By now, we’ve hopefully whetted your appetite with this simple design. Most of the mechanical detail here is "roll your own" but there is an alternative commercial rain sensor available if you don’t have the time or inclination to make one. For those who do . . .

The rain sensor uses a tipping bucket mechanism, consisting of a rain collection funnel that feeds water into a pivoting bucket divided into two opposing halves. When one side of the bucket fills and tips, it empties itself, at the same time positioning the opposing side under the funnel outlet where it can fill, tip and repeat the cycle.

The main funnel is glued using silicon sealant into a 100mm PVC pipe end-cap which has its end removed. Note the mesh leaf and insect trap on the bottom end.

The tipping back and forth can be detected in various ways. Since power consumption was important for this project, a passive sensing mechanism was chosen. This involves momentarily closing a switch each time the bucket tips. As shown in the circuit diagram, the switch closure pulls the rain sensor line low (it is normally held high by a 220kΩ pull-up resistor).

To keep the design simple and maximise reliability, I used a magnetic reed switch that is mounted on a bracket next to the tipping bucket.

The switch is closed by a tiny rare-earth magnet (measuring only 3x2mm) that is mounted on the tipping bucket. As the bucket swings, the magnet moves past the reed switch, closing it for a brief moment. A similar mechanism is found in many commercial rain sensors and has the advantage that the reed switch is practically immune to moisture, corrosion, etc.