Temprature Controlled Fan Circuit


Description
This circuit is self explanatory and it is fairly simple, the temperature sensor used in this circuits are not thermistors but they are nothing but the signal diodes, the variation of the resistance according to increase or decrease in temperature sets the operational amplifier which drives the transistor Q1 which inturn gives the base voltage to transistor Q2 and their by the fan is switched on and off. 
Circuit Diagram



Source-http://www.electronguide.com/circuits_pages/TEMPRATURECONTROLLEDFAN.html

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