How I can add a cooler with this circuit?

[HerohetoChakma]

This is the circuit diagram for hole system.

 

[Electro nS]

what do you mean by cooler ?
heatsink ? , it looks like a small PIC ,lcd keypad , what do you need that for ?

 

[HerohetoChakma]

Cooler means a fan that will switch on when temperature exist my set point.

 

[nikhilsigma]

if u want to drive a 12V dc fan....u can use a simple transistor driver circuit...
1. read the resistance of fan from multimeter...
2. calculate value of resistances...

http://www.bristolwatch.com/ele/transistor_drivers.htm
**broken link removed**

 

[Electro nS]

ah ok , offcourse a transistor works , but if current draw is a problem or load is AC (for some reason) use a relay to switch the fan .
by the way we will need a transistor anyway to drive the coil of the relay