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Sensitive
Temperature Switch
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temperature switch has a high sensitivity and is quite
reliable. Here, in place of a single transistor, a Darlington
pair has been used for switching. At normal room temperature
germinum diode D1 (OA79 or equivalent) has a back resistance
value of about 10 kilo-ohms. As a result Darlington pair,
comprising transistors T1 and T2, conducts and keeps the
anode terminal of diode D2 at ground potential. Consequently,
transistor T3 does not get base bias and thus relay RL1
is not activiated. But when temperature increases, the
back resistance of diode D1 decreases sharply, which results
in cutting off of Darlington pair and forward biasing
of transistor T3 via resistor R2 and diode D2. As a result,
relay RL1 energises and switches on the alarm. Potmeter
VR1 may be adjusted for required sensitivity. This simple
circuit can be used as an overheat indicator, fire alarm,
or it can be used in a constant temperature circuit for
switching on a fan etc. The circuit can be easily assembled
on a piece of veroboard. Diode sensor D1 must be of germanium
type and not silicon. |
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