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The Hydreon RG-9 Solid State Rain Sensor is a rainfall sensing device intended to detect and communicate when a pre-selected rain intensity has been reached or exceeded. The RG-9 is rugged, reliable, and maintenance-free. 
The RG-9 uses beams of infrared light within a plastic lens about the size of a tennis ball. The round surface of the lens discourages collection of debris, and the RG-9 has no moving parts to stick, and no water-pathways to clog. The device features an open-collector output that indicates when the selected rain intensity has been reached. The sensitivity levels are: Rain Drops, Very Light, Medium, Heavy. The duration of the positive detection is also selectable: No Hold, Hold Time 5 Minutes, Hold Time 10 Minutes, Hold Time 15 Minutes.
The RG-9 has multiple outputs: visual output via an on-board LED, Open Collector output and RS232 serial communications that provide more detailed data, including 8 near real-time rain intensity levels and allow for reading the status and configuration of the device. Power consumption of the RG-9 is very low, and the device is well-suited to solar-power applications.
| Parameter | Value |
| Input Voltage | Range 5-15 VDC on J1 Reverse polarity protected to 50V or 3.3VDC through pin 8 on J2 |
| Current Drain | 110 μA nominal. (No outputs on, dry not raining) 2-4 mA when raining |
| Output | NPN Open Collector Output 100 mA / 80V Max |
| Operating Temperature | -40°C to +60°C (Will not detect rain when freezing) |
| DIP Selectable Sensitivity | Rain Drops Very Light Medium Heavy |
| RS232 Port | 3.3V |
| Supported Baud Rates | 1200, 2400, 4800, 9600, 19200, 38400, 57600 |
Technical Document can be found here