Here is the circuit diagram of an electronics thermometer using operational amplifier and silicon diode having range of -20oC to 125oC. Various types of temperature measuring and controlling circuit either using microcontroller or discrete component is already posted in bestengineeringprojects.com.
Circuit Diagram of Electric Thermometer Using OP-AMP 741
The entire circuit of electric thermometer is built around two op-amp 741 IC, silicon diode and few passive components, where silicon diode D1 is used as temperature sensor. The diode 4148 have voltage drop of 0.7V at room temperature and has a linear characteristic as the voltage change is very small (2mV/oC with increase in temperature) over a wide range of temperature. Linear characteristics of sensor diode eventually eliminates the problem of scale calibration.
The working of this electronics thermometer can be understood in two phases:
Here, the op-amp IC1 works as a temperature to voltage converter whereas sensor diode D1 provides a feedback path. Thus, output of op-amp depends upon the voltage across the diode which eventually depends on the temperature. The non-inverting pin of op-amp IC1 is maintained at constant voltage by VR1, R1 and R2.
The output of 1st op-amp (IC1) is connected to the input of 2nd op-amp (IC2) which works as a voltage amplifier. Gain is adjusted by VR4 in such a way that the voltage change of 2mv at pin 2 of IC2 cause an increase of 10mV at the output. The output is observed at analog meter and its sensitivity is adjusted by VR3. Meter reading can be adjusted by keeping the diode at various known temperature.
PARTS LIST OF ELECTRIC THERMOMETER USING 741 IC
|Resistor (all ¼-watt, ± 5% Carbon)|
|R1 = 650 Ω
R2 – R5 = 1 KΩ
R6 = 6.8 KΩ
R7 = 10 KΩ
VR1 = 2.2 KΩ
VR2 – VR4 = 10 KΩ
|C1 = 0.1 µF (Ceramic Disc)
C2 = 10 µF,16V (Electrolytic Capacitor)
C3 = 0.1 µF (Ceramic Disc)
C4 = 10 µF,16V (Electrolytic Capacitor)
|IC1, IC2 = 741 (Operational Amplifier)
D1 = 1N418