Electronic thermometer

The scale of a thermometer used for measuring the temperature of liquids is normally graduated from 40°C to 100°C. The circuit described here operates within this range and uses the recently introduced KTY-10 temperature sensor from Siemens. The current produced (up to a maximum of 20 mA) is directly proportional to the temperature, allowing simple calibration without the need for 1 complicated calculations. The circuit can be used to measure the temperature of a number of things including; car oil, bath water, baby food etc. (but not all at the same time!). As can be seen from figure 1, the electronic thermometer is made up of a bridge circuit consisting of resistors R1 ... R3 and the sensor RT. The voltage across the bridge is stabilised by the zener diode D1. The bridge circuit is followed by an opamp, IC1. Any voltage difference at the input is amplified and fed to transistor Ti. This determines the amount of current flowing through the load circuit RL. This type of temperat...