Temperature Monitor, For Industrial Use, Voltage : 220V

The Temperature Monitor is a miniature temperature controller device up to 100^oC. This simple circuit of temperature monitor uses differential amplifier IC and temperature sensor. The three different colors of LED indications are used to indicate different temperatures. The temperature at which LEDs will glow set by preset P1. Generally LED D1 will glow at normal temperature. As the temperature increases LED D2 glows. At highest temperature LED D3 glows.
About the Circuit :

• As described earlier circuit uses differential amplifier IC, IC1. The reference voltages are applied to pin2 and pin6, which are set by preset P2 and P3 respectively. The differential voltages developed at pin3 and pin5 according to the voltage produced by R1/P1 and temperature sensor, which gives an output voltage of 10mV/^oC. Accordingly output at pin1 and pin7 changes and LED indication changes.
• At normal temperature, voltage produced by sensor is smaller than either of the reference potentials. The output at pin1 and pin7 are low: D1 will then light.
• When the ambient temperature rises, the output of sensor rises proportionally. When the level of the sensor output lies between the two reference levels, the output at pin1 is high and that of pin7 is low. Diode D2 will then light, showing that the critical temperature has been reached.
• At even higher temperature, the output at pin7 also goes high and D3 lights, while the other two LEDs will go out. At the same time, the relay will be energized via T1. Also LED D6 lights which gives visual indication.
• Zener diode D4 ensures that T1 does not come on when D2 lights. The temperature at which the LEDs should light can be set with preset P1, P2 and P3. The precaution should be taken that the monitor is intended for normal temperatures between 25^oC and 100^oC.