May 022022

2 LED Temperature change indicator

This 2 LED Temperature change indicator shows us, by means of two LEDs, when the sensed temperature is above or below a set limit. When the temperature is above the preset temperature level, LED D1 will light up and if it is below, LED D2 will light up.

How does 2 LED Temperature change indicator work?

The temperature sensor used is the LM35. The LM35 increases its output voltage by 10mV for each degree Celsius (°C) increase in the temperature sensed.

The potentiometer R2 is used to set the voltage that will change the operational amplifier to the positive saturation state and that will turn on the red LED. When the output voltage of the LM35 is equal to or greater than the voltage set by the potentiometer connected to the operational inverter pin.

2 LED Temperature change indicator with lm35 & 741

2 LED Temperature Change Indicator with LM35 & 741

The operational amplifier changes to the positive saturation state, saturating the transistor Q1 as well. With transistor Q1 in saturation mode, LED D1 turn ON, transistor Q2 enters the cut-off mode and LED D2 turn off.

When the output voltage of the LM35 is less than the voltage set by the potentiometer connected to the operational amplifier inverter pin, it changes to the negative saturation state and cause transistor Q1 to go into cut off mode (not conducting), and LED D1 turn off.

The cutoff mode of Q1 causes the transistor Q2 to go into saturation mode and LED D2 lights up.

741 Op Amp Pin Out

For example: We want to detect when the temperature is equal to or greater than 50 °C

If the reference voltage is 0.5V (inverter pin set to activate the operational amplifier when the temperature reaches 50 °C) and the voltage at the non-inverting input of the operational amplifier (LM35 output) is 0.5V (the temperature is 50 °C), then:

The output of IC2 goes to positive saturation. This causes transistor Q1 to saturate and D1 to turn on. As the collector of Q1 is connected to the base of Q2, it will cause it to enter into cut.

When the temperature is below 50 °C, the operation of the operational amplifier and transistors is reversed. So D1 LED turns off and D2 LED lights up.

List of circuit components

  • 1 LM35 temperature sensor (IC1)
  • 1 741 Operational Amplifier (IC2)
  • 2 2N2222 NPN transistors or similar (Q1, Q2)
  • 3 10K resistors (R1, R3, R5)
  • 2 220 ohms resistors (R4, R6)
  • 1 5K potentiometer (R2)
  • 1 10 uF (microfarad) electrolytic capacitor (C1)
  • 2 LEDs, 1 red (D1), 1 green (D2)

Note: The circuit must use a 5 VDC power supply.

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