temperature transmitter

There may be instances where temperature measurements have to be made electronically, but where it is not possible to have long leads floating about. In such cases the circuit described here may offer a solution. The temperature sensor is an NTC (negative temperature co-efficient) resistor, R7 in the circuit diagram. As the temperature of the NTC rises, its resistance decreases and capacitor C2 is charged more rapidly. This capacitor is repeatedly dis- charged by transistors T1 ... T3. As the temperature rises the repetition rate will also increase. During the discharge period of C2 a short positive pulse appears at the base of T4, thereby causing it to oscillate at the frequency determined by L1 and C1 (approximately 1.3 MHz). The circuit is therefore a "real" transmitter wich is switched on for very short periods and more fre- quently according to the rise in temperature — 60 Hz at 25°C with 7 temperature transmitter an increase of 5 Hz/°C. To prevent interfering with the ...