A simple and low-cost dew sensor circuit

This is a simple and low-cost dew sensor circuit that can be used to automatically shut down any gadget in case of excessive humidity. Dew (condensed moisture) can adversely affect the proper performance of sensitive electronic devices.

The main part of this circuit is the low-cost dew sensor element (resistive type). Although dew sensor elements are widely used in cassette players and VCRs, they may not be easily available in the local market. However, the same can be purchased from the authorized service centers of reputed companies. The author used a dew sensor of FUNAIVCP model VIP3000A (Part No: 6808-08-04, Reference No. 336) in his prototype. In practice, it can be observed that all dew sensors available for video applications have the same electrical characteristics regardless of their physical shape/size, and therefore are interchangeable and can be used in this project.

This simple and low-cost dew sensor circuit is basically a switch type circuit made with the popular dual operational amplifier ICLM358N, configured here as a comparator. (Note that only half of the IC is used here.) Under normal circumstances, the dew sensor has a low resistance (around 1Kohm), so 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 correspondingly low, so nothing happens in the circuit.

When the humidity exceeds 80%, the sensor resistance increases rapidly. As a result, the non-inverting pin becomes more positive than the inverting pin. This pushes the output of IC1 to high. As a result, the LED inside the optocoupler is energized. At the same time, LED1 provides a visual indication. Optocouplers can be used in any electronic device for switching.

This circuit uses a low voltage, low current power supply unit. Diode D1, resistors R8 and R6, and capacitor C1 do the job. This simple power supply module eliminates the need for a bulky and expensive step-down transformer.