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Build Your Fire Alarm: Thermistor Circuit Diagram (10K)
Description
Within this fire detection circuit, a Thermistor functions as the heat sensor. As temperature rises, its resistance decreases, and conversely, when temperature drops, its resistance increases. At ambient temperatures, the resistance of the Thermistor (TH1) is approximately 10 kilo-ohms, diminishing to just a few ohms when the temperature exceeds 100 degrees Celsius. The design utilizes commonly available components and can be readily constructed upon any general-purpose Printed Circuit Board.
Integrated Circuit Details: NE555
The NE555 is a versatile integrated circuit frequently employed as an astable multivibrator. This IC generates a continuous square wave signal, making it ideal for timing circuits and oscillators. Its operation relies on internal comparator circuits and a discharge transistor, providing precise frequency control. Its widespread availability and ease of use contribute to its popularity in various electronic applications.
Circuit Diagram:
Parts:
- R1 = 470R
- R2 = 470R
- R3 = 33K
- R5 = 560R
- R4 = 470R
- R6 = 47K
- R7 = 2.2K
- R8 = 470R
- C1 = 10uF-16V
- C2 = 0.04uF-63V
- C3 = 0.01uF-63V
- Q1 = BC548
- Q2 = BC558
- Q3 = SL100B
- D1 = Red Led
- D2 = 1N4001
- IC1 = NE555
- SPKR = 1W-8R
- TH1 = Thermistor-10K
Circuit Operation:
The timer IC NE555 (IC1) is configured as an astable multivibrator, oscillating at an audio frequency range. Switching transistors Q1 and Q2 drive the multivibrator IC1. The output of IC1 is connected to an NPN transistor Q3, which in turn drives the loudspeaker (SPKR) to produce a sound. The frequency of IC1 is determined by the values of resistors R6, R7 and capacitor C2. When Thermistor TH1 becomes hot, it provides a low-resistance path, extending a positive voltage to the base of transistor Q1 via diode D2 and resistor R3. Capacitor C1 charges up to the positive voltage, increasing the ‘on’ time of the alarm. A higher value of capacitor C1 translates to a greater forward voltage applied to the base of transistor Q1. Since the collector of transistor Q1 is connected to the base of transistor Q2, transistor Q2 delivers a positive voltage to the reset pin 4 of IC1. Resistor R5 is utilized to ensure IC1 remains inactive when no positive voltage is present. Diode D2 prevents the discharge of capacitor C1 when the Thermistor connected to the positive supply cools down and provides a high-resistance (10k) path. It also inhibits the conduction of Q1. To safeguard the Thermistor from overheating, it is recommended to wrap it in mica tape. The circuit operates on a 6V-12V regulated power supply. Diode D1 indicates the presence of power to the circuit.