Photodiode Alarm

This photodiode-based alarm system is designed to provide a warning signal when an individual passes through a designated protected area. The circuit remains in standby mode by utilizing a laser beam or infrared (IR) beam directed onto the photodiode. When the beam is interrupted, the alarm is activated. The circuit employs a PN photodiode operated in reverse bias mode to detect variations in light intensity. In the presence of laser or IR rays, the photodiode conducts electricity and supplies base bias to transistor T1. As a result, the NPN transistor T1 turns on, grounding the reset pin (pin 4) of integrated circuit IC1. IC1 is configured as an astable oscillator using resistors R3, variable resistor VR1, and capacitor C3. The astable oscillator functions only when its reset pin receives a high signal. Upon interruption of the laser or IR beam, the current through the photodiode stops, causing T1 to switch off. Consequently, the collector voltage of T1 rises, enabling IC1. The output pulses generated by IC1 drive the speaker, producing an alarm tone. Additionally, a straightforward IR transmitter circuit is provided, which emits continuous IR rays. This transmitter can project IR rays up to 5 meters, especially when the IR LEDs are housed in black tubes.

The photodiode-based alarm system operates through a sequence of events triggered by the interruption of a light beam. The photodiode, when exposed to the laser or IR light, allows current to flow, which is essential for the operation of the entire circuit. The reverse bias configuration of the photodiode enhances its sensitivity to light changes, making it effective for detecting beam interruptions.

Transistor T1 plays a crucial role in the circuit. When the photodiode conducts, T1 is activated, pulling the reset pin of IC1 low, which inhibits the oscillation. The astable oscillator, configured within IC1, generates a square wave output when enabled. This output is responsible for driving the alarm speaker, producing audible signals that alert individuals to the breach of the protected area.

The design includes additional components such as resistors and capacitors that determine the frequency and duty cycle of the output pulses from the astable multivibrator. The use of variable resistor VR1 allows for fine-tuning of the oscillation frequency, enabling customization of the alarm tone.

For the IR transmitter circuit, the inclusion of black tubes around the IR LEDs serves to focus the emitted light, enhancing the effective range of the transmitter. This design consideration ensures that the IR rays can reach distances of up to 5 meters, providing flexibility in the placement of the alarm system and the protected area.

Overall, this photodiode-based alarm system is a practical solution for security applications, leveraging simple electronic components to create an effective and reliable detection mechanism.This Photodiode based Alarm can be used to give a warning alarm when someone passes through a protected area. The circuit is kept standby through a laser beam or IR beam focused on to the Photodiode. When the beam path breaks, alarm will be triggered. The circuit uses a PN Photodiode in the reverse bias mode to detect light intensity. In the prese nce of Laser / IR rays, the Photodiode conducts and provides base bias to T1. The NPN transistor T1 conducts and takes the reset pin 4 of IC1 to ground potential. IC1 is wired as an Astable oscillator using the components R3, VR1 and C3. The Astable operates only when its resent pin becomes high. When the Laser / IR beam breaks, current thorough the Photodiode ceases and T1 turns off. The collector voltage of T1 then goes high and enables IC1. The output pulses from IC1 drives the speaker and alarm tone will be generated. A simple IR transmitter circuit is given which uses Continuous IR rays. The transmitter can emit IR rays up to 5 meters if the IR LEDs are enclosed in black tubes. 🔗 External reference