Car Engine Overheating Alarm Circuit

To develop an overheating alarm for cars is simple and it doesn’t take much effort. Furthermore, this is one effective way to detect if your car engine is getting over-heated. While most of the new range automobiles do have that features but not all has the benefit.

However, the following section will detail the process behind development of an overheating alarm system.


car engine over heating alarm circuit


The aforesaid circuit diagram shows the way the circuit need to be designed. As per the figure, when the engine temperature rises the thermistor decreases. The thermistor is further attached to R1 in order to distribute the power-supply voltage to Pin 2 [ICI-LM741]. Furthermore, the voltage decreases as the temperature increases. The adjustment of the voltage is done by VR1 – 100K on Pin 3 [IC1]. When the voltage on Pin 2 go low than Pin 3, the IC1 gets switched to make the voltage output on Pin 6 in order to send to IC2.

The IC2 is connected and used as an astable multi-vibrator to the circuit that generates sound signal. The level of temperature that makes the sound can be adjusted with the help of VR1, whereas, on the other hand, the VR2 will manage sound frequency. The thermistor used in this circuit act as beads and is negative temperature coefficient [NTC]. A thermistor is such a device that is resistant to heat. Here the circuit current is set up-to 10mA.

List of Components

Following are the components that you need to design the circuit:


  • R1 [4.7k] – 1 piece.
  • R2 [22k] – 1 piece
  • RTH1 [NTC Type Thermistor] – 1 piece.
  • VR1, VR2 [Variable Resistor of 100k] – 2 pieces


  • C1 [0.01 uF 63V Polyester – 1 piece
  • C2 [100uF 16v Electrolytic Capacitor] – 1 piece


  • ZD1 [Zener Diode 5.1v] – 1 piece
  • IC1 [LM741 op-lamp] – 1 piece
  • IC2 [NE555 timer] – 1 piece

Loudspeaker [8 ohms] – 1