Two transistor siren

Posted on Feb 4, 2014

Here is the circuit diagram of a simple two transistor alarm circuit that can be operated from a 9V PP3 battery. Here the two transistors are wired to form an oscillator whose frequency increases when switch S2 is pressed and decreases when S2 is released. In order to attain this the base of Q1 is biased from an RC circuit comprising of R2 and C1.

Two transistor siren
Click here to download the full size of the above Circuit.

When S2 is pressed C1 is charged through the resistor R2. As the voltage across the C1 increases the time constant decreases and this results in an increases in the frequency. When S2 is released capacitor discharges and the frequency of the tone decreases. The sound heard from the speaker will be almost like that of a siren.

Leave Comment

characters left:

Related Circuits

  • New Circuits



    Popular Circuits

    16 watt amplifier
    Locked Loop Synchronous Demodulator
    5V 0.5A power supply
    Wireless Ir Security System
    Simple digital frequency meter
    Solar Charger for 6V 4.5Ah Battery
    Basics and implementation of capacitive proximity sensing
    Fog Lamp Sensor Circuit
    Parallel crystal oscillator