TINY DEW SENSOR

This is a circuit for a dew sensor that turns off electrical appliances in case of excessive humidity. -- T.K. HAREENDRAN

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Dew (condensed moisture) adversely affects the normal performance of sensitive electronic devices. A low-cost circuit for a dew sensor described here can be used to switch off any gadget automatically in case of excessive humidity.

Dew Sensor: B79_Tiny_efy

Dew sensor: Circuit explanation

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 ofIC1 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 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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