In some cases of temperature measurements, it is more advantageous to convert the measured value into a frequency than into a voltage. A temperature to
frequency converter can be directly connected to a frequency counter or it can be connected to a computer without using any A/D interface to display the
measured value. The conversion factor
of the circuit featured here is 10 Hz per degree centigrade. The measurable range is from 5 to 100°C with a maximum error of (plus/minus) 0.3°C.
A single IC LM335 is used as the temperature sensor. When the temperature sensed by LM335 is zero, its output is exactly 2.73 volts. The output
signal comes out in the form of a squarewave.
Calibration: Put crushed ice in a glass of water and place the temperature sensor in it. Let the sensor cool down for a few minutes. Using a voltmeter, measure the
voltage level between the positive (+) pin of the temperature sensor (LM335) and pin 2 of the LM331.
Afterwards, adjust P1 to output a voltage level of 2.73V. After this initial calibration, heat a glass of water to 50°C (use a normal thermometer to measure the
temperature). Connect the display device (e.g. frequency counter) to the circuit's output. Place the sensor into the warm water, and adjust P2 to generate the desired frequency
representing this temperature. For example: Adjust P2 so that the frequency is 50 Hz representing the temperature of 50°C.
You must choose the output frequency so that you can easily adapt it to your display instrument (like frequency counter or computer).
Electronic Circuits volume 1.0 - Circuit Nr. 64
The complete data of the electronic circuit described above can be found in the following book and is available from Amazon.com.
Click on the image to view the book.