Fire alarm using thermistor & NE555
Description.
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.
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.
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.
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Hi Richa this sensor is sensing the temperature of the ambient, hence once the fire severity is more only it will sense. Hence smoke detector is a better choice.
may i know the advantages n disadvantages of using fire alarm using thermistor.
thanq…
Hi TRHaslim you replace the speaker with 6 volt relay. remove NE555. delete R5 R6 C2 and C3. connect T2 R4 junction to R7. use the relay contact to operate sprinkler’s solenoid.
can i get the circuit for automatic sprinkler…..
please
how can i test the circuit when am finished with the connections.
wastage of my money n time….
this thing is nt working.
Hi Keybal any available 1K to 5K NTC (film / bead type thermistor with proper mica insulation)can be used.
what is the thermistor value?
can i use smoke detector as detector in this fire alarm ckt?
any general purpose thermistor like DO35 from US Sensors can be used here.
circuit can sense temperature changes as low as 1 degree Celsius.
My greetings to the author for posting a simple& smart circuit,but 4 things stoping from constructing this ckt.1.how much realiable is this circuit ?? 2.what value of thermistor is required ??.3.what is the sesitivity of the circuit ??4.what is the intention behind using pin no 4 instead of 2 ??
what is the thermistor value?