the output sound of the siren. First picture is the schematic of the rate VCO and the other is a scope plot of the Wail and Yelp tones.

3: Coast-down cutoff comparator circuit

Frequencies below 350Hz can damage the amplifier part of the PA-300 siren. This circuit makes sure that the 4046 oscillator turns off if the siren tone fall below 350Hz which is 3.2V. The output amplifier is an older design using audio transformers and the power transistors are not happy with lower frequencies. First picture is the Coast-down circiut in the upper part and the lower part is the VCO. The second picture shows the Yelp tone.

4: Voltage Controlled Oscillator circuit

According to the manual the siren operates between 725Hz and 1500Hz which seems to be the norm (most seem to operate between 500 and 1500Hz). The range is generated by the 4046's internal VCO together with C21, R46, R47 and R57. The 4046 is actually a complete PLL but only the VCO part is used. The VCO can be inhibited by pulling pin 5 high and this is what the Coast-down cutoff circuit does. First picture is the Wail tone and the second the Yelp tone.

Here is a small video of the project so far. In the video I have changed the Air Horn frequency and I have added a Phaser/Piercer sound. All the switches will eventually be replaced by some kind of single switch circuitry. This was all just to see if I could get the right police sound of a 555 timer based circuit.

Today (29th of August) I received a Whelen SA314 siren speaker that I got on eBay. Wow...this is a BIG speaker. I hooked it up and fired a modest 1W siren tone into the speaker and I most say that this is a potent "little" speaker. I had consired building a 100W amplifier but that's not gonna happen! I will probably make a MUCH smaller 10W amplifier for this siren. There is no need to attract the police. As a bonus I also got some small bug from Arizona. :)

Today (3rd of September) I got yet another Whelen speaker. This time it is a Whelen Howler speaker and again I got it from eBay. To make a Howler or a Rumbler (Federal Signal's version) it seems that only one part needs to be changed. Capacitor C21 that is connected to IC13 sets the frequency of the VCO. According to an article about the Rumbler siren it operates 2 octaves lower than the normal siren. So I added a 47nF over C21 and it is almost two octaves lower. It seems to be working fine as can be seen here.

Here is a video of the siren using the standard SA314 speaker from Whelen. I made a crude 60W amplifier so that I could the "test-feel" the Howler speaker...and it works just fine. The sound on my Canon IXUS 230 HS is ridiculous...sorry.

[Oct 22nd]

The PEAK tone in the PA300 siren does not have the characteristic sound we all know from the movies. So I thought it would be fun to make a circuit that could produce that sound. And I actually think it is pretty close...if not spot on. The normal PA300 siren PEAK tone has the coast-down of the tone until the comparator turns off the VCO. But this circuit starts the voltage at ca. 2.6V so that the sound comes immediately but pressing the button. The capacitor is discharge immediately when the button is released. This way you get the sound in the video found below.

And here is the finished PEAK circuit in action.

I have also started building my own siren or should I say a different version then the PA300. I have added and removed some things. This is what it looks like at the moment. The schematics will be added here soon...!

[Nov 23rd]

I just finsihed my prototype siren today. The results can be seen on here. The only thing I need to add is an amplifier. Since I already made a small 20W amplifier I will of course use that. I will also add a small power supply section with an LM317 so the 12V input is regulated down to 8V. After that I think I have played enough with sirens. ;)

Naahhhh...I will probably make a Rumbler / Howler add-on to this siren. And the schematics for all this will be available soon.

[Nov 24th]

CTRL_SCH Description:

The control circuit is based on the CD4013 D-Type Flip Flop. Instead of using a rotary switch I thought it would look better using push-buttons. When power is applied the RC network R19 and C4 keeps the 3 Flip Flops reset so that the WAIL, YELP and PHASER outputs are low (not siren output). To activate a siren tone one just pushes the desired button. This will both activate the tone and reset the 2 other Flip Flops so that it is only the desired siren tone coming from the speaker. The Q_NOT output of the Flip Flops are all connected to a MOSFET which drives an indicator LED.


SIREN_SCH Description:

The siren is based on the CD4046 voltage controlled oscillator which is driven by a voltage from either the WAIL/YELP/PHASER part or the PEAK and AIR HORN parts of the circuit. U4a and U4b controls which siren tone is outputted. If no tone is selected the output is silent because U1 is held in the reset position. U4a and U4b are connected so that U1's output is sent through the switches which mean the WAIL, YELP and PHASER tones will be passed to the VCO. If the AIR HORN button is pushed U4b switches the AIR HORN tone through to the VCO. And when the PEAK button is pushed U4a opens for the PEAK tone which is send to the VCO.

SW1 reduces the output to the power amplifier which is practical for testing.

The WAIL/YELP/PHASER selection circuit has also been changed from the original PA-300 schematic. Federal Signal used a CD4066 switch but I have had some trouble with the ON-resistance of these switches. Maybe it was a bad 4066 but it changed its resistance significantly so that the tones did not sound right. I decided to use relay's instead.

I don't think the AIR HORN circuit is the best. I had some problems with the sound it produces and currently I'm thinking about a redesign of that part. But the PEAK circuit is a great addition to the this siren. The original PA-300 peak circuit does not have the cool sound from the movies...but this circuit does. :)


R17 needs adjusting before the siren sounds right. Connect an oscilloscope to the output and adjust R17 until the output frequency is 1550Hz (this is from the PA-300 siren manual).


Audio Amplifier

The audio amplifier is a TDA2009A from ST and the specific circuit was found in the datasheet. It is an easy amplifier and the output is pretty loud (20W). On the Siren schematic I added switch SW1 which reduces the output power.


...more amplifier stuff

Oh how this amplifier has annoyed me. I want more power but there is only 12VDC available in a car. The TDA2009 gets very hot and yes maybe I should have used a bigger heat sink. But still it gets so hot that I think something may be wrong with the chip. So I decided to take a different approach. Somewhere on the internet I found a guy that used a MOSFET driver as a power amplifier.

This actually works pretty good. And since the output of the siren is a square wave this kind of amplifier is perfect for the job. It is near to perfect efficiency since the MOSFETs are either on or off. With a 12VDC power supply I get about 4.5W output power to the SA-314 speaker...which is fine for my basement projects.