Transistor Siren Circuit – B1P18

Link to the PCB version of this kit.

We will now learn how to build a two transistor siren that is manually operated with a switch. First, let’s try to understand the basics behind a speaker.


A speaker converts an electrical signal to sound by using a magnet. We already know that when a varying current flow through a coil, it generates a magnetic field. A speaker uses this principle to generate sound.


A speaker has two terminals to which two wires are connected and wound to form a coil – as seen in the centre of the speaker. One end of this coil is connected to a very light material which is called a diaphragm. The other end is suspended into a magnetic field that is created by a permanent magnet. When a varying electric current flows through the coil, it generates its own magnetic field which interacts with the magnetic field produced by the permanent magnet and this causes the diaphragm to move according to the current – producing sound waves. The speaker coil contained in the kit has a low resistance value of about 8 Ohms.

Transistor Siren - Schematic

This is what the circuit looks like and it is made up of two main sections – an RC charging and an oscillator section. Let’s simulate the oscillator section.

Transistor Siren - Simulation

If we apply a voltage at R2, it will cause C2 to start charging and as this voltage builds up, it will cause Q1 to switch ON. When Q1 switches ON, it will cause Q2 to switch ON as well. This will cause the voltage across C2 to reverse its polarity, switching Q1 OFF, which switches Q2 OFF and the cycle repeats when a positive voltage is applied across R2 again.

Transistor Siren - Simulation

The positive voltage applied will determine the rate of charging or oscillation. A higher voltage will increase the oscillation frequency. We’ve applied a constant voltage at R2 which gives us a steady oscillation frequency but a siren has a varying frequency which starts low and increases with time. In order to create this effect, we add the RC charging circuit that is made up of R1 and C1.

Transistor Siren - Simulation

When the switch is pressed, C1 starts charging from 0 and this causes an increase in the oscillator frequency giving us the rise effect for the siren. The voltage reaches its peak when the capacitor is fully charged and when the switch is released, it starts discharging giving us the fall effect for the siren.

Transistor Siren - Layout

Let’s use the breadboard layout to build and test the circuit. This is a very nice project for beginners and if you’ve received the Kickstarter edition of BBox 1, then you will have also received a PCB version of this that you can solder together for something more permanent.

Transistor Siren - Demo

Let’s move on to the next project.