Dew
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.
Download Skema :
Detector Air Humidity
Password : asinan
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.
