If you are thinking about making a temperature sensing project the easiest way to do it is to use a sensor based on the diode temperature coefficient. This coefficient is a measure of how any diode junction reacts to temperature and every electronic engineer knows that it is -2.1mV per degree Celsius.
There’s two problems in using it directly:
1. It’s going down.
2. It’s very small and inconvenient to read.
3. It is not calibrated.
Direction
Of course using opamps you can correct these problems and for the first problem you can use an opamp in it’s inverting mode so now you have an output that goes up by 2.1mV per degree Celsius.
Output Size
For the second problem it would be much easier to use if the actual output could be read directly using a volt meter e.g. 25 degrees Celsius represented by 250mV is much easier to read than 52.5mV. (52.5/2.1=25).
What you really want is an output that increases in linear steps for each degree increase e.g. 10mV per degree is a good value for convenient reading on a volt meter.
You can do this using an opamp with a gain of 4.76191.
If you are an engineer you’ll immediately see the problem – it’s all those decimal place after the 4 and it means that you will need super accurate resistors to do the job. Of course you can use a 4.7k standard resistor to come close an approximate result but for temperature measurements you really want an accurate output.
For instance using a gain of 4.7 at 25 degrees Celsuis results in an output of 246.75mV i.e. It’s already in error by half a degree.
Calibration
With all discrete diode based circuits you have to calibrate the unit at a known temperature e.g. By holding the diode in freezing water either that or subtract the equivalent voltage from the diode output again using another opamp.
Convenient IC : LM35
Although you can do all these separate actions and make up a discrete version of a temperature measurement project a very convenient IC exists that does all of them for you.
That is the LM35 temperature measurement IC addressing all these problems that:
1. Generates an output that increases with temperature.
2. Produces a 10mV output for every degree Celsius increase.
3. Is fully calibrated.
Whichever method you use either discrete project or measurement IC an easy way to use the output is to feed it into the ADC input of a microcontroller.
You can then use the microcontroller to make a series of temperature readings over any time period you want to and at any time interval you need i.e. It’s easy to make a temperature recorder project.
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