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Dew
(condensed moisture) ad- versely affects the normal per- formance
of sensitive electronic devices. A low-cost circuit described here
can be used to switch off any gadget automatically in case of
excessive humidity. At the heart of the circuit is an inexpensive
(resistor type) dew sensor element. Although dew sensor elements
are widely used in video cassette players and recorders, these may
not be easily available in local market. However, the same can be
procured from authorised service centres of reputed companies. The
author used the dew sensor for FUNAI VCP model No. V.I.P. 3000A
(Part No: 6808-08-04, reference no. 336) in his prototype. In
practice, it is observed that all dew sensors available for video
application possess the same electrical characteristics
irrespective of their physical shape/size, and hence are
interchangeable and can be used in this project. The circuit is
basically a switching type circuit made with the help of a popular
dual op-amp IC LM358N which is configured here as a comparator.
(Note that only one half of the IC is used here.) Under normal
conditions, resistance of the dew sensor is low (1 kilo-ohm or so)
and thus the voltage at its non-inverting terminal (pin 3) is low
compared to that at its inverting input (pin 2) terminal. The
corresponding output of the comparator (at pin 1) is accordingly
low and thus nothing happens in the circuit. When humidity exceeds
80 per cent, the sensor resistance increases rapidly. As a result,
the non-inverting pin becomes more positive than the inverting
pin. This pushes up the output of IC1 to a high level. As a
consequence, the LED inside the opto-coupler is energised. At the
same time LED1 provides a visual indication. The opto-coupler can
be suitably interfaced to any electronic device for switching
purpose. Circuit comprising diode D2, resistors R5 and R6 and
capacitor C1 forms a low-voltage, low-current power supply unit.
This simple arrangement obviates the requirement for a bulky and
expensive step-down transformer.
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