Temperature-frequency converter made with astable multivibrator and thermistor

Abstract

The temperature can be measured electrically by several methods: with thermoresistive transducers, with thermocouple transducers, with thermistors, with the pn junctions of semiconductor devices, etc. Thermistors are electrical resistors made of metal oxides with semiconductor properties, which are small in size, have very small time constants and can measure temperatures locally, without disturbing the temperature range in the measured area. In order to conveniently transmit the temperature information (without errors) at a distance, the paper uses an astable multivibrator circuit RC type, in which the resistor is replaced with a thermistor with negative characteristic, whose resistance changes depending on the temperature in a range allowed by this type of oscillator. After several experiments, it was established that for a certain value of a capacitor, using a thermistor with a negative characteristic, the frequency of the oscillator (rectangular signal) changes quasi-linearly (in the range of tens of kHz). The measured temperature (tens of 0C) corresponds to the environment in which the thermistor is mounted. It was possible to determine the value of the astable multivibrator circuit capacity for which the temperature and frequency ranges in which the converter is considered to operate quasi-linearly were established. The signal obtained from the output of the astable multivibrator circuit is transmitted remotely to a digital port of a microcontroller development board with an LCD shield that displays the measured temperature. Also, it is present the flow chart of program implemented in microcontroller.