Control Relay using IR Sensor, Ready Kit, 100% Tested


Control Relay using IR Sensor circuit is presented here is based on the readily available IR sensor, comparator IC LM393 and relay. The photo diode is used as the light intensity detector while IC LM393 used as comparator. IR beam is continuously falling on photodiode. On interruption  the movement is detected. Hence,  no IR beam falls on the photo diode. As a result photo diode stop conducting and alarm sound. However, when IR beam falls on the photo diode, photo diode starts conducting and alarm sound stays off.

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Complete working video can be seen from

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Application of the circuit:

Most of the IR based circuits available in market use one or another kind of IR sensor which increases the cost and also require complicated circuitry around it. But this IR based circuit uses readily available IR sensor as IR beam detector. The functioning of this Control Relay using IR Sensor is to detect interruption in IR beam falling on photo diode and automatically activate the alarm in absence of beam. Alarm sound continuously indicating the absence of beam till beam falls on the photo diode.

Why this particular circuit:

IR sensor used here as a beam detector. The variation in beam wanted can be easily done by interrupting IR beam using hand palm. The circuit operates normally over a DC voltage of 5V which can be obtained by DC supply cable.

Explained working of circuit:

The basic idea behind the circuit operation is to turn ON alarm in absence of IR beam. The photo diode is needed for this purpose. Photo diode is photoelectric device as it converts light energy to electric energy. Here, Control Relay using IR Sensor works as IR beam detector while photo diode working as light sensor that alarm. IC LM393 is used as comparator. Photo diode output is connected as input to comparator.  Pin no. 4 of LM393 is pulled towards ground. Relay is used as a switch. When relay is activated, normally closed contact becomes open and normally open contact becomes closed.  In normal condition, IR beam  falls on  photo diode and alarm stays off. Now consider second scenario when IR beam is irrupted, the alarm sound in the absence of light.

How to build:

First of all read the given manual thoroughly and study the circuit given in the figure. Also have look at PCB and components supplied along with the kit. Each component has to be soldered in its position on PCB.

Identification of resistors is done by color coding. The color band on each resistor corresponds to its exact value.

There are different methods in which values are defined on capacitors. But usually values are specified numerically on them.

Can you make out the whole working of circuit and are you able to identify each component separately as to where each of them has to be placed?

If yes only then proceed further to actually mounting and soldering the parts.

Not ICs but their sockets are to be soldered on PCB. This is to make mounting and dismounting of ICs easy while troubleshooting.

Start from left most corner of PCB and solder the components one by one on their correct position on PCB.

Before soldering any component see that you have placed it at its right position and with correct polarity. Give due attention to diodes and electrolytic capacitors as they are polarity dependent.

Do the soldering of other components in the same way while keeping in mind that components with long and sensitive leads like capacitors and transistors are soldered last.

Testing the kit:

After assembling and soldering of components on PCB, connect external DC power source of 5V.

For testing purpose manually hold hand palm over IR sensor. Alarm sound in this case.

Now set the desired intensity at which you want detection of light. This can be done by changing the resistive value of R2 variable resistor.

Install the unit at ceiling of room.

ICs used here:

The only IC used here is LM393

It is dual differential comparator IC.

This 8 pin IC is popular and commonly used IC

Part List


U1               – LM393

Base            – 8 pin

Resistors(all ¼ watt +/-5% carbon unless stated otherwise)

R1              – 100 Ohm

R2               – 10k Ohm(variable)

R3               – 1k Ohm

R4               – 1k Ohm

R5               – 10k Ohm

R6               – 10k Ohm

Capacitor C1               – 1uF


D4:              – IR Sensor

D3:              – Photo diode

D1:              – Red LED

D2:              – Green LED

5 V USB Cable