A classic digital-watch case with new internals: an OLED display, Bluetooth and custom firmware. It shows your Android phone’s notifications and runs on a single coin cell.
1Welcome
This manual explains what the Kepler F91 does and how to use it.
The Kepler F91 sits in a classic digital-watch case, but the internals have been replaced. In place of the old movement are a crisp OLED display, a Bluetooth Low Energy radio and custom firmware. The everyday basics of a digital watch remain — time, alarm and stopwatch — and to them it adds what it receives from your phone over Bluetooth: it shows notifications and calls, keeps its time in sync automatically, and acts as a music remote for your phone.
It runs on a single CR2016 coin cell — nothing to charge, and no cable.
- You replace the battery yourself. A coin cell lasts several weeks to months depending on use; your settings are kept when you change it.
- No account, no cloud. The watch talks directly and encrypted to your phone. Nothing goes to a server.
- Notifications, weather and time-sync run through a free Android app (Section 6).
Please read Section 3 (Important safety information) first. It covers the coin cell and water — both behave differently on a Kepler than on the original watch.
2What’s in the box
The package should contain:
- Kepler F91 watch, with battery fitted
- This user manual
If anything is missing or damaged, please contact us (Section 18).
3Important safety information
Please read this section before use and keep it for reference.
This watch contains a CR2016 lithium coin cell, and a spare may be included. Swallowing a coin cell can cause severe internal chemical burns in as little as two hours and can be fatal. Keep new and used batteries out of the reach of children and pets. If you suspect a battery has been swallowed or placed inside any part of the body, seek medical attention immediately.
After a battery change, make sure the cover is firmly closed, and dispose of the old cell safely and out of reach (see Section 16).
Converting the watch involves opening the case, so the original water seal may have been compromised and can no longer be guaranteed. The case may still be marked “WR” (water resistant) from the factory — don’t rely on it. Treat the watch as not water resistant and keep it dry:
- Do not wear it while swimming, showering, bathing or washing your hands.
- Do not submerge it or expose it to rain, water jets or steam.
- If it gets wet, dry it immediately. Moisture inside the case can damage the electronics and corrode the battery contacts.
- Not a medical device. Kepler measures neither heart rate nor any other health metric and must not be used for medical purposes.
- Not for navigation, diving or hazardous activities where exact or fail-safe timekeeping is required.
- Temperature. Avoid extreme heat or cold and prolonged direct sunlight. Battery performance drops sharply below 0 °C.
- Do not destroy it. Do not crush, heat or incinerate the watch or the battery. Open the watch only to change the battery (Section 11) and do not dismantle it further. Use only a CR2016 cell of the correct type.
- Skin irritation. If you notice redness or irritation where the watch touches your skin, stop wearing it.
4Your watch at a glance
Kepler has three buttons and one OLED display. Everything on the watch is done with these three buttons; setup and notifications come from your phone.
The three buttons
The watch has two buttons on the left side (top and bottom) and one button on the right side.
- 1 MODE (top left)
- Switches between screens. Hold to jump back to the clock (on the clock: keep the display on).
- 2 Right button (main action)
- The main action on each screen (start, next, +5 min, alarm on/off). On the clock it switches between 12- and 24-hour time.
- 3 Lower-left button (secondary action)
- The secondary action on each screen (back, +1 min, reset, play/pause).
Wake the watch and press each button once. The button that changes the screen is MODE (top left). Of the other two, the right button switches the clock between 12- and 24-hour time; the remaining one (lower left) is the secondary action. This manual uses these names throughout.
Short press, hold, and the display
- A short press is a quick tap. A hold means keeping the button down for about a second.
- To save power, the display turns off after about 5 seconds, but only on the clock screen. MODE or the right button wakes it again.
- On every other screen (Timer, Stopwatch, etc.) the display stays on. As soon as you return to the clock, it goes back to sleep.
- On the clock you can hold MODE to keep the display on continuously (handy in low light). Hold it again to release.
The display
Depending on your setup, the clock face can show:
- Time, large in the centre (with PM in the afternoon and evening in 12-hour mode).
- Date as month/day.
- Weather: a small icon and the temperature, if your phone sends it.
- Notification icons: a small row for waiting email, text, voicemail or missed-call alerts.
- “A” (alarm) appears when an alarm is armed.
5Getting started
Three short steps: wake it, learn the buttons, connect your phone.
1. Wake the watch
The watch ships with the battery fitted. Press MODE or the right button and the display lights up. If it stays dark, the battery is flat or was removed for shipping. See Section 11 for how to fit or replace the CR2016 cell.
Until you connect a phone, the watch shows a default date and starts in 1994. You don’t need to set anything by hand. Once the app is paired (Section 6), the watch takes the time, date and time zone automatically.
2. Get to know the buttons
Take a quick look at Section 4. The idea: MODE changes the screen, and the right and lower-left buttons perform the actions on whatever screen is shown.
3. Connect your phone
To set the time and receive notifications, pair the watch with the free Gadgetbridge app on an Android phone. This is covered in Section 6.
6Pair with your phone
Kepler connects to an Android phone using a free app. Once paired, your watch automatically shows the correct time and your phone’s notifications.
Apple does not allow third-party watches the features that are needed, so there is no iPhone app. You need an Android phone running Android 6.0 or newer (Android 12 or newer recommended). You can also use the watch without the app: connect once so the time is correct, after which it keeps running on its own and needs neither the app nor a phone nearby. Only notifications and a fresh time-sync require the Android phone to be nearby.
