Heat / Temperature Sensor Alarm using NE555

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Today we are going to demonstrate one of the easiest heat or temperature sensor alarms. The most interesting feature of this circuit is that it gives visual as well as sound indications when the temperature is sensed. This circuit is using a 555 timer IC which is one of the most popular ICs in the world of electronics. It provides oscillations and short/long time delays in a circuit. You can easily find this IC in stores and online and it is inexpensive as well therefore it is widely used.

The sensing is done by a thermistor. The thermistor is a resistance thermometer whose resistance decreases with the increase in temperature. Other components used in this circuit are a variable resistor for adjustments, an LED for visual representation, a piezo buzzer for sound indication, resistors, and a capacitor.

Hardware Components

The following components are required to make Temperature Sensor Alarm Circuit

S.no	Component	Value	Quantity
1.	DC Supply	9-12V	1
2.	IC	NE555 Timer	1
3.	Thermistor	1M	1
4.	Resistor	1K, 470Ω	1, 1
5.	Variable Resistor	1K	1
6.	Electrolytic Capacitor	100µF	1
7.	LED	–	1
8.	Buzzer	–	1

NE555 IC Pinout

For a detailed description of pinout, dimension features, and specifications download the datasheet of 555 Timer

Temperature Sensor Alarm Circuit

Working Explanation

The operating voltage of this circuit is 9 to 12V. When the thermistor senses temperature or heat, its resistance is decreased and it lets the current pass through the circuit. This activates the 555 timer IC and the circuit produces a beeping signal and flashes the LED in the gap of a few seconds. When the temperature increases near the thermistor the beeping signal and LED flashing rate will also increase.

The time period of the flashing LED and beeping buzzer is selected by the value of the resistor and capacitor used at the input of the 555 IC. A variable resistor of 1K is used to adjust the volume of the piezo buzzer