Transistor as Control Switch

Today i'm going to continue our discussion about light control switch by using transistor & LDR.

Before we go further, firstly u must understand the characteristic of LDR?
LDR is a type of resistor known as Light Dependent Resistor which is very sensitive towards light.
Important point about LDR that u MUST remember:
In a BRIGHT condition, its resistance DECREASE
In a DARK condition, its resistance INCREASE.

Now lets have a look at the circuit below, describe what happen at NIGHT???
At night,

• LDR's resistance increase much higher than the fixed resistor,
• A higher voltage is supplied across the LDR,
• This increases the flow of base current and switch ON the transistor,
• The bulb lights up

So what if we want to switch ON the bulb in the morning & close it at night?
Its simple, u just need to change the position between the LDR & the 10k fixed resistor.

The same concept applies in Heat Control switch, except instead of using LDR we use THERMISTOR.
Again, its important for u to memorise the chracteristic of a thermistor, i.e;
HIGH temperature - Resistance DECREASE
LOW temperature - Resistance INCREASE