This temperature meter uses the precision micro power centigrade sensor IC LM35. The output voltage of the IC is linearly equal to +10Mv per degree centigrade. The temperature level is displayed through LED readout.
The circuit uses the precision temperature IC LM35. This three pin transistor like IC give output linearly equal to +10mV per degree rise in temperature. It can measure temperature between -4 degree to +110 degree centigrade. Its related type LM34 is Fahrenheit sensor and its output is equal to -10mV per degree Fahrenheit. Output of IC1 is directly given into the input of the display driver IC LM3914. It is a monolithic integrated circuit with 10 active low outputs that can drive 10 LEDs directly without a current limiting resistor.
The internal circuitry of the IC adjusts the current passing through the LEDs. The input of LM 3914 is very sensitive and its outputs 18 – 10 sinks current one by one as the input receives an increment of 125 milli volts. Here only 6 outputs are used to drive 6 LEDs. More LEDs can be included in the remaining outputs if required. As the IC LM35 senses temperature rise, LEDs one to six light up. If the sensitivity is not high, VR2 can be omitted. Then output of IC1 should be directly connected to the input of IC2
When power is applied, some of the LEDs will glow. Calibrate the circuit by giving different temperature to IC1. For this a thermometer and hot water of different temperature is required. Sock some cotton with warm water of around 37degree (normal room temperature) and gently make contact with IC1. Adjust VR1and VR2, till LED1 glows.