In this tutorial, we are going to make a “Fire Alarm Circuit”.
Things that are necessary for doing work anywhere, like workplaces, homes, or streets; safety matters the most. A safe workplace reduces injury and illness expenses and increases productivity and quality. Safety comes with precautions that’s why people are installing alarms on every level of the home, industries, etc.
The fire alarm is one of those alarms, to immediately protect surroundings from fire. Here we design a simple fire alarm circuit by using a 10K NTC (Negative Temperature Coefficient) thermistor and timer IC 555. This simple fire alarm circuit can be used as an indicator or warning circuit for excessive fire and temperature, to connect or disconnect the target device you can connect a relay at the output. It’s a circuit that detects a fire, or smoke and immediately activates the siren. Thus, detection at the right time can prevent human injuries, damages, and properties. When the temperature near the thermistor raises beyond the threshold, it will produce a buzzer sound and this threshold limit can be varied by using a variable resistor.
Hardware Required
| S.no | Component | Value | Qty |
|---|---|---|---|
| 1. | Thermistor NTC | 103 | 1 |
| 2. | IC | NE555 Timer | 1 |
| 3. | Buzzer | 5V | 1 |
| 4. | Transistor | BC547 | 1 |
| 5. | Variable Resistor | 100KΩ | 1 |
| 6. | Resistor | 100KΩ | 2 |
| 7. | Resistor | 1KΩ, 4.3KΩ, 10Ω | 1,1,1 |
| 8. | Capacitor | 10μF / 16V, 0.01μF | 1,1 |
| 9. | Connecting Wires | – | – |
| 10. | Battery | 9V | 1 |
Circuit Diagram
Working Explanation
The key component of the circuit is the thermistor, which has been used as a fire detector or fire sensor. A thermistor is temperature sensitive resistor, whose resistance changes according to the temperature, its resistance decreases with the increase in temperature and vice versa. In this circuit, we have used a 10K value thermistor that comes in NTC (Negative Temperature Coefficient) which means this thermistor resistance across two terminals is greatly reduced by heat. And thermistor sensitivity level can be varied by VR1 and you need to calibrate it according to your need. Here the 555 timers IC has been configured in the astable mode so that the alarm (Buzzer) can produce an oscillating sound. Timer IC 555 circuit reset pin is connected to the thermistor through transistor Q1. In astable mode, capacitor C charges through resistance R2 and R3, till 2/3 Vcc, and discharges through R3 till it reaches to 1/3Vcc. During the charging time, OUT PIN 3 of 555 IC remains HIGH and during discharging it remains LOW, that’s how it oscillates. We have connected a buzzer to the OUT pin so that it produces a beep sound. We can control the oscillation frequency of the alarm by adjusting the value of R3 and/or capacitor C. Now when there is no fire and the temperature is near the thermistor, the timer IC 555 remains reset. When the temperature rises, the NTC thermistor changes its resistance, allowing current to flow. The timer IC 555 oscillates square pulses and the buzzer element produces sound. For proper working, this circuit sensing element thermistor should be exposed to fire or temperature.
Applications
Can be installed in homes, offices, museums, and everywhere where safety is concerned
