Two-Tone Patrol Car Siren
Here is a loud siren which is so much like the real sirens on some police cars or ambulances that you will have to be careful you don't confuse people.
The initial tone is at a high pitch, but when the Key is closed the pitch decreases. You can control the cycling of the pitch the same as the police and ambulance drivers do. The circuit used is the basic pulse oscillator type. The Transistor is periodically turned ON and OFF to produce strong pulses of current in the Speaker. The Key is used to insert a Capacitor into the circuit to lengthen the pulse repetition rate and therefore lower the pitch of the tone.
This circuit contains the two basic requirements for an oscillator: output greater than the input, and feedback from output to input in such a manner (regenerative) that oscillations are sustained. Switching occurs in this oscillator as the Transformer is driven into saturation by the current which flows through the Transistor when it is ON. When saturation is reached, the Transistor is turned OFF until the magnetic field collapses to such a low level that the bias current through the 22K Resistor can turn it ON again.
Meanwhile the collapse and build up of magnetic flux induces a voltage across the secondary winding which produces the output power into the Speaker. The addition of the 0.02uF Capacitor slows down the switching action as its charge tends to both hold the Transistor ON and then hold it OFF.
Hear is a other version