e-Gizmo Raindrop Detector Module

Technical Specifications
* Operating Voltage: 3.3 V−5 V DC
* Main Comparator IC: LM311 Voltage Comparator
* Output Signals: Analog (ANA) and Digital (DIG)
* Default Digital State: Normally HIGH (VCC​) when dry; active LOW (0 V) when wet
* Connections: 4-pin header (VCC, GND, DIG, ANA)

Theory of Operation
1. Resistive Rain Sensing PCB
The external sensor board consists of two set sets of exposed, interleaved nickel/copper traces.
* Dry State: Air acts as an insulator between the traces, creating a very high resistance across the sensor terminals (J1 IN to GND).
* Wet State: Water droplet accumulation acts as a conductive bridge across the traces, significantly decreasing resistance proportionally to the amount of liquid bridging the gap.
2. Analog Output (ANA)
The rain board forms a voltage divider circuit with onboard resistors (R1​, R2​). As water contacts the sensor pad and decreases its resistance, the node voltage directly reflects this change. This variable voltage signal is routed straight to the ANA output pin, allowing microcontrollers to read analog values (via ADC) to estimate rainfall intensity or surface wetness levels.
3. Comparator & Digital Output (DIG)
The module utilizes an LM311 voltage comparator to generate a clean, chatter-free digital threshold output:
* A reference voltage is set at the non-inverting input (V+​) using the onboard trimpot (RV1​).
* The variable sensor voltage from the rain pad is fed into the inverting input (V−​).
* When dry, V−​ remains higher than V+​, keeping the comparator output High-Z/pulled HIGH (Normally HIGH).
* When raindrops lower the sensor resistance, V−​ drops below V+​. The LM311 pulls the output pin (DIG) LOW to ground, simultaneously turning on the red OUTPUT INDICATOR LED (D1​) to signal wet conditions.

Adjustment Procedure (Digital Sensitivity Calibration)
1. Power Up the Circuit:
* Connect VCC to 3.3V or 5V and GND to Ground.
* Confirm that the green Power Indicator LED (D2​) lights up.
2. Set the Dry Baseline:
* Ensure the sensor PCB surface is completely clean and dry.
* If the red Output Indicator LED (D1​) is ON while dry, turn the Threshold Adjust Potentiometer (RV1​) counter-clockwise until D1​ just turns OFF.
* Verify that the digital output pin (DIG) reads HIGH (VCC​).
3. Calibrate Rain Detection Sensitivity:
* Place a few drops of water (or a moist cloth) onto the interleaved copper traces of the sensor pad.
* Adjust RV1​:
* Turn Clockwise: Increases sensitivity (triggers easily with small water drops or light mist).
* Turn Counter-Clockwise: Decreases sensitivity (triggers only with larger amounts of water).
* Adjust until the red Output Indicator LED (D1​) turns ON and DIG drops to LOW at your desired wetness level.
4. Verify & Test:
* Wipe the sensor pad completely dry with a paper towel or cloth.
* Confirm that D1​ turns OFF and DIG returns to HIGH.
* Re-apply water drops to test repeatability and fine-tune RV1​ as necessary.