Ambulance Siren

This article is about an ambulance siren made from transistors.

You can see the circuit working in this video:

This is the power bank that I used (6 V AA battery holder):

Figure 2: 6 V battery holder.

Step 1: Design the Circuits

I have drawn the circuit in PSpice simulation software:

Figure 3: Oscillator Circuit 1.

You can also implement this circuit if you do not have enough NPN transistors:

Figure 4: Oscillator Circuit 2.

C1 is the frequency sweep control capacitor. You can select C1 of 100 uF. I used C1 of 470 uF.

The power amplifier is not linear because it is not used for audio (it would NOT work for audio signals) but square wave only.

Figure 5: Power Amplifier.

Step 2: Simulations

Simulations show that the two oscillator circuits are similar:

Figure 6: Oscillator Circuit 1 Simulations.

Figure 7: Oscillator Circuit 2 Simulations.

You can see the maximum current output of the power amplifier:

Figure 8: Power Amplifier Simulations.

Step 3: Make the Circuits

The three LEDs are connected to the power supply in series with a 100-ohm resistor.

Figure 9: Make the Oscillator.

The two black 100 uF electrolytic capacitors are bipolar.

I used high power resistors just for the image:

Figure 10: Make the Power Amplifier.

Step 4: Testing

Testing showed that the circuit is working:

Figure 11: Testing.

I used InstruStar USB Oscilloscope to acquire generated siren signals.

Figure 12: Testing Minimum Frequency.

Figure 13: Testing Maximum Frequency.

Video C1 100 uF:

Video C1 470 uF, number 1:

Video C1 470 uF, number 2:

Step 5: Assembly

Connect two circuits together with metal rods:

Figure 14: Assembly.

Conclusion

Let's just hope that you (the reader of this article) will not need to hear this sound.