---
type: concept
tags: [parking, device, indicator, radar, camera, aux-output, barrier-not-a-door, event-relay]
sources: []
updated: 2026-06-28
status: settled
---

# Alert relays (radar × camera disagreement lamp)

A relay on the [[dingtian-relay|Dingtian]] controller is uniformly **"when EVENT X happens, do
action Y"** — see [[entry-exit-points|relays carry an event]]. The barrier events (`entry`/`exit`/
`both`) **pulse** a barrier; the **`radarAlert`** event drives a non-barrier **indicator lamp**
(blink + camera-lock) on a spare relay. The entry button's **12 V light** is the canonical alert
relay, a 3-state indicator that combines a **[[hikvision-radar|radar]]** trigger input with the
**camera "car in zone"** signal:

| Trigger input (radar) | Camera (lane entry busy) | Alert lamp |
| --- | --- | --- |
| active | **free** — no car confirmed | **BLINK** (~1 Hz) |
| active | **busy** — camera confirms a car | **SOLID on** |
| inactive | — | **OFF** |

It is a **disagreement indicator**: the radar sees *something* but the camera hasn't confirmed a
real vehicle → blink (attention / "pull forward"); both agree → solid; nothing there → off.

Because it's just another relay row, a controller can carry **several** alert relays (e.g. R3 and a
future R4), each with its own trigger input — no new config shape, no code change.

## Signals

- **Trigger** = the alert relay's own `triggerInput` edge (the [[hikvision-radar|radar]]). When
  unset, it falls back to the controller's entry-relay `presenceInput` — the same edge the
  [[entry-double-press|one-car-one-ticket]] gate observes, so the lamp and the gate agree on "a car
  is here".
- **Lock (camera "car in zone")** = the existing **[[lpr-camera|lane status]]** (`LaneStatusEvent`,
  from camera vehicle detection). Already advisory; already drives the booth's barrier lights. Each
  lamp picks **which lane's camera** locks it via `relays[].lockLane: "entry"|"exit"` (default
  entry) — so an **exit radar's lamp locks on the EXIT camera**, not the entry one. (Lane-busy is the
  only lock *kind* wired today; the model leaves room for others later.)

## Config

An alert lamp is a `config.relays[]` row with `direction: "radarAlert"`, carrying
`{ relay, triggerInput?, blinkOnMs?, blinkOffMs? }`. No separate `buttonLight` block (that was the
pre-2026-06-28 shape — barriers and the lamp were two different configs; now they're one list).
Blink defaults to 500 ms / 500 ms. The operator picks a **spare** relay (an alert relay never opens
a barrier; every barrier resolver skips `radarAlert` rows).

## Implementation

`apps/server/src/button-light.ts` — `ButtonLightController` reads the `radarAlert` rows
(`alertRelaysOf()` in `device-resolve.ts`), subscribes to `deviceEvents.onInput` (radar) +
`onLaneStatus` (camera), computes the target state **per lamp** (keyed `controllerId:relay`, so
several alert relays on one controller are independent), and drives each lamp via a **device-agnostic
aux-output** capability.

- **Aux-output capability.** `AuxOutputDevice { setAux(channel, on) }` on the device interface (the
  Dingtian driver implements it as a latch). Business logic drives the lamp through this — **never**
  the driver's barrier methods.
- **Barrier-not-a-door is preserved.** The lamp is **not a barrier**, so holding / blinking it on a
  timer is fine — the [[barrier-not-a-door]] rule forbids timing a *barrier* closed, and barriers
  still only ever `pulseOpen`. The lamp uses the separate `setAux` latch.
- **Fails OFF.** On host loss, shutdown, or a `setAux` error the lamp defaults OFF — a dead lamp is
  "no hint", never a misleading solid "go". SOLID is only ever held while busy + present is actively
  true (never latched on through a crash path).
- **Serialized sends (must — UDP is unordered).** The first cut fired fire-and-forget `setAux` every
  500 ms; over **unordered UDP** the on/off packets reordered/overlapped and the relay **latched on
  whichever packet the device processed last** — the lamp got stuck on/off at random (observed on
  hardware). Fix: a **desired-state + serialized worker** (`#pump`). The blink timer only flips a
  `desiredOn` flag; the worker guarantees **one in-flight send per lamp** and, on completion,
  re-converges to the latest desired state. So the **final state is always authoritative** and a
  lost/stale packet self-corrects. This also de-dupes (it skips a send when `confirmedOn === desiredOn`),
  so the input stream never spams the controller.
- **Hot-reloads the config (no restart).** The lamp map is reconciled against the live device config
  at start AND before each event (mirroring [[device-status-monitoring|DeviceMonitor]], which re-reads
  the device set each tick) — adding/updating/dropping lamps. So a button light added or re-pointed in
  the setup UI takes effect on the **next radar edge**, not after a server restart. (The first cut
  loaded the map once at boot, so a just-saved lamp silently did nothing until restart.)

## Status

Built 2026-06-24 for the first booth (button I1, radar I2, lamp on a spare relay); the serialized-send
+ hot-reload fixes landed the same day after the lamp stuck on/off on hardware. **Reframed
2026-06-28**: the dedicated `config.buttonLight` block was folded into the unified `relays[]` list as
a `radarAlert` event-relay (carrying its own `triggerInput`), so the operator can add arbitrary
event-driven blinkers (e.g. R4) without code changes; the 3-state machine itself is unchanged.
Covered by `apps/server/src/button-light.test.ts` (the truth table, blink toggling asserted on the
device's *confirmed* state, fail-OFF, de-dupe, lamp-added-after-start reconcile, and two independent
alert relays on one controller).
Related: [[hikvision-radar]], [[entry-double-press]], [[lpr-camera]], [[dingtian-relay]],
[[entry-exit-points]], [[barrier-not-a-door]].