Simple Infrared Alarm Barrier Circuit

This circuit explains how to use invisible light to create a tripwire alarm.

Think of it as a fence like beam of light.

A loud alarm is sent off to warn of the presence of someone and to scare them away if they walk across the light and interrupt it.

This is an enjoyable project but keep in mind that actual security systems are also more advanced and important for safety of our house.

Circuit Working Infrared Light Alarm Transmitter Circuit:

Parts List of Infrared Light Alarm Transmitter Circuit:

This infrared security system is designed to identify people entering through tiny gates, corridors and doors.

When any human passes across the invisible infrared light beam that the transmitter generates the buzzer in the receiver is triggered.

This is how an infrared barrier alarm operates:

This infrared alarm systems transmitter and receiver circuits are designed for a range of several meters making them almost insensitive to changes in ambient light.

Extra filtering processes are required in the rare cases where the receiver sensor is exposed to strong direct sunlight.

To turn ON and OFF the Infrared Emitting Diode (IRED) the transmitter controls a 36 kHz carrier.

Since most infrared sensors do not respond well to constant incidence of infrared light this modulation takes place at a rate of around 300 Hz.

The IR detectors can recover and limit their reactions to ambient light by quickly turning off the IR source.

The transmitter uses two CMOS oscillators either the TLC555 or the 7555.

A substitute to the two 555 ICs is a single TLC556 or 7556.

The 300 Hz generator is IC1 and the 36 kHz source is IC2.

Driver transistor T1 pulses the IRED type LD274 at a high peak current.

A carrier frequency of exactly 36 kHz is ensured by adjusting preset P1.

One can reduce current consumption by increasing the value of resistor R5 if the situation calls for a shorter IR beam distance.

Formulas:

The following formula is used for a 555 integrated circuit (IC) in astable multivibrator mode which is frequently used to produce pulse width modulation (PWM):

Duty Cycle = (THIGH / T) * 100

where,

• The duty cycle shows the percentage of a single output waveform cycle that the 555 IC output is in the HIGH state.
• During a single cycle THIGH is the duration in seconds that the 555 IC output stays in the HIGH state.
• T is the total amount of time (in seconds) required for the output waveform to finish one cycle.

Understanding the link:

In simple terms the formula calculates the proportion of the cycle that the output is HIGH compared to the whole cycle duration.

By multiplying by 100 one can represent it as a duty cycle or percentage.

Why is this important in PWM circuits?:

PWM signals can control the average power delivered to a load (such an LED or motor) by adjusting the duty cycle.

Greater Duty Cycle (THIGH nearer T):

This means that the output remains in the HIGH state for a longer period of time giving the load more power (brighter LED and quicker motor).

Duty Cycle Reduction (THIGH is shorter than T):

This indicates that the load (slower motor and dimmer LED) receives less power since the output is in the HIGH state for a shorter amount of time.

How does duty cycle affect the 555 IC circuit?

A standard 555 astable multivibrator circuit has a duty cycle of around 50% (THIGH is roughly equal to T).

But the circuit can be modified to control the duty cycle:

A potentiometer is used to modify the resistance of resistor R2 (refer to a schematic of a 555 IC astable multivibrator) and therefore the charging time of the capacitors (THIGH).

This results in a change in the duty cycle.

Including other elements:

Over 50% fixed or adjustable duty cycles are made possible by certain circuit designs that include additional components.

Take note:

Finding the percentage of a full cycle in which the output of a 555 IC operating in astable mode is HIGH is made easier with the use of the duty cycle calculation above.

This figure is important for understanding and controlling the power supplied to a load in PWM applications.

Circuit Working for Infrared Alarm Barrier Receiver:

Parts List of Infrared Alarm Barrier Receiver Circuit:

The CMOS 555 based receiver generates no noise as long as the sensor is able to pick up the transmitters infrared light.

By acting as low frequency rectifiers components D1 and C2 reduce the impact of the 300 Hz modulation on the transmitter signal.

A warning tone is produced by the IC 555 oscillator when the infrared light beam is cut off.

The test values of circuit diagrams show the average DC levels obtained using a digital voltmeter (DVM) in both light and dark situations.

Waveforms at the majority of the test points are sawtooth or rectangular.

How to Build:

For Building an infrared barrier alarm circuit includes assembling the transmitter and receiver circuits.

Transmitter Circuit:

• Connect the IC TLC555 IC1 and IC2 to the breadboard.
• Set up the 300Hz generator and connect pins 2, 6 and 7 of IC1 together.
• Pin 4 and pin 8 of IC1 should be connected to positive supply.
• Connect pin 5 of IC1 and IC2 to ground via C2 and C4.
• Connect pin 7 of IC2 to P1 with one end connected to Vcc and the other to ground.
• Connect pins 2 and 6 to resistor R2 100k and preset P1.
• Connect pin 3 of IC1 to pin 4 of IC2
• Connect pin 3 of IC2 to the base of transistor T1 through resistor R4 330k.
• Connect T1s emitter to IR diode D1 through R5 and its collector to the positive supply.
• Connect the LD274 IREDs cathode to ground and its anode to T1s emitter.
• To limit the current connect a resistor R5 18Ω in series with the LD274 IRED.
• For stability connect a C5 10μF capacitor between the positive supply and ground.

Receiver Circuit:

• Set up the 36 kHz source and connect another IC TLC555 IC2 to the breadboard.
• Connect pins 2 , 6 of IC2 together with R3.
• Connect pin 4 of IC2 to the output of IC1 through diode D1 BAT85.
• Connect pin 5 via to ground through a C4 100nF capacitor.
• Connect pin 3 of IC2 to piezo buzzer

Testing:

• Supply the circuit with power.
• To keep the buzzer quiet make sure the infrared beam is not stopped.
• Observe as the alarm goes off when the beam is cut off.

Note:

• When working with electronic circuits and components take care and make sure all of the connections are correct before switching on the power.

Conclusion:

In security applications the Infrared Alarm Barrier Circuit is often used to keep an eye on restricted areas, stairs and gates.

It provides a dependable and affordable way to identify unwanted movement or entry in a restricted area.

The device can easily used both indoors and outdoors because to the modified infrared signal which also enables it adjust to changes in the surrounding light conditions.

References:

Simple IR barrier sensor for arduino