Why a second rain sensor?

The CloudWatcher's capacitive rain sensor is reliable and proven, but sensitive equipment in a remote observatory warrants redundancy. The Hydreon RG-9 uses a fundamentally different sensing principle (optical, not capacitive), which means one sensor can detect rain that the other might miss in edge cases: light drizzle, dust coating, or a failure.

By wiring the Hydreon RG-9 directly into a DragonFly sensor input, the observatory gains a fully independent second rain detector that feeds into the same automation rules as the CloudWatcher.

What you need

  • DragonFly observatory controller
  • Hydreon RG-9 optical rain sensor
  • 4.7 kΩ resistor (other values may work as well)
  • A screwdriver to unscrew the lid of the RG-9, and one to screw in the screw-on terminals
  • Three short wires with suitable connectors

Installation

Steps 1–2—Open the RG-9 and prepare the wiring

Remove the four screws from the base of the RG-9 to open the enclosure. Route three wires through the cable gland, connecting one to each of the screw-on terminals in the J1 connector. Screw in the GND wire, but not the other two just yet.

Hydreon RG-9 internals—step 1 Hydreon RG-9—wiring step 2

Steps 3–4—Install the resistor

Connect your resistor between the OUT and V+ pins. Screw the terminals in, making sure there is good contact between the resistor, the cable, and their slot; try pulling to ensure they're sufficiently tight. Taking a picture of the colour you've chosen for each pin may be useful now, for when you connect the wires to the DragonFly.

Hydreon RG-9—resistor installation step 3 Hydreon RG-9—connections step 4

Step 5—Close the RG-9

Re-seat the silica gel bag inside the enclosure to control internal humidity. Tighten the cable gland to make the entry waterproof, then replace the four base screws. The RG-9 must be well-sealed: it sits outdoors exposed to the same rain it is detecting.

Step 6—Connect to the DragonFly

Connect the three wires to the DragonFly terminal block following this mapping:

  • OUT → DragonFly sensor input (IN)
  • V+ → DragonFly 5 V supply
  • GND → DragonFly ground

Step 7—Set up the sensor in the DragonFly application

Set up the sensor in the DragonFly application. For a 4.7 kΩ resistor, the settings shown below work well.

DragonFly software—sensor configuration

Result: With this setup, the observatory has two independent rain detectors working in parallel: the CloudWatcher's capacitive sensor and the DragonFly-connected Hydreon RG-9. An Automata rule or macro can trigger a roof closure if either sensor reports rain, giving maximum protection with minimal false negatives.

Step 8: there is no step 8—you're all set. Enjoy having an extra-safe setup!

Think we should build a dedicated safety monitor driver for the DragonFly? Let us know.