The circuit diagram given is Scream generator circuit. This is ideal for alarm or car horn. The core part of this circuit consists of two transistors Q1 (BC 107) and Q2 (BC 177). A switch S2 is used in this circuit as siren controller. When switch S2 is pressed, frequency of the siren starts to increase and the reverse will happen when the switch is released. That is the frequency of siren depends on the pressing and releasing of switch S2. If S2 is pressed before completely releasing it, then also frequency increases. Frequency decreases in the reverse case.
- Use a good quality PCB to assemble this circuit.
- By giving different values for C2, tone quality can be adjusted.
- The switch S1 is used as power switch.
- Make sure that transistors are not leaky. If transistor is leaky then alarm oscillates before S2 is pressed.