Simple Fire Alarm Circuit using Thermistor and IC 555

In the event of a fire, fire alarm circuits are essential for identifying and warning residents.

This specific design uses an IC 555 and a thermistor to detect temperature variations that might be signs of a fire.

One important kind of temperature sensitive resistor for detecting changes in temperature is the thermistor.

Building the circuit is simple, the resistance of a thermistor varies with temperature.

When it is hot, its resistance is low, and when it is cold, it is high, this characteristic helps us identify fire.

Circuit Working:

Parts List:

The above circuit diagram, the IC1 555 functions as a free running oscillator at an audio frequency.

To power IC1, transistors T1 and T2 are used.

Pin 3 of IC1s output is linked to transistor T3s 2N2222 base, which powers the speaker and emits the warning sound.

Capacitor C2 and resistors R5 and R6 determine the oscillation frequency of the IC 555.

The positive voltage travels via resistor R2 and diode D1 to reach the base of transistor T1 via the low resistance route created by the heating thermistor Th1.

By charging to the positive supply voltage, capacitor C1 prolongs the time that the alarm sounds.

A stronger positive bias is given to transistor T1s base when C1 is bigger.

When transistor T2s base and collector are connected, transistor T2 produces a positive voltage that is applied to pin 4 of IC1 555, which is the reset pin.

In order to keep IC 555 inactive in the absence of a positive voltage, resistor R4 is selected.

When the Th1 thermistor, which is coupled to the positive supply voltage, cools down and provides a high resistance route, diode D1 stops capacitor C1 from discharging.

To shield the thermistor from harm from extreme heat, it can be installed atop a substance that resists heat, such as mica.

It also prevents transistor T1s forward biasing.

When the power supply is turned on, the LED serves as a signal.

Either a 6V power supply or a 6V battery can power the circuit.

Formulas:

With the help of above circuit diagram, we can create a basic fire alarm circuit that uses an IC 555 and thermistor.

We can also specify the criteria for functionality and components.

To construct such a circuit, use the following fundamental formulas:

IC 555 Setup as a Monostable Multivibrator:

For IC 555, get the values of the timing capacitor C and resistor R.

The following formula may be used to find the output pulses time period T in monostable mode:

T = 1.1 × R × C

where,

To specify the desired alarm duration, select resistors R5, R6 and capacitor C2.

For instance, the time period T may be computed using the above method if R5 = 47k, R6 2.2k and C2 = 0.04μF.

Note:

A fire alarm circuit using an IC 555 and thermistor is designed with the fundamental design principles outlined in this formulaic method.

Based on the intended sensitivity and the circuits operational characteristics, the resistor, capacitor and transistor biasing values will be determined.

The circuits deployment sites environmental factors and results from real world testing may call for modifications.

How to Build:

To build a Simple Fire Alarm Circuit using Thermistor and IC 555 you need to follow the below mentioned assembling steps:

• Assemble all the components required for above mentioned circuit design.
• Connect pin 1 of IC 555 to ground,
• Connect pin 2 of IC 555 to pin 6 of IC 555.
• Connect pin 3 of IC 555 to base of transistor T3 through resistor R7.
• Connect pin 4 of IC 555 between collector of transistor T2 and resistor R4.
• Connect pin 5 of IC 555 to ground through capacitor C3.
• Connect pin 7 of IC 555 to pin 6 of IC 555 through resistor R5 and capacitor C2 to ground, connect a resistor R6 from pin 7 to positive supply.
• Connect pin 8 of IC 555 to positive supply of +6V.
• Connect resistor R1 and LED1 in series from positive supply to ground.
• Connect transistor T1s collector pin to base pin of transistor T2, connect base pin of transistor T1s to ground via a resistor R2 and capacitor C1, connect emitter pin of transistor T1 to ground.
• Connect collector pin of transistor T2 to ground through resistor R4, connect base pin of transistor T2 to collector pin of transistor T1, connect emitter pin of transistor T2 to positive supply.
• Connect a diode through resistor R2 and a resistor R8 to ground, connect a thermistor from diode D1 to positive supply.
• Connect a collector pin of transistor T3 to one end of 8 ohm speaker and other end of speaker to positive supply, connect base pin of transistor T3 to pin 3 of IC 555, and connect a emitter of transistor T3 to ground.

Conclusion:

To conclude, the fire alarm circuit that uses an IC 555 and thermistor provides a dependable and affordable way to identify fire threats.

The circuit efficiently sounds an alarm as the temperature rises, signaling a possible fire, by utilizing the temperature sensitive characteristics of the thermistor and the timing capabilities of the IC 555.

This design strikes a good mix between utility and simplicity, making it appropriate for a range of uses where early fire detection and warning are vital for public safety.

References:

Performance characteristics and assessment of fire alarm system