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Simple Entrance Alarm Circuit

This easy and Simple Entrance Alarm Circuit is made to protect door or way where people enter.

It makes loud sound when someone walk through invisible IR light.

It works good at day and night by not giving much wrong alarm.

The circuit is simple and good for hobby people during nighttime.

Circuit Working:

Parts List:

CategoryItemQuantity
Resistors100Ω 1/4 watt2
Preset 1k1
SemiconductorsPhoto transistor L14F1 or similar type1
Transistor BC5471
LED Green 5mm 20mA1
LED Red 5mm 20mA1
IR transmitter LEDs3
Buzzer1

This circuit use IR diodes to send out infrared light all time but not like remote that send in short pulses.

It uses one NPN darlington phototransistor to see the light.

The L14F1 is very sensitive phototransistor with big gain.

Its collector connects to positive side through VR1 and emitter have green LED to show standby.

Base of T1 is not connected as the light comes through T1 window to hit base and make it work.

When there is more light more it works.

T2 work as driver for alarm and its base connect to T1s collector so it depends on T1 working.

When IR light fall on phototransistor it works and pull T1 base to ground and T1 stay OFF

In this case the red LED and buzzer ON T2 emitter as it stays OFF

When someone break the IR beam the T1 stops working and so its collector voltage goes up.

Then T2 turns ON and red LED + buzzer also turn ON.

We can build this circuit on small PCB board.

Put R1 and IR LEDs on different small board and keep IR LEDs on one side of door and phototransistor on other side.

Point IR light properly to phototransistor so buzzer stays quiet when no one passes.

Wave hand in middle to test and check buzzer should make sound.

Use VR1 to adjust T2 base bias, so buzzer stays OFF when no one is around.

How to Build:

To build a Simple Entrance Alarm Circuit we need to follow the below mentioned steps:

Make IR LED Board:

Make Phototransistor Board:

Build Main Circuit:

Connect LEDs and Buzzer:

Connect T2 Base:

Give Power:

Adjust VR1:

Test the Circuit:

Mount Everything:

Finish:

Note:

Conclusion:

To conclude this Simple Entrance Alarm Circuit is easy and useful way to protect door or entrance.

It work good in day and night and does not gives any wrong alarms.

Because circuit is simple it is very good small project for hobby people who like home security ideas.

References:

Datasheet LI4F1

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