LM358N
The schematic below depicts a simple DEW sensor circuit diagram using the LM258N IC. As the humidity around the circuit rises, so does the resistance in the DEW sensor. When the resistance exceeds a predetermined threshold, the circuitry displays a warning message and turns the external device off.
Under typical conditions, the dew sensor's resistance is low (1 kilo-ohm or so), therefore the voltage at its non-inverting terminal (pin 3) is lower than that at its inverting input (pin 2) terminal. As a result, the comparator's output (at pin 1) is low, and nothing happens in the circuit. When the humidity level exceeds 80%, the sensor resistance rapidly increases. As a result, the non-inverting pin now has a higher potential than the inverting pin. This raises IC1's output to a high level. As a result, the LED within the opto-coupler is turned on. LED1 offers a visual indicator at the same time. For switching purposes, the opto-coupler can be interfaced to any electronic equipment. A low-voltage, low-current power supply unit is formed by diode D2, resistors R5 and R6, and capacitor C1. This simple design eliminates the need for a large and costly step-down transformer.
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