SIMPLE AUDIO ALARM CIRCUIT USING BC547 AND S8550

SIMPLE AUDIO ALARM CIRCUIT USING BC547 AND S8550

Parts Required:

Capacitors

C1 – 0.02uF ceramic disc

C2 – 1000uF, 12V or 25V

Resistors (All 1/4Watts)

R1 – 100K

R2 – 2.2K

R3 – 47E

Transistors

T1 – BC547 (NPN)

T2 – S8550 (PNP)

Loudspeaker – 8 ohms

Battery – 9V

This is a simple audio alarm which operates from 3 to 9 volts and consumes less power. Transistors T1 and T2 form a direct coupled complimentary amplifier which oscillates because of positive feedback via capacitor C1. The frequency of the oscillator is given by F = 1/R1C1

Where F is in Hz, R1 is in megaohms, C1 is in microfarads.

Resistor R3 increases the effective loudness by increasing the duty cycle of the output oulses. The current consumption is about 250mA at 6 volts. The loudness increases with the applied voltage. Capacitor C2 helps in maintaining the loudness with a weak battery.

Care should be taken to observe the correct polarity.