The light badge

The Light Badge is an electronic kit which utilizes an LDR( light dependent resistor) to detect lowering light levels and light a LED once it gets dark.This is a great example of an LDR in action.

A few Project goals:

  • Implement the circuit myself and explain about it .
  • Build a tiny PCB.

Let me explain little bit Theory ……….

An LDR(Light Dependent Resistor) or Photo resistor is a resistor where the resistance decreases with the strength of the light .

When it is dark,the LDR has high resistance .

What are these terms ?

  • Rated Dark resistance – This means when exposed to total darkness,you can measure it by using a multi meter.
  • Cell resistance @ illuminance (light resistance) – it is the resistance of photoresistor when it exposed to bright light.

In this way we can find minimum and maximum resistance of LDR.

So photo resistor is basically a device that give off high and low resistance based on amount of light falls on it.With this characteristics , this can be used as a sensor for light or as a photo sensor.


With the photoresistor acting as the light sensor,the other significant component is the BC547 NPN transistor .Here transistor will amplify the current so that sufficient current is produced to light the LEDs.

How it will amplify ?

okay ,okay I will explain that too……………………..

Here 3V coin cell is powering the circuit,This 3 Volts is in parallel to a 100 kΩ and the photoresistor. In the middle of these two component is connected to base of NPN transistor.

This is how the circuit works:
When exposed to bright light,the photoresistor’s resistance is very low.It drops to around 20-30 KΩ .Current travels through the 100 KΩ resistor and then has 2 paths.It can either go through the base of the transistor or go through the photoresistor.
The base of transistor to the collecor has a resistance of around 400 KΩ.Current always takes the path of least resistance.When photoresitor is exposed to bright light,its resistance is about 20-30 kΩ ,which is significantly less than the 400 kΩ of resistance the base of the transistor has .Therefore,most of the current will go through the photoresistor and very little will go to the base of the transistor.
So the base of the transistor is bypassed,Thus,the transistor does not receive enough current to turn on and power on the LED.Thus the LED is off when there is lot of light in the surroundings.

It is very important to build stuff,not just read about it,so get all component you need and start building it .
checkout photoresistor resistance at bright and dark light condition,also checkout collector-base resistance of transistor.In this way you
will learn what is happening in the circuit.

When it is dark,photoresistor’s resistance becomes very high.It’s resistance goes up to 2 MΩ.This creates a very high resistance path, due to this , most of the current will go through the base of the transistor.Means that current does not go through the photoresistor when it is dark.

In the above circuit LDR and the 100 k resistor form a simple ‘potential divider’ circuit,where the middle point of the potential divider is fed to the base of the NPN
transistor.Therefore based on light intensity the brightness of LEDs will be changed.

The light Badge Kit includes :

  • CR2032 coin cell and Holder
  • slide Switch -11.6×4 mm
  • LED,1206 SMD package -6 (Any colour)
  • 100K Resistor ,1206 SMD Package
  • LDR
  • BC547 Transistor
  • Custom PCB: I designed a key chain sized PCB for this project and got it manufactured by

/* HAPPY 11111100100 */ is nothing but “HAPPY 2020 ”

Tools:Tweezers,Soldering iron,Solder wire,Wire Cutter and a multi meter.

This PCB was sponsored by ,Huge thanks to them for sponsoring PCBs
They provide PCB manufacturing service for customers with many customized product options such as quantity,layer ,PCB thickness ,dimension, color and all that .If you are interested checkout

The board outline were made using Autodesk Fusion 360.And PCB design was done using KiCad.

On the PCB I have represented anode of led with small white dotted silkscreen.

From initial idea to final assembly ,this project took about 3 weeks time.

All the Project files are up on Github, go make !

Do you have any questions ? Leave a comment down here .

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  • 12/02/2020 : Light Badge Certified by OSHWA

Reference links:


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