turfy · irrigation sidecar · v0.1

An AI sprinkler brain that fails back to dumb.

Turfy is a weather-informed, camera-and-sensor irrigation controller — but the clever part isn’t the AI, it’s that it only touches your lawn when it’s provably healthy. It rides alongside a 1990s Rain Bird as a fail-safe sidecar: take authority while the watchdog is fed, and hand control straight back the instant anything goes wrong. In hardware. Because a stuck valve is a flooded yard and a very large water bill.

Rain Bird ESP-6Si controller faceplate — The subject: a Rain Bird ESP-6Si. Reused, not replaced — Turfy borrows its transformer, its valves, and its job as the always-there fallback brain.

Safe by construction

Rain Bird keeps authority

The 1990s controller stays wired exactly as it is. Turfy is a sidecar on the station outputs — it never replaces the brain, it borrows it.

A hardware dead-man

A 555 missing-pulse watchdog must be fed by the daemon's own main loop every ~24s. Miss a beat and the transfer relays physically drop.

Fail back to dumb

Power loss, kernel panic, daemon hang, wedged I²C — every failure reverts to the Rain Bird with zero software involved. A stuck valve floods a yard; this can't stick.

The invariant the whole design hangs on

A de-energized transfer bank leaves the Rain Bird wired exactlyas it is today. It’s the first line of the acceptance test — proven with a continuity meter before anything is ever powered on.

The transfer-switch architecture

It’s the classic automatic-transfer-switch pattern applied to sprinklers. An SPDT relay per zone: Rain Bird output on NC, Turfy output on NO, valve wire on the common pole. One authority line pulls the whole bank — unpowered, crashed, or watchdog-tripped, the relays drop and the Rain Bird is back in control.

signal path — v0.1

                     24VAC (Rain Bird VT terminals, shared transformer)
                          │
                ┌─────────┴──────────┐
                │ fuse 1A + MOV       │
   ┌────────────┐   ┌─────────────┐   │
   │ Rain Bird  │──▶│ ZONE BANK   │◀──┼── MCP23017 (I2C 0x20) ◀── Pi
   │ ESP-6Si    │   │ (Turfy out) │   │
   │ ST1..ST6   │   └──────┬──────┘   │
   └─────┬──────┘          │ NO       │
         │ NC              ▼          │
   ┌──────────────────────────────┐   │
   │ TRANSFER BANK  8ch SPDT relay │◀──┴── AUTHORITY line
   │ COM ─▶ valve field wires 1..6 │       (Pi GPIO ∧ watchdog)
   └──────────────────────────────┘            ▲
                                    555 missing-pulse detector
                                                ▲
                                    HEARTBEAT GPIO (daemon-toggled)

The one rule that must survive every revision:never run a second 24VAC transformer while the NC path to the Rain Bird exists. Out-of-phase secondaries through the transfer relay = circulating current and dead triacs. Turfy steals 24VAC from the Rain Bird’s own terminals so both controllers switch the same hot leg.

Reverse-engineering the box

Before you can sidecar a sealed 1990s controller, you have to map it. Board by board: what’s the tap point, where does 24VAC come from, and is the fallback actually trustworthy?

Rain Bird ESP-6Si faceplate with its analog scheduling dial — Rain Bird ESP-6Si — an analog dial, six stations, an ‘Adjust Water %’ knob. About as simple as irrigation controllers get, which makes it perfect to augment rather than tap.

The opened controller: green logic board over an orange power board, with a soldered-in lithium battery — Inside: a 24VAC transformer, a COB micro, one triac per station, and a hobbyist coin-cell hack that holds the schedule through outages — i.e. what makes the fallback trustworthy.

Power board showing station triacs TH1–TH8, red MOVs, a glass fuse and the field terminal block — Power board (949-26330): eight station triacs TH1–TH8, per-output MOVs, and a fused 24VAC feed. Field wires land on this screw terminal — zone wires lift straight off, nothing gets cut.

COB daughtercard with epoxy-blob microcontroller and a station-count solder-jumper legend — COB daughtercard: the epoxy blob is the micro, and that S1/S2 jumper table is the 4/6/8-station config for the shared module. ‘OPEN:ACTIVE HIGH’ is the jumper-sense legend, not an I/O spec.

The safety core

“The heartbeat must prove applicationliveness, not kernel liveness.”So it’s toggled from the daemon main loop — never cron, never hardware PWM. A cron job would keep ticking after a crash and defeat the entire point.

Authority = liveness ∧ intent

Two 2N2222s in series pull the transfer bank low. It engages only when the 555 says the daemon is alive and the Pi explicitly asserts yes. Either transistor off → the line floats high → the bank drops → Rain Bird.

authority ⇔ (heartbeat alive) ∧ (Pi says yes)

Boot sequence — safe at every step

1Pi boots. AUTHORITY GPIO defaults pull-down → Rain Bird stays in control regardless of any GPIO chatter.
2MCP23017 powers up all-inputs (POR default) → zone relays held off. Defined state, no init race.
3turfy.service self-checks (I²C ack, config sane, time synced), starts the heartbeat, then raises AUTHORITY.
4Watchdog charges within one timeout; the transfer bank engages and Turfy is driving.

Every way it can fail

Failure	Result
Pi power loss / kernel panic	fails safeHeartbeat stops → watchdog drops → Rain Bird
Daemon crash or hang	fails safeSame — the heartbeat is owned by the daemon loop
Pi reboot GPIO chatter	fails safeAUTHORITY pull-down + MCP POR = no valve action
I²C bus wedge	fails safeDaemon detects NAK → drops AUTHORITY
Relay 5V supply dies	fails safeTransfer coils drop → Rain Bird
Both controllers fire one zone	fails safeHarmless — shared transformer, same phase
Healthy daemon, latched zone	software invariantNot caught by the watchdog → hard-capped by a per-zone max-runtime in the driver layer, plus flow-meter alarm in v0.2

Validate each layer

Phase 0

Passive learn

An H11AA1 opto per station output logs when the Rain Bird actually waters. Validates zone mapping and collects a baseline schedule — zero risk, Turfy never asserts.

instrumentation

Phase 1

Take authority

Transfer relays + watchdog go live. Turfy replicates the dumb schedule first, proving the fail-safe handover with a meter before anything smart happens.

the sidecar

Phase 2

Flow metering

An inline pulse flow meter learns each zone's baseline GPM — the killer feature. Detects stuck valves, broken heads (flow too high) and clogged lines (flow too low), per zone.

closing the loop

Phase 3

Weather + vision

ET-based scheduling (Penman-Monteith / Hargreaves) scales runtime to replace the daily deficit, minus rainfall. A cheap camera adds an NDVI-ish turf-stress index on top.

the AI layer

v0.1 build package

Two stock relay boards, an MCP23017 for defined power-on state, a 555 watchdog, and an H11AA1 sense bank — a parametric schemdraw source renders the four schematic sheets, so a board photo becomes a one-line edit and re-render.

2× 8-ch SPDT relayMCP23017NE555 + 2N39062× 2N2222 AND7× H11AA1 sense1A fuse + 39V MOVRaspberry Pi