Less Annoying Intruder Alarm System Using Basic Circuits

In this project, I will be building a multipurpose device that contains an intruder alarm, dim light, emergency light, and backup power supply.

The alarm circuit only beeps twice when the door is opened. To achieve this a monostable multivibrator circuit was used. However, the NE555 multivibrator needs only a pulse for activation. It gets really annoying If the buzzer keeps buzzing until the door is closed. Thus, for this project, I created a unique circuit using PC817 optocouplers which only produces a pulse when the door opens not when the door is closed. The bell only beeps two times and then turns off automatically.

I made this project almost 4 years ago, but it still works!! I will go into different parts about the circuit in this article so bear with me :)

Tools needed for this project:

2 x PC817 optocouplers

4 x BC547 transistor

1 x BC557 transistor

3 x 1k ohm resistor

2 x 470ohm resistor

1 x 100ohm resistor

1 x 100k resistor

2 x 10k resistor

1 x 10k potentiometer

1 x 10k LDR

1 x 47uF capacitor

1 x 160uF capacitor

1 x NE555 multivibrator

2 x 47k resistor

2 x 10uF capacitor

1 x 1000mAh Li-ion battery

1 x TP4056 lithium battery charger

2 x 5V buzzer

1 x 5V old mobile charger

1 x green led with lens.

1 x blue led with lens.

Plastic box and glue gun

This project has 3 different sections:

1) It produces a high beep sound when someone opens the door

2) It contains an emergency white light that can run for hours during load shedding

3) It contains a dim light that turns on when it's dark

The above picture shows a draft drawing for the device.

Diagram 1: In this part, when the door is closed the switch is also closed. At this moment the Q4 transistor is ON and thus current cannot flow through the U4 optocoupler. This allows the 47uF capacitor to charge as shown in the first diagram.

Diagram 2: When the door opens the Q4 transistor stops conducting. The U4 optocoupler is now active, it closes the connection between the second optocoupler and the capacitor. This enables the U5 optocoupler to run for a short time through a 100ohm resistor. This eventually creates a pulse that is sent to the NE555 circuit.

Diagram 3: after a short time, the capacitor is completely discharged. the U5 optocoupler stops conducting and no further signal is sent to the multivibrator part.

The whole process is repeated when the door is closed again.

This section contains a NE555 circuit is a monostable configuration. It receives the pulse signal from the optocoupler section and switches the output high on pin 3.

The length of the output signal can be adjusted by varying the 10k potentiometer. Basically, you can control how many beeps would be produced each time the door opens.

This is a simple astable vibrator configuration. This circuit is directly powered by the output of NE555. However, it is recommended to use a transistor to power this circuit. The output from this circuit is a square wave. When the output is high the buzzer beeps and vice versa.

Change the value of capacitance to change the duration of beeps.

This is a simple circuit that turns on a blue and green led when it gets dark. The sensitivity can be adjusted by varying the value of the 10k resistor. Use a potentiometer to control the sensitivity.

This portion basically contains a 3.7V Li-ion battery that is charged by a TP4056 module. The battery powers the led. During load shedding, the light can be turned on using a switch. Since this portion is separate from the other circuits, I did not add any simulation.

When the door opens, the capacitor discharges, and the buzzers beep twice, and when door is closed the capacitor gets charged.

Check the attached simulation file, tweak the circuit to build more interesting outputs!

Proteus File: LINK

Build the circuit and test if it works as the diagram. Power it using a 5V supply.

I used an opaque plastic box to hold everything. This helps me to see the charging state of the battery.

Used some glue sticks to hold the reed and magnet in place. When the door closes the contacts get connected and vice versa.

The input switch to the optocoupler in step 3 is connected directly to this Reed switch.

And you are done! I hope you learned something from this project. Cheers!!