This simple and economical Dew Detector Probe circuit works on 5V dc supply. It is useful for checking the presence of dew (condensed moisture) inside sensitive electronic devices like VTR, Camera,C opier,Computer etc. The circuit utilises a ready-made dew sensor element HDP-07 (from “Hokuriku”). However, you can use any other type (carefully lifted from a discarded VTR or similar equipment) without any circuit modification.
The principle of the ready made dew sensor element is based on the change in resistance of a conductive polymer in a thin film on a small ceramic substrate. As the sensing polymer wet (90 to 95% Relative Humidity), its resistance will increase drastically because the polymer expands and therefore causes a larger distance between the conductive particles. After successful construction, enclose the whole unit in a small aluminium tube as shown in the figure.
This circuit has an on-board voltage regulator wired around LM78L05 (IC1). Although we can safely feed dc supply upto +35 V to this circuit, dc input in +9 to 12 V range is recommended. Similarly an opto-coupler PC817 (OC1) is used to isolate the detector circuit from the rest of the external switch/control unit. As you may noticed, this is an open-collector output which can be easily interfaced to any analogue or digital switch/control circuits like relay switches,alarm units etc.
In idle state resistance of the dew sensor element is very low and thus most of the base current of transistor T1 (2N2222) finds an alternative path via dew sensor and T1 remains in cut off. Condition. When Relative Humidity (RH) exceeds and touches the 90-95% level, dew sensor element behaves almost like an open component (very high resistance), T1 is forward biased and the opto-coupler is energised. Note that the adjustment of threshold set preset pot P1 is very critical. Use theoritical & empirical methods to calibrate the detection mechanism.