Temperature gauge circuit using LM324 IC
On this temperature gauge circuit, 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 741 operational amplifiers (Op Amp) in a single package.
The temperature gauge circuit diagram shows a voltage divider network made with R2, R3, R4, R5 and R6 resistors. 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 thermistor upper terminal.
The thermistor upper terminal voltage is compared to the non-inverting terminal voltage on each comparator. If this is less than the voltage obtained by the voltage divider, the output of the corresponding comparator will light the LED.
The higher the temperature, the lower the thermistor voltage will be, and more LEDs will light. When the temperature is too 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.
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 components list
- 1 LM324 IC (four 741 operational amplifiers in a single package)
- 1 10 K NTC Thermistor (R10)
- 5 5 K resistors (R2, R3, R4, R5, R6)
- 1 10 K resistor
- 4 220 ohms resistors (R7, R8, R9, R11)
- 4 LEDs (1 green, 2 yellow, 1 red) (D1, D2, D3, D4)
- 1 buzzer (BZ)
- You have to enlarge the thermistor terminals, so that it can be placed where you want the temperature to monitor.
- NTC (negative temperature coefficient)