---
type: concept
tags: [parking, device, indicator, radar, camera, aux-output, barrier-not-a-door]
sources: []
updated: 2026-06-24
status: settled
---

# Button-light indicator (radar × camera disagreement lamp)

The entry button has a **12 V light**. It is driven by the host on a **spare relay** of the
[[dingtian-relay|Dingtian]] controller as a 3-state indicator that combines the **[[hikvision-radar|
radar]]** input with the **camera "car in zone"** signal:

| Radar input | Camera (lane entry busy) | Button light |
| --- | --- | --- |
| detecting | **free** — no car confirmed | **BLINK** (~1 Hz) |
| detecting | **busy** — camera confirms a car | **SOLID on** |
| clear | — | **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.

## Signals

- **Radar** = the presence input edge on the entry relay (`relays[].presenceInput`, the same edge
  the [[entry-double-press|one-car-one-ticket]] gate observes — so the lamp and the gate always
  agree on "a car is here").
- **Camera "car in zone"** = the existing **[[lpr-camera|lane status]]** (`LaneStatusEvent` entry
  busy/free, from camera vehicle detection). Already advisory; already drives the booth's barrier
  lights. No new camera plumbing.

## Config

A controller-level `config.buttonLight = { relay, blinkOnMs?, blinkOffMs? }` (the operator picks a
**spare** relay — not a barrier relay; the setup UI warns if it overlaps one). Blink defaults to
500 ms / 500 ms.

## Implementation

`apps/server/src/button-light.ts` — `ButtonLightController` subscribes to `deviceEvents.onInput`
(radar) + `onLaneStatus` (camera), computes the target state per controller, and drives the 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. 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, and a lamp-added-after-start reconcile case).
Related: [[hikvision-radar]], [[entry-double-press]], [[lpr-camera]], [[dingtian-relay]],
[[barrier-not-a-door]].