|
Tiny
Dew Sensor
|
| 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. |
|
|
Click on the Image for its larger version
|
