Fire Alarm Circuit using Thermistor and IC 555

In fire time, the Fire Alarm Circuit using Thermistor and IC 555 help to find and warn people.

This design uses IC 555 and thermistor to check temperature change and maybe fire.

Thermistor is special part which feels the temperature change.

Circuit is easy to make and thermistor changes resistance with heat.

If hot then resistance goes low and if cold then resistance goes high and this help to know if fire happens.

Circuit Working:

Parts List:

In this circuit IC1 555 timer work like sound oscillator.

IC1 get power from transistors T1 and T2.

IC1s pin 3 connect to transistor T3 2N2222 base.

T3 make speaker give alarm sound.

Capacitor C2 and resistors R5, R6 control how fast IC1 oscillate.

When thermistor Th1 gets hot its resistance goes low.

Then positive voltage goes through R2 and diode D1 to transistor T1 base.

Capacitor C1 charges and keep alarm sound longer.

Bigger C1 means longer sound time.

T2 give positive voltage to pin 4 of IC1 which is reset pin when base and collector connect.

Resistor R4 keep IC1 OFF when there is no positive voltage.

If Th1 cools down then resistance goes high.

And Diode D1 stops C1 from losing charge.

Thermistor should be on heat proof material like mica to avoid damage.

It also stops T1 from turning ON when not needed.

LED turns ON to show power is ON.

We can use 6V supply or 6V battery to run this circuit.

Formulas:

Using this circuit we can make this simple fire alarm with IC 555 and thermistor.

We can also decide how it works and which parts to use.

To build this we should use basic formula:

IC 555 in Monostable Mode:

To find output time T we can use this formula:

T = 1.1 × R × C

Choose values for resistor R5, R6 and capacitor C2 to set alarm time.

Example:

If R5 = 47k, R6 = 2.2k and C2 = 0.04μF then we should use formula to get time T.

Note:

This alarm is made using basic rules and formula.

Part values like resistors, capacitors, transistors depend on how sensitive and how it should work.

We might need to change some parts based on real test and environment where circuit is used.

How to Build:

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

• Put all parts together like circuit design shows.
• Pin 1 of IC 555 goes to ground.
• Pin 2 connect to pin 6 of IC 555.
• Pin 3 connect to base of transistor T3 using resistor R7.
• Pin 4 connect between collector of T2 and resistor R4.
• Pin 5 connect to ground through capacitor C3.
• Pin 7 connect to Pin 6 through resistor R5 and capacitor C2 goes to ground.
• Also resistor R6 goes from pin 7 to +6V supply.
• Pin 8 connect to +6V power.
• Resistor R1 and LED1 goes in series between +6V supply and ground.

T1 Transistor Connections:

• Collector of T1 connect to base of T2.
• Base of T1 goes to ground through resistor R2 and capacitor C1.
• Emitter of T1 goes to ground.

T2 Transistor Connections:

• Collector goes to ground through resistor R4.
• Base connect to collector of T1.
• Emitter goes to +6V supply.
• Diode D1 connects through resistor R2 and resistor R8 to ground.
• Thermistor (Th1) goes between D1 and +6V supply.

T3 Transistor for speaker Connection:

• Collector goes to one side of 8 ohm speaker and other side of speaker to +6V.
• Base of T3 connect to pin 3 of IC 555.
• Emitter goes to ground.

Conclusion:

This Fire Alarm Circuit using Thermistor and IC 555 is cheap and works well.

When temperature goes high then thermistor change resistance and circuit turns ON alarm sound.

It is easy to make useful and good for places where early fire warning is important.

References:

Performance characteristics and assessment of fire alarm system