The Opamp IC 741 is used here as non-inverting non-zero reference comparator. Its inverting terminal is connected to variable voltage which can be adjusted with the help of variable resistor VR1.
The non inverting terminal is connected to the output of temperature sensor LM35.
The prototype works very effectively and responds to quick changes in temperature of LM35.
- Pin 1 and pin 5 of IC 741 are left open.
- Pin 2 is connected to voltage divider using one fixed resistor of 2.2k and one variable resistor of 10k.
- Pin 3 is connected to the output pin i.e. middle pin of IC LM35.
- Pin 7 and pin are connected to +ve and ground supply of 6V, single power supply.
The circuit is fully tested in our laboratory and works very nicely to measure the temperature from room temperature value to maximum of hot soldering iron temperature.
Please note that the circuit works only on single power supply. Don’t try it on dual power supply as the output of the IC gets saturated and both LEDs remain ON, if you connect dual power supply.
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