How to Replace a Rain Sensor with a Soil Moisture Sensor

Warning: Make sure your irrigation timer uses a contact closure input, before proceeding with this tutorial.

Many sprinkler timers have a rain sensor input. The problem with these sensors is that they are not very sophisticated, as they normally consist of a sponge and two plates. When the sponge becomes wet it presses the two plates together forming a contact closure, which tells the irrigation timer to prevent watering. When they dry out the plates recede and the switch opens up again signaling to the controller to resume normal watering.

The problem with a traditional rain sensor is that it doesn't sense the actual moisture in the ground. The sponge can dry out in the air, while the ground is still saturated. A better approach is to measure the actual moisture in the soil with a soil moisture sensor.

This article will explain how to replace rain sensors with soil moisture sensors for irrigation timers that have contact closure rain sensor inputs.

To replace a rain sensor, we need to make the moisture sensor have the same electrical interface. Moisture sensors output a voltage that is related to the moisture of the soil, and so we need to convert this information into a contact closure output, in order to simulate a rain sensor. We do this by using a relay controller board. A relay is a mechanical switch. The relay board monitors the sensor, and when the moisture passes a threshold the relay's switch will flip on or off, depending if the soil is dry or wet.

Vegetronix caries several types of relay boards: The VG-RELAY-DC is powered from a wall power adapter, and the The VG-RELAY-LATCH is battery powered. Either can be used. We also offer a wireless WiFi sensor hub with a relay that can be used, which has additional benefits of allowing you to view data over the Internet, and allowing you to place your sensor without any wires. All of these boards are hooked up to the sprinkler timer in the same way, as shown in the following diagram.

Hookup Diagram of Replacing Rain Sensor with a Soil Moisture Sensor.