This optical burglar alarm uses two 555 timer ICs (IC1 and IC2). Both the ICs are wired as astable multivibrators. The first astable multivibrator built around IC1 produces low frequencies, while the second astable multivibrator built around IC2 produces audio frequencies. General-purpose Darlington photo-transistor T1 is used as the light sensor. To increase the sensitivity of the circuit, NPN transistor T2 is used.
Place phototransistor T1 where light falls on it continuously. Phototransistor T1 receives light to provide base voltage to transistor T2. As a result, transistor T2 conduct to keep reset pin 4 of IC1 at low level. This disables the first multivibrator (IC1) and hence the second multivibrator (IC2) also remains reset so the alarm (LS1) does not sound.
Circuit Diagram
When light falling on Darlington phototransistor T1 is obstructed, transistor T2 stops conducting and reset pin 4 of IC1 goes high. This enables the first multivibrator (IC1) and hence also the second multivibrator (IC2). As a result, a beep tone is heard from speaker LS1. The beep rate can be varied by using preset VR1, while the output frequency of IC2 can be varied by using another preset VR2.
This circuit works off a simple 6V-12V DC power supply.
Parts List
- Resistors (all ¼-watt, ± 5% Carbon unless stated otherwise)
- R1, R5 = 1 KΩ
- R2 = 100 KΩ
- R3 = 4.7 KΩ
- R4 = 10 KΩ
- VR1 = 1 MΩ
- VR2 = 100 KΩ
- Capacitors
- C1 = 1 µF/16V
- C2 = 0.01 µF
- C3 = 0.047 µF
- C4 = 0.01 µF
- C5 = 47 µF/25V
- Semiconductors
- IC1, IC2 = NE555
- T1 = 2N5777 Photo Transistor
- T2 = BC547
- LED1 = RED LED
- Miscellaneous
- LS1 = 8Ω / 0.5W
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