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
  • Two screwdrivers (one Phillips, one flat)
  • 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 to reach the J1 connector screw terminals inside. Secure the GND wire first to prevent accidental shorts during assembly.

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

Steps 3–4 — Install the resistor

Place the 4.7 kΩ resistor between the OUT and V+ pins on the J1 connector. This pulls the output high so the DragonFly sensor input reads correctly. Tighten all terminal screws firmly, then photograph the wire colours for reference — you'll need this when connecting 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 — Configure sensor 8 in the DragonFly software

Open the DragonFly configuration software and navigate to the sensor settings. Assign the RG-9 to sensor input 8 and configure the threshold values appropriate for the 4.7 kΩ pull-up resistor. The sensor reading will be near its maximum when dry and will drop sharply when rain is detected.

DragonFly software — sensor 8 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.