LEDs Temperature Indicator

This is a LEDs temperature indicator which will provide output indicator of the temperature around the circuit. The indicator is just 2 pieces LEDs (D1 and D2) which indicates the heat of above 80 degrees Celsius and below 80 degrees Celsius.

A well-known IC LM35 is chosen to be the temperature sensor in this circuit. Output voltage of LM35 will increase by 10mV per degree rise in temperature. Output of LM35 is joined to the non inverting input of the operational amplifier (op-amp) of CA3130. The inverting input of the same op-amp could be provided using the required reference voltage using variable resistor R2.

If the reference voltage is 0.8V, then the voltage at the non inverting input (output of LM35) becomes 0.8V when the temperature is 80 degree Celsius. At this point the output of IC3 goes to positive saturation. This will make the transistor Q1 On and LED D1 (red LED) lights up. Because the base of Q2 is connected towards the collector of Q1, Q2 will be switched OFF and LED D2 (green) remains OFF. When the temperature is below 80 degree Celsius, the reverse condition will happen. IC1 generates a stable 5V DC operating voltage from the available 9V DC source. In case you already have got a 5V DC source then you may work with it directly.

Circuit notes:

• The circuit could be assembled on a Veroboard.
• The temperature trip point can be set by fine-tuning variable resistor of R2.
• IC3 should be mounted on a holder.
• The type of Q1 and Q2 aren’t crucial. Any common purpose NPN transistors can do it.