kairos

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Kairos is an open source hardware trigger that starts a camera at a set time. It was built to record a rocket launch automatically, and it does one thing well: be ready, and press record at the right second, without anyone standing next to the camera.

Why Kairos

• Exact timing. Plan each attempt around one moment, T-0. Kairos wakes the camera, starts recording before T-0 and stops after it, on schedule. Up to 30 attempts per device.
• Works with your camera. Trigger any camera with a standard remote trigger port, or a Z CAM over its HTTP API on a wired Ethernet link.
• Controlled from your phone. Native iPhone and Android apps connect over Bluetooth Low Energy. You set the schedule, check the countdown, run a test and watch the log, in the field with no network.
• Hard to misuse. Pair once with a physical button press, so only your phone can control the device.
• Small and low power. It runs on an ESP32-S3 or ESP32-C3 with a real-time clock, starts in under two seconds and draws a fraction of a watt, with Wi-Fi switched off. (The original Raspberry Pi Zero W version still works and is now legacy.)
• Open. The device software, both apps and the Bluetooth protocol are MIT licensed and documented in this repository.

How it works

Each launch attempt has one moment that matters, T-0. Kairos works backwards and forwards from it:

 wake camera       start recording          T-0           stop recording
      ●─────────────────●───────────────────●───────────────────●
      │◄─ wakeUpTimeout ─►│◄──  startBefore ──►│◄───  endAfter ───►│

You set the three timings once and list your attempts. The device keeps the schedule, and the phone sets its clock every time it connects.

Quick start

You need an ESP32-S3 or ESP32-C3 board, a camera with a remote trigger port (or a Z CAM with a W5500 Ethernet module), and ideally a DS3231 real-time clock module. Pairing uses the board's BOOT button, so there is no extra button to wire. Then, with PlatformIO installed:

git clone [email protected]:nprail/kairos.git
cd kairos/firmware/esp32
pio run -e device -t upload      # ESP32-S3 (use -e device-c3 for an ESP32-C3)

Install the app on your phone, press the BOOT button on the board, then tap Pair within two minutes. Wiring is in Hardware, and the full steps are in Setting up a device.

[!NOTE] The original Raspberry Pi Zero W version is legacy. It still works with the same apps and protocol, and its setup is at the end of Setting up a device.

The apps

With the apps you can find and connect to devices, pair with the button, see live status with a countdown to the next event, edit the schedule and camera settings, run a test, manually wake, start, stop and sleep the camera, and watch the device log live.

• iPhone: apps/ios/, a native SwiftUI app for iOS 17 and later.
• Android: apps/android/, a native Jetpack Compose app for Android 8.0 and later.

Get the apps

• iPhone: the app isn't on the App Store, so you build it yourself with Xcode 16 or later and install it on your iPhone (iOS 17 or later):

  1. git clone https://github.com/nprail/kairos.git, then open kairos/apps/ios/Kairos.xcodeproj.
  2. In Signing & Capabilities, choose your team (a free Apple ID works) and change the bundle identifier to one that is unique to you.
  3. Connect your iPhone, turn on Developer Mode (Settings › Privacy & Security), choose it in Xcode and press Run.

  Bluetooth doesn't work in the Simulator. An app signed with a free Apple ID stops opening after 7 days: run it from Xcode again to renew it.

• Android: the app is in a closed test on Google Play and is by invitation only. Email [email protected] with the email address of your Google account to be added. You can also build it from the source (apps/android/).

Documentation

Read it online at nprail.github.io/kairos/docs, or browse the files here.

Read	For
Setting up a device	Flashing an ESP32, Bluetooth setup, pairing (and the legacy Raspberry Pi setup)
Hardware	ESP32 parts, pins, wiring for generic cameras and Z CAMs (and the legacy Pi)
Configuration	Every setting, and the timeline of an attempt (and the legacy config.json)
Bluetooth protocol	The protocol between the phone and the device
Development	Repository layout, running the tests, test vectors
App specification	What the apps do, and the rules for working in this repository
ESP32 firmware	The recommended device: design, status, and what it changes from the legacy Pi version
Brand assets	The Kairos mark, colors and fonts

License

MIT. See LICENSE.