to leave it connected with the car battery.
This simple circuit will be a great idea to get alert before the engine overheats so one can check what actually happening or if the vehicle needs to be cool down. Normally there is a temperature gauge in all vehicles but sometimes we forget to notice it so it's always good to have a sound indication.

The circuit shown here is using two ICs which are an LM358 that is a dual operational amplifier IC and a NE555 timer IC. The circuit can be divided in two sections, the first section of the circuit is temperature sensor built around an LM358 IC, the LM358 IC is working as comparator in the circuit and the second section is buzzer circuit built around a NE555 IC which is working as an astable multivibrator here. The beeping rate and the LED's flashing rate can be adjusted with the help of 100K variable resistor used between NE555 IC's pin 6 & 7, and the desired level of temperature on which you want to activate the buzzer can be selected with the 100K variable resistor connected with the LM358 IC. For detecting changes in the temperature a 10K NTC thermistor is used.

The circuit needs to be calibrated before using it for the first time. For doing so you will need an infrared non-contact temperature meter. After completing the circuit connect it with a 12V fully charged battery (but not with the car battery just connect it with a separate battery). Then attach the thermistor with some kind of iron (metal Sheet) or metal scale and provide heat to the scale from few inches away with a lighter etc. during this also measure the temperature of the metal near the thermistor. When the temperature of the metal reaches 200 then adjust the 100K variable resistor connected with the LM358 until the buzzer become activated. You can also select any desired temperature level between 100 to 200 centigrade on which you want to activate the buzzer. After completing the calibration the circuit will be ready to use. Fit it in a suitable enclosure and attach the thermistor with the engine's body.  

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This circuit alerts when the engine of a vehicle like, car, jeep, bus etc. will get overheat or its temperature rises above the safe level. The circuit will alert by beeping a buzzer and flashing an LED. The circuit drains a very low amount of current that is around 400uA in the standby mode so it's ok
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