Smart Irrigation Controller

Overview

A smart irrigation controller is the timer at the head of a sprinkler system, upgraded with a network connection and sensor inputs so that watering tracks actual conditions instead of a fixed calendar. The electrical contract it serves is decades old and entirely standard: each irrigation zone is a 24 VAC solenoid valve buried in a valve box, wired back to the controller with a shared common wire. Energise a zone wire and that valve opens; remove power and it closes. Everything the product does reduces to deciding when to energise which output on the Zone Output Stage stage.

The "smart" layer earns its name through water savings. A fixed timer waters through rainstorms and waters identically in April and August. Connected controllers that adjust run times from weather data (evapotranspiration, or ET, scheduling) typically cut outdoor water use by 20–30%, which is why utility rebate programmes and the US EPA WaterSense certification exist for this category.

How it works

The Main Controller Board runs the schedule. An Microcontroller holds the zone programmes, up to four independent programmes with six start times each, in EEPROM, and a battery-backed Real-Time Clock keeps real time through outages so a power blip does not shift watering into the afternoon. The Compute SoC Module handles 802.11 WiFi through a PCB trace WiFi Antenna; through it the controller fetches weather forecasts and ET estimates, and exposes the app interface used for remote control.

When a start time arrives, the MCU fires one Zone Triac through an optical trigger, switching 24 VAC onto that zone's terminal at the Terminal Block. A Relay closes in parallel to drive a master valve or a pump-start contactor, common on systems fed from a well or lake rather than mains pressure. Zones run one at a time because the plug-in transformer and the hydraulics are both sized for a single valve's flow.

The Current Sense Circuit circuit watches solenoid current on the active output. A reading near zero means a broken wire or failed coil; a reading far above 300 mA means a shorted solenoid that would eventually cook the transformer. Either condition is logged, the zone is skipped, and the fault Status LED and app are notified, wiring faults in buried runs are the dominant failure mode of installed systems.

Sensors and weather logic

Three local inputs complement the cloud weather feed through the Sensor Interface, each isolated by an Optocoupler because sensor wires run outdoors and pick up surges.

The Rain Sensor is the simplest and most mandated device (several US states require one): hygroscopic discs swell after measurable rain and hold a switch open, vetoing all watering until they dry out at roughly the rate soil does. The Soil Moisture Probe reports volumetric water content from the root zone, letting the controller skip cycles while moisture is above a set threshold rather than estimating from weather. The Flow Meter Input counts pulses from an inline turbine meter; flow far above the zone's learned baseline indicates a broken head or lateral pipe, and the controller closes the master valve rather than empty a few thousand litres onto the lawn.

Power and protection

Power arrives from the standard 24 VAC wall transformer (Power Supply) used across the irrigation industry, protected by a Thermal Fuse. The Bridge Rectifier produces an unregulated DC rail, and an Buck Converter steps it to 3.3 V for logic and radio. Valve outputs stay AC because solenoid valves are AC devices, the impedance of the energised coil limits holding current, and AC avoids the DC electrolytic corrosion that would otherwise attack splices in wet valve boxes.

Lightning is the environmental threat that shapes the output stage. Valve wires act as long buried antennas for nearby strikes, so every field terminal carries a metal-oxide Surge Suppressor clamping transients before they reach the triacs.

Packaging and local control

The electronics mount in a polycarbonate enclosure: the Case Back carries keyhole wall mounts and knockouts, field cable enters through Cable Glands, and the gasketed Case Door brings the assembly to IP44 for garage or sheltered outdoor walls.

The Front Panel exists because irrigation is maintained by people standing at the valve box, often without the homeowner's phone: an LCD Panel and Keypad allow manual zone runs for head flushing and winterisation blow-outs, a rain delay button, and full programme editing. The controller is fully functional with WiFi down, the cloud adjusts schedules, but the schedule itself always lives locally in EEPROM and executes from the RTC.