Hello friends, today we are going to make one simple mini project on fire alarm. In this mini project we will make a fire alarm circuit using timer IC (IC555) and thermistor. Here we are using a thermistor sensor to sense the fire. The working principle of fire alarm circuit depends upon working principle of thermistor (i.e. thermistor changes its resistance as temperature changes). Thus when temperature of thermistor increases because of fire its resistance also changes.
Circuit diagram:
Following figure shows the circuit diagram of fire alarm using IC555 mini project:
Components:
Following components are required for making fire alarm circuit (mini project).
- VR1 = 10 K ohms Variable Resistor which is used for varying the sensitivity of circuit.
- R3, R7, R8 = 470 Ω
- R2 = 33 K Ω
- R4 = 560 Ω
- R5 = 47 KΩ
- R6 = 2.2 KΩ
- C1 = 10 µF/16V
- C2 = 0.04 µF
- C3 = 0.01 µF
- IC1 = NE555 (timer IC)
- T1 = BC548
- T2 = BC558
- T3 = SL100B or (NPN transistors like 2N4922 , 2N4921,2N4238)
- D1 = 1N4001
- TH1 = Thermistor 10 KΩ
- LS1 = 8 Ω, 1W speaker
Working of fire alarm circuit:
In this fire alarm mini project three components are very important i.e. thermistor, IC 555 (which is timer IC) and third one is transistor which is used to drive the circuit.
The timer IC used in this circuit works as an astable multivibrator oscillator which is used to oscillate in audio frequency band. As stated earlier timer IC is driven by two transistors T1 and T2. The frequency of timer IC 555 depends upon the values of resistors R5 and R6 and capacitor C2. The loudspeaker which is used for fire alarm is connected to the pin no 3 of timer IC through transistor T3.
When temperature of thermistor increases it causes a low resistance path for a current flowing towards base of transistor.
For this type of fire alarm circuit we can use a power supply of 6V to 12V.
Tips:
1) Thermistor do not have polarity.
2) Following figure shows the pin out diagram of timer IC 555:
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