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.

This PCB can be used as a Wearable device.

Step 1: Watch Video

Step 2: Schematics:

Step 3: Theory :

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.

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.