Tiny Dew Sensor

Dew (condensed moisture)
adversely affects the normal performance 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 authorized service cent res 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.


Circuit diagram

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 energized. 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 D1,
resistors R8 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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