Circuit Diagram
Description
An reverse biased germanium diode is used here as a heat sensor at
normal room temperature the reverse resistance of the diode is very high
in the order of over 10 kilo ohms so it produce no effects on the
transistor Q1 which conducts and keeps the reset pin 4 of ic 555 at its
ground level, and so the alarm doesn’t get activated.
When the temperature in the vicinity of the diode (the sensor)
increases in case of fire, the reverse resistance of the germanium diode
drops at about 70 degree its resistance drops to a value below 1 kilo
ohms this stops Q1 conduction and the 555 ic pin 4 becomes positive
through the resistor R1 which activates the alarm.
Note:
For
installation of the alarm two or three reverse biased germanium diode
can be connected in parallel at different locations in case of fire any
of the diode can sense the heat and can activate the alarm.
DR25 Germanium diode is recommended for use but still it can be replaced with base emitter junctions of germanium transistors such as AC128, AC188 OR 2N360.
DR25 Germanium diode is recommended for use but still it can be replaced with base emitter junctions of germanium transistors such as AC128, AC188 OR 2N360.
+ comments + 2 comments
What is DR25 germanium diode?Does DR25 indicates it model as 1n4007 is one of PN jucntion diodes
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