A very simple electronic thermometer can be designed using electronic diagram below. Thermometer displays temperature radiator not only on two displays, but also provides an output for switching can be used to connect a fan.
This circuit consists of four parts: a reference voltage source IC1, IC4 sensor, IC2 and IC3 the display, the switching IC5.
Temperature sensor generates a voltage temperature dependence of 10 mV / ° K. The display is built around IC 3162 CA 3161 and CA. IC2 contains A / D converter and multiplexer circuit for display circuit IC3 is a BCD decoder / seven segments
IC2 measures the difference between the voltage supplied by the sensor and the fixed reference voltage potentiometer P1.
IC5 compare the voltage supplied by a voltage sensor obtained with the help of R9 and R10, the reference voltage of IC1. When the voltage sensor reference voltage rises above the secondary's output changes state and transistor IC5 lead
The values of R9 and R10 data, the comparator changes state at about 80 degrees C. The thermometer must be independent supply with a transformer containing two isolated side.
Sensor used in this project is a LM335Z noted in circuit with IC4 .
Circuit must be powered from a 2x9-10v transformer which can provide an output current of 400mA.
Output connector from T3 can be used to activate a fan or to deactivate power from speakers or something else .
This thermometer must be calibrated using a precision multimeter . First of all you need to connect together Y and Z points and turn P4 potentiometer until display show 00. Next step is to connect at point Y a 0.9V DC signal and set P3 until display show the same value . The final calibration is to connect points Y and X together an measure voltage on C5. If the voltage it's not around 2.73 volts you can adjust it using P1 potentiometer .The sensor must be mounted on the radiator, as close as possible to the power transistors.