Disadvantage-to-Opportunity Convertor: An Ultra (Simple, Sensitive and Low Cost) Body Thermometer Based on High Thermal Tolerance Ceramic Capacitor and Negative Differential Resistance (NDR) Oscillator

Temperature measurement is one of the most important and practically encountered measurements. Measuring and monitoring body temperature is one of the routine tasks in hospitals and clinical examinations to check the general health status of a patient. Generally, mercury thermometers, adhesive forehead thermal strips or non-contact infrared thermometers are used for this purpose. Mercury thermometers are prone to breakage and releasing toxic mercury; forehead strips are disposable, and infrared thermometers are generally expensive. In this research, a simple RC relaxation oscillator circuit with negative differential resistance (NDR) element for measuring and monitoring body temperature in the range of +۳۴ to +۵۰ °C has been built and studied. The temperature-sensitive element is a ceramic lentil capacitor and the NDR element used is the emitter-collector junction of a typical universal negative (TUN) unijunction (UJT) transistor. The circuit works with a supply voltage of about ۱۲ volts and its oscillation output is in the range of ۴.۵ to ۷.۲ KHz which is noise-immune and can be easily monitored by a frequency counter with an accuracy of ۱ Hz. A ۱۰ nF lentil capacitor was used in this study. This cheap capacitor had high temperature tolerance which created a temperature sensor with high sensitivity (about ۱۴۵ Hz/°C) or (۰.۰۰۷ °C/Hz). The trend of increasing oscillator frequency with temperature comes from the inverse dependence of dielectric coefficient on temperature. In other words, for lentil capacitors, especially cheap commercially available ones, the dielectric coefficient decreases sharply with increasing temperature. It should be noted that the curve of frequency changes with temperature was built using a Pt-۱۰۰ reference thermometer (۰.۰۱ °C resolution) in an air thermostat. The obtained curves of frequency changes vs. temperature for studied lentil capacitor could easily be fitted with a third order polynomial with high accuracy in the temperature range of +۳۴ to +۵۰ °C. The stability and reproducibility of performance of all studied capacitors as temperature sensors were excellent without any clear hysteresis behavior. Therefore, this ultra-simple and very inexpensive circuit can be used to measure and monitor body temperature and convert it into frequency that is easily comprehensible for logic circuits and microprocessors.