Fire Alarm Circuit Using NTC Thermistor, BC547 and 555 Tone Generator
This project is a heat-activated alarm. An NTC thermistor senses temperature, a BC547 transistor decides when the temperature has crossed a set limit, and an NE555 timer wired as an astable oscillator produces a loud tone in a small speaker. It is a good way to learn how a thermistor behaves, how to calculate a threshold from a voltage divider, and how the 555 reset pin can enable or silence an oscillator.
Components Required
| Part | Value / Type | Quantity |
|---|---|---|
| NTC thermistor TH1 | 10 kilo-ohm at 25 C (bead or disc type) | 1 |
| VR1 (threshold preset) | 47 kilo-ohm trimmer | 1 |
| R1 (Q1 base) | 1 kilo-ohm | 1 |
| R2 (reset pull-up) | 10 kilo-ohm | 1 |
| NPN transistor Q1 | BC547 | 1 |
| Timer IC U1 | NE555 with 8-pin socket | 1 |
| RA, RB (timing) | 1 kilo-ohm and 10 kilo-ohm | 1 each |
| C1 (timing) | 47 nanofarad ceramic or film | 1 |
| C2 (control pin) | 10 nanofarad | 1 |
| C3 (speaker coupling) | 100 microfarad, 16 V | 1 |
| R3 (speaker series) | 47 ohm | 1 |
| Speaker | 8 ohm, 0.5 W | 1 |
| C4 (decoupling) | 100 microfarad | 1 |
| Power | 9 V battery or 9 V adapter | 1 |
Circuit Explained Stage by Stage
Stage 1: Thermistor divider
An NTC (negative temperature coefficient) thermistor's resistance falls as it heats. A common 10 kilo-ohm part with a B constant near 3950 K reads roughly 3.6 kilo-ohm at 50 C and roughly 2.5 kilo-ohm at 60 C. Your part's B value may differ, so use these only as a guide. VR1 goes from +9 V to node A, and the thermistor goes from node A to ground. When the thermistor is cool and its resistance is high, node A sits well above 0.7 V. As it heats, node A falls.
Open-circuit voltage at node A: V_A = 9 x R_TH / (R_TH + VR1) For Q1 to switch off at about 60 C (R_TH = 2.5 k), set V_A = 0.65 V: VR1 = R_TH x (9 / 0.65 - 1) = 2.5 k x 12.8 = about 32 k At 25 C (R_TH = 10 k) with VR1 = 32 k: V_A = 9 x 10 / 42 = 2.1 V (Q1 firmly on)
Stage 2: Transistor switch
Node A drives Q1's base through R1. While the sensor is cool, Q1 is on and its collector pulls the 555 reset pin (pin 4) low against R2. A low reset pin forces the 555 output low, so the speaker is silent. When the thermistor heats past the threshold, node A can no longer hold Q1 on, Q1 switches off, R2 pulls pin 4 high and the oscillator starts.
Stage 3: 555 astable tone generator
With RA, RB and C1 connected in the standard astable arrangement, the 555 output on pin 3 is a square wave. The frequency follows the datasheet formula below. Pin 3 drives the 8 ohm speaker through R3 and C3; R3 limits the peak current and C3 blocks DC so the speaker does not sit with constant current through it.
f = 1.44 / ((RA + 2 x RB) x C1) = 1.44 / ((1 k + 20 k) x 47 nF) = about 1.46 kHz
Connections
| Point | Connects to |
|---|---|
| VR1 | +9 V to node A |
| TH1 | Node A to GND |
| Node A | R1 to Q1 base; Q1 emitter to GND |
| Q1 collector | 555 pin 4 and R2 to +9 V |
| 555 pin 8 | +9 V, C4 to GND |
| 555 pin 1 | GND |
| 555 pin 7 | RA to +9 V, RB to pins 2 and 6 |
| 555 pins 2 and 6 | Joined, C1 to GND |
| 555 pin 5 | C2 to GND |
| 555 pin 3 | R3, then C3 (positive towards R3), then speaker to GND |
How It Works
At room temperature the alarm is silent and the circuit draws a few milliamps, mainly through Q1, R2 and the 555 itself. When a flame or hot surface warms the thermistor, its resistance drops, node A falls below what Q1 needs, Q1 turns off, the 555 reset pin goes high and a tone of roughly 1.5 kHz sounds. When the thermistor cools, Q1 turns back on and the alarm stops.
Because a single transistor switches gradually, the tone may start weakly just at the threshold. In practice heat from a real fire rises quickly, so the transition is fast. For a sharper threshold, replace Q1 with an LM358 comparator.
Assembly and Testing
- Build the 555 oscillator first with pin 4 tied directly to +9 V. You should hear a steady tone. Then remove that link.
- Add Q1, R2 and the thermistor divider. Mount the thermistor on two wires so it can reach the place you want to monitor, away from the board.
- Set VR1 to about 32 kilo-ohm, measured with the power off. At room temperature the speaker should be silent.
- Warm the thermistor with a hair dryer or by holding it near, not in, a soldering iron. The alarm should sound within seconds.
- Fine-tune VR1: increasing it lowers the trip temperature, decreasing it raises it.
- Check the alarm stops once the sensor cools.
Troubleshooting
- Alarm sounds all the time: node A is below 0.7 V even when cool. Decrease VR1, and check the thermistor is NTC, not PTC.
- Alarm never sounds: measure pin 4 while heating the sensor. If it stays low, increase VR1 so Q1 turns off at a lower temperature.
- Tone is weak: reduce R3 to 22 ohm or use a piezo transducer, which is louder for the same power.
- Alarm triggers in hot weather: Q1's base-emitter voltage also changes with temperature. Keep the board away from the sensor and set the trip point well above the highest room temperature.
- No tone even with pin 4 high: check the 555 timing parts, pin 5 capacitor and supply decoupling.
Summary
A thermistor divider converts temperature into voltage, a BC547 turns that voltage into an on or off decision, and the 555 reset pin gates an astable tone generator. You calculated the trip point from the divider equation and the tone frequency from the 555 astable formula, then calibrated the real circuit with gentle heat.