LDR & Relay based Street Light Controlling, Ready Kit, 100% Tested


LDR & Relay based Street Light Controlling circuit is presented here is based on the readily available LDR, comparator IC LM393 and relay. The LDR is used as the light intensity detector while IC LM393 used as comparator. In the absence of light, no light falls on the photo sensor. As a result photo sensor stop conducting and Street light glows. However, when light falls on the photo sensor photo sensor starts conducting and Street light stays off.

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Complete working video can be seen from


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Application of the circuit:

Most of the LDR based circuits available in market use one or another kind of LDR which increases the cost and also require complicated circuitry around it. But this LDR based circuit uses readily available LDR as light detector. The functioning of this LDR & Relay based Street Light Controlling circuit is to detect interruption in light and automatically activate the Street light in absence of light. Street light glows continuously indicating the absence of light till light falls on the LDR.

Why this particular circuit:

LDR used here as a light detector. The variation in intensity of light wanted can be easily done by interrupting light 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 Street light in absence of light. The LDR  is needed for this purpose. LDR is photoelectric device as it converts light energy to electric energy. Here, LDR & Relay based Street Light Controlling circuit works as light detector while LDR working as light sensor that glows street light. IC LM393 is used as comparator. LDR 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, light falls on  LDR and street light stays off. Now consider second scenario when light irrupts, the street light glows 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 LDR. LED glows 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)

R2               – 10k Ohm(variable)


R3               – 1k Ohm


R4               – 1k Ohm


R5               – 10k Ohm


R6               – 10k Ohm


Capacitor C1               – 1uF


D4:              – LDR


D1:              – Red LED


D2:              – Green LED


5 V USB Cable