Fire alarm using in4148

Tuesday, June 16, 2015





Here is a simple fire alarm circuit built around 555 timer ic, A reverse biased germanium diode i used here as a heat sensor. At room temperature the reverse resistance of the diode-being very high (over 10 Kilohm), it produces no effect on transistor T1 which conducts and keeps the reset pin (4) of IC1 at "ground"level, and so he alarm does not sound When temperature in the vicinity of diode D1 (the sensor)increases in case of a fire the reverse resistance of D1 drops.At about 70 c its resistance drops to a value below 1 -kiliohms. This stops conduction and the IC's reset pin (4)becomes positive through resistor R2,which sounds tha alarm.


For installation of the alarm,two or three reverse biased germanium diodes connected in parallel can be kept at different locations. In case of a fire any of the diodes can sense the heat and raise the alarm. DR 25 diode works as a sensor but base-emitter junction of such germanium translstors as AC128 AC188 or 2N360 can also be used.

 

Parts list-

R1  -  1K
R2  -  4.7K
R3  -  1K
R4  -  47E
VR1- 100K(PRESET)

C1,C2  -  0.01UF
C3        -  100UF/16VOLT 

IC1     -   IC 555
T1       -  BC548
LS       -  8E SPK
D1       -  IN4148

                                                    Circuit diagram
























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