Feb 192019
 

How to make a car temperature alarm?

This car temperature alarm check the car engine temperature and warns us, by means of a sound signal of a certain frequency, when the temperature exceeds a seted value. This alarm uses the well-known integrated circuits 741 operational amplifier and 555 timer

How the car temperature alarm works?

The circuit has two parts:

The first part has an operational amplifier that works as a comparator. The operational amplifier compares the two voltage signals that are explained below:

1. The voltage obtained from the junction of the thermistor (R2) and the resistance R3 (voltage divider), which is applied to the inverting input of the comparator. This voltage is proportional to the engine temperature and …

2. The voltage obtained from potentiometer P1, which is applied to the non-inverting input of the comparator. P1 is used to adjust the temperature level that activates the sound in the speaker.

When the temperature is equal to or higher than the maximum set for the engine, the output voltage of the comparator goes “high” and enables the operation of the 555 timer (the oscillator).

Car Temperature Alarm using 741 & 555 ICs

741 Op-Amp Pinout

741 Op-Amp Pinout

The second part has a 555 timer that works in astable mode (as an oscillator). The 555 outputs a square wave to a small speaker through an electrolytic capacitor at its output. (C2)

The sound of the speaker is what gives the warning signal. Potentiometer P2 allows us to change the frequency of the speaker sound.

555 timer IC Pinout

555 timer Pinout

The thermistor used in this circuit has a negative temperature coefficient (NTC).

List of components for the car temperature alarm

  • 1  4.7K resistor (R1)
  • 1  33K resistor (R3)
  • 1  NTC thermistor (R2)
  • 2  100K potentiometers (P1, P2)
  • 1  0.01uF 63V polyester capacitor (C1)
  • 1 100uF, 16V electrolytic capacitor (C2)
  • 1  741 operational amplifier (IC1)
  • 1  555 timer (IC2)
  • 1  8 ohms miniature speaker.

The thermistor (R2) can be placed close to the car’s temperature sensor.

This circuit can also be used as an alarm for devices that operate at high temperatures such as ovens, which must not exceed a maximum temperature.

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