This Simple Entrance Alarm Circuit designed to safeguard doors and entry passages.
It emits a loud alarm when someone passes through the invisible infrared barrier.
The system operates reliably during both day and night, minimizing false alarms.
The circuit is simple making it an ideal evening project for hobbyists.
Circuit Working:
Parts List:
| Category | Item | Quantity |
|---|---|---|
| Resistors | 100Ω | 2 |
| Preset 1k | 1 | |
| Semiconductors | Photo transistor L14F1 or similar type | 1 |
| Transistor BC547 | 1 | |
| LED Green 5mm 20mA | 1 | |
| LED Red 5mm 20mA | 1 | |
| IR transmitter LEDs | 3 | |
| Other | Buzzer | 1 |
The circuit employs infrared diodes to continuously emit infrared rays unlike pulsed emissions from remote handsets.
An NPN darlington phototransistor serves as the light sensor.
The L14F1 is an ultra sensitive darlington phototransistor with high gain.
Its collector connects to the positive rail through VR1 and the emitter features a green LED indicating standby mode.
The base of T1 is left unconnected with this junction exposed through T1s window to allow light photons to bias the base.
The level of biasing varies with the photon intensity.
T2 acts as the alarm driver, with its base connected to T1s collector, so T2s base bias depends on T1s conduction.
When infrared rays illuminate the phototransistor, it conducts pulling T1s base to ground and keeping it off.
In this state, the red LED and buzzer connected to T2s emitter remain off.
When someone crosses the IR beam, T1 turns off causing its collector voltage to rise.
This turn on of T2 activates the red LED and buzzer.
The circuit can be assembled on a small piece of PCB.
Place R1 and IR LEDs on a separate board, with IR LEDs on one side of the entry and the phototransistor on the opposite side.
Align the IR beam carefully on the phototransistor to ensure the buzzer remains silent in standby mode.
Wave your hand across the beam to test, the buzzer should sound.
VR1 adjusts T2s base bias to keep the buzzer off in standby mode.
Electrical characteristics of phototransistor L14F1:
| PARAMETER | TEST CONDITIONS | MIN | TYP | MAX | UNIT |
|---|---|---|---|---|---|
| Input offset voltage | VIC=0,RS=10 kΩ | 0.8 | 2 | mV | |
| Average temperature coefficient of input offset voltage | VIC=0,RS=10 kΩ | 10 | μV/°C | ||
| Input offset current | VIC=0 | 2 | 200 | pA | |
| Input bias current | VIC=0 | 50 | 200 | pA | |
| Common mode input voltage range | -11.5 | 11 | V | ||
| Maximum peak output voltage swing | RL=10 kΩ | ±12 | ±13.5 | V | |
| Large signal differential voltage | VO=±10 V,RL=2 kΩ | 25 | 200 | V/mV | |
| Input resistance | TJ=25°C | 1012 | Ω | ||
| CMRR | RS≤10 kΩ | 70 | 100 | dB | |
| Supply voltage rejection ratio | 70 | 100 | dB | ||
| Supply current | 2 | 3.4 | mA |
How to Build:
To build a Simple Entrance Alarm Circuit you need to follow the below mentioned steps:
Prepare the IR LED Board:
- Assemble the IR LEDs infrared diodes and R1 resistor on a separate board.
- The specific resistor value will depend on the IR LEDs used.
- This board will emit the infrared beam.
Prepare the Phototransistor Board:
- Fix the phototransistor L14F1 on another board.
- Ensure that the IR beam from the IR LED board aligns with the phototransistor.
Assemble the Circuit:
- Place the NPN darlington phototransistor L14F1 on the main PCB.
- Connect its collector to the positive supply through preset VR1 and connect the emitter to the ground.
Connect the LEDs and Buzzer:
- Connect the green LED to the emitter of the phototransistor to indicate standby mode.
- Connect the red LED and buzzer to the emitter of a separate NPN transistor T2 used as the alarm driver.
Connect the Base of T2:
- Connect the base of transistor T2 to the collector of the phototransistor T1.
- This connection determines the activation of the alarm.
Power the Circuit:
- Connect the power source to the circuit, ensuring proper polarity.
Adjust VR1:
- Use VR1 to adjust the base bias of T2 so that the buzzer remains off in standby mode.
Test the Circuit:
- Wave your hand across the IR beam.
- The buzzer should sound indicating that the circuit is working correctly.
Mount the Circuit:
- Mount the assembled circuit in a suitable enclosure, ensuring that the IR LED board and the phototransistor board are placed facing each other with the IR beam aligned.
Finalize:
- Secure all components and wires in place, and your infrared broken beam alarm is ready to use.
Note:
- Remember to take necessary precautions when working with electrical circuits such as disconnecting power before making any changes and ensuring that all components are securely connected.
Conclusion:
To conclude, Simple Entrance Alarm Circuit offers a simple yet effective way to protect doors and entry passages.
It operates reliably day and night with minimal risk of false triggering.
With its basic circuit design it serves as an excellent evening project, for hobbyists interested in home security systems.