As above, In fact, we can use all semiconductor devices as a temperature sensor.
Inside semiconductors have resistances. Its resistances will reduce when it to be hot.
In the opposite way, when it is cold its resistance rises.
In fact, when the diode is in forward biased. It has voltage across about 400mV to 500mV at current about 1mA. This voltage level depends on the current and the substance used to generate these diodes.
In silicon diode, the voltage across it will reduce about 2mV in each one degree Celsius temperature.
We so can use it. Also, you can use a transistor, diode bridge and more. They are also silicon semiconductors.
In the circuit above is a simple temperature sensor diode, D1.
First, get a steady voltage power supply. The Dual +15V/-15V DC Regulator is suitable for here.
Second, a constant current comes to D1 through R1. When the temperature of D1 changes, the voltage across it changes, too.
But this changing makes voltage changes very very low. We cannot measure it on a normal voltmeter. We need a helping of the comparator.
Third, use op-amp comparator IC, LM741. We set it into a inverting compator mode.
Why use this mode?
Since we need to measure the output voltage and Temperature is in the same way.
Thus, we use this mode.
When Vin (pin 2) exceeds Vref(pin3), output switches from High to Low.
You do not worry if you do not understand it. You do not need to know all about it. Just you can use it is enough.
Fourth, we need to use VR1 and VR2 to adjust a right of this temperature sensor diode.
Last, when setting you should use the Digital Thermometer to compare real temperature. A digital voltmeter you may have it. The importance, you take an ice, hot water. but Diode cannot touch the water.
This circuit is not accurate full 100%. It just is good for learning in the Science experiments. Have fun!
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