• Anirban Samanta

How To Prepare A Simple Automatic Light Sensing Switch

There is saying that “necessity is the mother of invention”. Few months earlier I was working in a college to set up a small off grid solar power plant for lighting the college campus. Now the tradition system for this purpose is to set up solar street light and in this case the lights we are using are driven by dc and have an inbuilt light controlled switches. But in this case the college campus was full of trees and to set up solar street light in that campus I had to do nothing but cut the trees which were not very practical decision. So I decided to centralize the whole system and use ac instead of using dc to drive the lights. I searched in the market for that switch but it was not available in the market at that time and to order online it would take minimum a week to get the circuit, but I don’t have that much of time. So I decided to design a circuit and made a very simple low cost automatic light controlled switch.

Automatic light controlled switches can be divided into various group depending upon the circuit elements by which it is made. Here I have used LDR and Mosfet based circuit to drive a relay depending upon the light intensity. It is a very simple method to prepare an automatic light controlled switch. Below there is a video and snaps of the total circuit I have used to make this.

I have used

i) A LDR (Light Dependent Resistance).

ii) Two Mosfet (IRFZ44N).

iii) A dc driven ac relay (12V dc, 220V ac).

iv) A power source (24V dc).

v) Some resistance, a zener (15V, 1 mW), two capacitor (100 uF, 45V).

The circuit diagram I have used is given below:





Here I have given a 15V zener diode and a 100 uF capacitor to protect the relay from high voltage generated due to induction in time of switching the relay off or on.





Total cost to make this circuit is given below:

i) LDR – Rs. 50/

ii) Mosfet – 2 x Rs. 24/-, Heat sink – 2 x Rs. 10/-

iii) Relay – Rs. 50/-

iv) Zener – Rs. 5/-, Resistances – Rs. 35/-, Capacitor – 2 x Rs. 6/-, Circuit board – Rs. 30/-

v) Mislaneous – Rs. 30/- (Soldering, wires etc.

Total Rs. 280

How it works is shown in the video below:







Anirban Samanta

Co-founder & CTO, AlienSolar


He did his M.Sc. in Physics from IIT Madras, Chennai. Being passionate about solar energy he wanted to pursue it as his career and co-founded AlienSolar to make it affordable in a better way.

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