Showing posts with label thermistor. Show all posts
Showing posts with label thermistor. Show all posts
Thursday, April 11, 2013
Fire alarm using thermistor NE555
Many fire alarm circuits are presented
here,but this time a new circuit using a thermistor and a timer to do
the trick. The circuit is as simple and straight forward so that, it
can be easily implemented. The thermistor offers a low resistance at
high temperature and high resistance at low temperature. This
phenomenon is employed here for sensing the fire.
here,but this time a new circuit using a thermistor and a timer to do
the trick. The circuit is as simple and straight forward so that, it
can be easily implemented. The thermistor offers a low resistance at
high temperature and high resistance at low temperature. This
phenomenon is employed here for sensing the fire.
The
IC1 (NE555) is configured as a free running oscillator at audio
frequency. The transistors T1 and T2 drive IC1. The output (pin 3) of
IC1 is couples to base of transistor T3 (SL100), which drives the
speaker to generate alarm sound. The frequency of NE555 depends on the
values of resistances R5 and R6 and capacitance C2. When thermistor
becomes hot, it gives a low-resistance path for the positive voltage to
the base of transistor T1 through diode D1 and resistance R2.
Capacitor C1 charges up to the positive supply voltage and increases
the the time for which the alarm is ON. The larger the value of C1, the
larger the positive bias applied to the base of transistor T1 (BC548).
As the collector of T1 is coupled to the base of transistor T2, the
transistor T2 provides a positive voltage to pin 4 (reset) of IC1
(NE555). Resistor R4 is selected s0 that NE555 keeps inactive in the
absence of the positive voltage. Diode D1 stops discharging of
capacitor C1 when the thermistor is in connection with the positive
supply voltage cools out and provides a high resistance path. It also
inhibits the forward biasing of transistor T1.
Circuit diagram with Parts list. IC1 (NE555) is configured as a free running oscillator at audio
frequency. The transistors T1 and T2 drive IC1. The output (pin 3) of
IC1 is couples to base of transistor T3 (SL100), which drives the
speaker to generate alarm sound. The frequency of NE555 depends on the
values of resistances R5 and R6 and capacitance C2. When thermistor
becomes hot, it gives a low-resistance path for the positive voltage to
the base of transistor T1 through diode D1 and resistance R2.
Capacitor C1 charges up to the positive supply voltage and increases
the the time for which the alarm is ON. The larger the value of C1, the
larger the positive bias applied to the base of transistor T1 (BC548).
As the collector of T1 is coupled to the base of transistor T2, the
transistor T2 provides a positive voltage to pin 4 (reset) of IC1
(NE555). Resistor R4 is selected s0 that NE555 keeps inactive in the
absence of the positive voltage. Diode D1 stops discharging of
capacitor C1 when the thermistor is in connection with the positive
supply voltage cools out and provides a high resistance path. It also
inhibits the forward biasing of transistor T1.
Notes.
- The circuit can be powered from a 6V battery or a 6V power supply.
- Click Here ! for the circuit diagram of a power supply circuit for this project.
- The thermistor can be mounted on a heat resistant material like mica to prevent it from damage due to excessive heat.
- The LED acts as an indication when the power supply is switched ON.
Sunday, March 24, 2013
Fire Alarm Using Thermistor
Small and simple unit, Can be used for Home-Security purpose
In this fire alarm circuit, a Thermistor works as the heat sensor. When temperature increases, its resistance decreases, and vice versa. At normal temperature, the resistance of the Thermistor (TH1) is approximately 10 kilo-ohms, which reduces to a few ohms as the temperature increases beyond 100 C. The circuit uses readily available components and can be easily constructed on any general-purpose PCB.
Circuit Diagram:
Fire Alarm Using Thermistor Circuit Diagram| Parts | Description |
| R1 | 470R |
| R2 | 470R |
| R3 | 33K |
| R4 | 560R |
| R5 | 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 |
Timer IC NE555 (IC1) is wired as an astable multivibrator oscillating in audio frequency band. Switching transistors Q1 and Q2 drive multivibrator IC1. The output of IC1 is connected to NPN transistor Q3, which drives the loudspeaker (SPKR) to generate sound. The frequency of IC1 depends on the values of resistors R6, R7 and capacitor C2. When Thermistor TH1 becomes hot, it provides a low-resistance path to extend positive voltage to the base of transistor Q1 via diode D2 and resistor R3. Capacitor C1 charges up to the positive voltage and increases the ‘on’ time of alarm. The higher the value of capacitor C1, the higher the 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 provides positive voltage to reset pin 4 of IC1. R5 is used such that IC1 remains inactive in the absence of positive voltage. D2 stops discharging of capacitor C1 when the Thermistor connected to the positive supply cools down and provides a high-resistance (10k) path. It also stops the conduction of Q1. To prevent the Thermistor from melting, wrap it up in mica tape. The circuit works off a 6V-12V regulated power supply. D1 is used to indicate that power to the circuit is switched on.
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