Various types of switch circuit like light operated switch, sound operated switch etc is already posted in www.bestengineeringprojects.com. Now here is a thermal touch switch which automatic on and off due to increase and decrease of pre defined temperature. This types of circuit is useful to opeared fan, cooler etc.
Circuit description of thermal touch switch using operational amplifier 741
The circuit comprises operational amplifier IC (IC1), silicon diode (D1 and D2) and few passive components to complete the circuit. Diode D1 is used here as temperature sensor. OP-amp 741 is used here as comparator.
When temperature is low, diode D1 does not connect and output of IC1 is low, when temperature is increases D1 starts conducting and output of IC1 is high, which further glow LED LED2.
For adjustment of pre define temperature put switch SW1 on and adjust potentiometer VR1 so that output at pin 6 is half of supply voltage.
NOTE: To control appliance according to temperature use relay with driver circuit at output pin 6.
PARTS LIST OF THERMAL TOUCH SWITCH USING OPERATIONAL AMPLIFIER 741
Resistor (all ¼-watt, ± 5% Carbon)
R1, R2 = 10 KΩ
R3 = 4.7 KΩ
R4, R5 = 1 KΩ
R6 = 1 MΩ
R7 = 10 MΩ
R8, R9 = 470Ω
VR1 = 10 KΩ
C1 = 100 µF/10V
C2 = 10 KpF
IC1 = LM741
D1, D2 = 1N4148
SW1 = On/off Switch