Apr 282015

Temperature gauge circuit using the LM324 IC

The temperature level is displayed using 4 LEDs.

  • – A green LED indicates that the temperature is in the desirable level
  • – Two yellow LEDs indicate that the temperature is higher than normal, but not dangerous
  • – A red LED indicates that the temperature is very high and we need to do something.

To complement the visual warning of a high temperature, a buzzer is included in the circuit. The
buzzer will sound to alert if there is an emergency. The temperature gauge circuit is implemented using four comparators. We use the LM324 integrated circuit having four operational amplifiers (Op Amp) 741 in a single package

Temperature gauge using the LM324 IC

The temperature gauge circuit diagram shows a voltage divider network made with R2, R3, R4, R5 and R6 resistors. The voltages obtained are 2.4V, 4.8V, 7.2V, 9.6V. Each of these voltages is connected directly to the non-inverting pin (+) of the operational amplifiers used as comparators. The upper terminal of the thermistor (R10) is directly connected to all inverting terminals (-) of the operational amplifiers. If there is a temperature change, the voltage varies at the upper terminal of the thermistor.

The voltage, on the upper terminal of the thermistor, is compared to the voltage that the comparators have at its non-inverting terminal. If this voltage is less than the voltage obtained by the voltage divider, we will have at the output of the corresponding comparator, a voltage that will activate  the LED.

The higher the temperature, the lower the thermistor voltage will be and more LEDS will light. When the temperature is very high, the lower comparator turns on the red LED and activates the buzzer, which gives the audio alert that is very important.

Printed circuit board for the temperature gauge circuit

The second diagram shows the set of tracks (on both sides of the printed circuit board) and the
distribution of the components. This diagram is not to scale.

PCB for the temperature gauge circuit

The next two diagrams show the tracks on the copper and the component side of the circuit board. These are suggested diagrams.

Temperature gauge circuit PCB

Temperature gauge circuit components list

  • 1  LM324 IC (four operational amplifiers 741 in a single package)
  • 1  10K NTC Thermistor (R10)
  • 5  5K resistors (R2, R3, R4, R5, R6)
  • 1  10K resistor
  • 4  220 ohms resistors (R7, R8, R9, R11)
  • 4  LEDs, 1 green, 2 yellow, 1 red
  • 1  buzzer


  • Enlarge the thermistor terminals, so that it can be placed on the place where you want to know the temperature.
  • NTC (negative temperature coefitient)

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