How to Build a Fire Alarm Circuit Using NE555 Timer IC
Fire injuries can be devastating, but early detection can keep lives. This easy DIY fire alarm circuit uses the NE555 timer IC along with an LDR (Light Dependent Resistor) and a thermistor to hit upon smoke and heat. Let’s explore how it works and how you could build it your self!
1. How the Circuit Works
The system operates in two ways:
Smoke Detection (LDR): When smoke blocks mild from falling at the LDR, its resistance will increase, triggering the alarm.
Heat Detection (Thermistor): At high temperatures, the thermistor’s resistance drops, activating the NE555 oscillator.
The NE555 IC is configured in astable mode, producing an audible alarm through a buzzer.
2. Components Required
| Component | Quantity |
|---|---|
| NE555 Timer IC | 1 |
| LDR (Light Dependent Resistor) | 1 |
| NTC Thermistor | 1 |
| Transistors (BC548, SL100) | 2 |
| Buzzer | 1 |
| Resistors (10KΩ, 100KΩ, etc.) | 5-6 |
| Capacitors (10µF, 0.01µF) | 2 |
| Diode (1N4007) | 1 |
| Breadboard & Jumper Wires | As needed |
3. Circuit Diagram & Connections
NE555 in Astable Mode:
Connect Pin 1 (GND) to ground.
Pin 8 (VCC) to +5V.
Pins 2 & 6 (Trigger & Threshold) are linked via R5, R6, and C2 to set frequency.
Pin 3 (Output) drives the buzzer via transistor T3 (SL100).
Thermistor & LDR Setup:
The thermistor connects to T1 (BC548) through R2 & D1.
The LDR forms a voltage divider with a fixed resistor.
Buzzer Activation:
When heat/smoke is detected, T1 & T2 activate, sending a signal to NE555 Pin 4 (Reset).
The buzzer sounds until the circuit is reset.
4. How to Test the Fire Alarm
Smoke Test: Cover the LDR to simulate smoke. The buzzer should sound.
Heat Test: Use a lighter near the thermistor. The alarm should trigger.
5. Applications & Safety Tips
✔ Home Safety: Install near kitchens or fire-prone areas.
✔ Workshop/Office: Detects electrical fires early.
✔ Testing: Check monthly to ensure functionality.
Conclusion
This NE555-based fire alarm is an easy, low-cost DIY project that enhances safety. With just a few components, you can build a reliable smoke and heat detector.
Want to customize it? Try adding an LED indicator or a relay for a siren!
