Sound Operated Light and Alarm

Various types of security based projects are already available in Now here is sound operated light and alarm which is used as intruder alarm, which activated when any sound is detected (like opening of doors, knocking or door opening) and starts flashing with sound which alert you of any intruder. After second sound pulse it turned off.

Circuit Description of Sound Operated Light and Alarm

The circuit of sound operated light and alarm is designed using a pair of timer IC (NE555), a dual JK flip flop IC (CD 4027) and a op-amp IC (CA3140).

The input from the AC mains is given to the primary side of transformer X1 for stepped down. The stepped down voltage is further rectified and filtered by diode D1 and capacitor C1 respectively. The reference voltage is given to the non-inverting input (pin 3) of op-amp IC (IC1) and is set by variable resistor VR1.


Microphone (MIC1) is used as sensor which detects sound and converts it into electrical signal which is given to inverting input (pin 2). The voltage appeared at pin 2 is equal to the voltage across condenser microphone, which is set for high sensitivity of the sound. The reference voltage is kept high which means a small sound is enough to change the output of IC1. In order to keep output voltage unchanged for any false triggering the reference voltage is kept fix.

At initial when no sound is detected by microphone, the voltage appeared t inverting input is equal to the DC voltage from power supply and hence no output is appeared at output of IC1 (pin 6). Since IC2 (CD4027) is dual JK flip-flop and is wired in toggle mode thus no output is obtained at output of IC2 (pin 15). The low output at pin 15 of IC2 make reset pin 4 of IC3 low to reset the IC3 (NE555) which is wired in astable multivibrator mode. As a result the output of IC3 become low which drive transistor T1 in cut-off and keep relay RL1 remain de-energized. The both N/O (N/O1 and N/02) contacts remains open. The N/O2 contracts keep bulb B1 turned off, where N/O2 contact keep loudspeaker circuit open and hence no sound is appeared at loudspeaker.

When any sound is detected by microphone it reduces the inverting voltage because some voltage is dropped by microphone itself. Hence output of IC1 is high, which trigger IC2 and make output high (IC2). As output of IC2 is connected to reset pin of IC3 which enable astable multivibrator for 1.5 seconds. This process repeats until pin 15 of IC2 remains high. The high output of IC3 drive transistor T1 conduction region and further relay is also energized. The energized relay light up the bulb B1 and at the same time audio tone is generated.

When another sound is detected by microphone, the output of IC2 goes low and reset astable multivibrator (IC3) which again put transistor in cut-off region and further relay is de-energized.


Resistor (all ¼-watt, ± 5% Carbon)
R1 = 33 KΩ

R2, R9 = 1 KΩ

R3 = 470 Ω

R4, R7, R8 = 10 KΩ

R5, R6 = 470 KΩ

VR1 = 1 KΩ

C1 = 1000 µF/25V

C2 = 4.7 µF/35V

C3, C5 = 0.01 µF

C4, C8 – C11 = 0.1 µF

C6 = 100 µF/25V

C7 = 10 µF/25V

IC1 = CA3140

IC2 = CD4027

IC3, IC4 = NE555

D1, D2 = 1N4001

T1 = BC548

RL1 = 12V, 200Ω, 2C/O Relay

LS1 = 8Ω/0.5W speaker

MIC1 = Condenser Microphone

X1 = 230V AC primary to 12V, 250 mA secondary transformer

B1 = 60W, 230V Bulb