Step 1: Install the app
On your phone, scan the QR code or open kepler.watch/app in your browser and download the app.
Because the app is not on the Google Play Store, your phone will ask permission to “install from this source” (sometimes called “install unknown apps”). Allow it for your browser, then open the downloaded file to install.
The app is our fork of Gadgetbridge, an open-source app for connecting watches to your phone. See Section 17.
Step 2: Grant permissions
The first time you open the app, Android asks for a few permissions. The watch needs them to work; the app does not use them for anything else:
- Nearby devices / Bluetooth, to find and connect to the watch.
- Notification access, so your phone’s alerts can go to the watch. Android opens a settings page; switch the app on in the list.
- Phone (calls), so the watch can show who is calling.
Step 3: Connect the watch
- Make sure Bluetooth is on. The watch does not need to be awake for this.
- In the app, tap add device (the + button) to scan.
- Choose “F91 Kepler” from the list of found devices.
- Confirm the pairing on your phone. There is no PIN or code to enter, but you do have to allow the connection once. After that it is encrypted and remembered, so the two find each other on their own from then on.
- Within a few seconds the watch takes the correct time, date and time zone.
For reliable notifications, exempt the app from Android’s battery optimisation (Settings → Apps → Gadgetbridge → Battery → Unrestricted / Don’t optimise). Otherwise Android pauses the app in the background and notifications stop arriving.
7Setting the time, date & alarm
The time is set from your phone, not on the watch. This keeps it accurate to the second and automatically correct for your time zone and daylight saving.
Time, date & time zone: automatic
Whenever the watch is connected, the app sends the time, date, time zone and daylight saving. There is nothing to set by hand. If the time ever drifts, open the app with the watch nearby and it resyncs on its own.
12- or 24-hour: on the watch
The one time setting you can change on the watch itself is the time format. On the clock face, press the right button to switch between 24-hour and 12-hour (with PM in the afternoon and evening). Your choice is remembered.
The alarm
Set the alarm time in the app. On the watch you can turn the alarm on or off from the Info screen (press the right button). See Section 10 for how the alarm behaves.
Time format, time zone, daylight saving and the alarm setting are stored in the watch and survive a restart. After a battery change, only the current time returns to its default until the next time you connect your phone — everything else is kept.
8Using your watch: the eight screens
Press MODE to move through the screens in order. Hold MODE at any time to jump straight back to the clock.
Clock → Notifications → Timer → Music → Stopwatch → Info → Flashlight → Find my phone → (back to Clock)
Button reference
| Screen | MODE (top left) | Right button | Lower-left button | Right + lower-left |
|---|---|---|---|---|
| Clock | Next screen hold: keep display on | Switch 12 / 24-hour | hold: clear notification icons | Open flashlight |
| Notifications | Next screen hold: clock | Next (newer) | Back (older) | — |
| Timer | Next screen hold: clock | +5 minutes | +1 minute | Start / Reset |
| Music | Next screen hold: clock | Next track | Play / Pause hold: previous track | — |
| Stopwatch | Next screen hold: clock | Start / Stop | Reset | — |
| Info | Next screen hold: clock | Alarm on / off | — | — |
| Flashlight | Next screen hold: clock | — | — | — |
| Find my phone | Next screen hold: clock | Stop | Ring | — |
Clock
Your home screen: time, date and, when your phone sends it, the weather. Notification icons and the alarm “A” appear here too. The screen sleeps after a few seconds to save power.
Notifications
Browse the most recent alerts forwarded from your phone. Each shows the app, its position in the list (e.g. 1/5) and the sender. Use the right and lower-left buttons to move through them.
Timer
A simple countdown. Use the lower-left button to add minutes one at a time and the right button to add five at a time. Press the right + lower-left buttons together to start; press them together again to reset. When time runs out, a full-screen alert flashes. Any button dismisses it.
Music
A remote for the music on your phone. Lower-left button = play/pause (hold for previous track); right button = next track. The centre icon toggles between ▶ (play) and ❙❙ (pause) on each press. It controls the music app on your phone; the watch itself plays no sound.
Stopwatch
Counts up. The right button starts and stops; the lower-left button resets to zero.
Info
An at-a-glance summary: current weather, the watch battery level and the next alarm. Press the right button here to turn the alarm on or off.
Flashlight
Lights the whole display white to act as a small torch. Press MODE to leave it. Because the entire display is lit, the flashlight uses more power than usual — use it only briefly.
On the clock, the right + lower-left buttons together open the flashlight instantly, even if it is hidden in the app’s screen order. MODE takes you back to the clock.
Find my phone
Makes your phone ring so you can find it. The lower-left button makes the phone ring; the right button stops it (the screen shows a short prompt). This feature needs the paired app running on your phone.
9Notifications & calls
When your phone is connected, Kepler tells you about messages and calls without your having to reach for your phone.
The notification icons
A small row of icons on the clock face shows what is waiting. Kepler knows four kinds: email, text message, voicemail and missed call. To clear the icons on the watch, hold the lower-left button on the clock face. They also clear as you deal with the alerts on your phone.
Pop-up alerts
For an incoming call or message, the watch shows a full-screen pop-up for a few seconds: the app icon, the caller’s or sender’s name and the action (“CALL” for a call, “TEXT” for a message). It then returns to the clock. Any button dismisses it early.
Choosing what comes through
Which apps and which kinds of alert reach the watch is set in the app on your phone. It also has a quiet-hours (do-not-disturb) schedule so the watch stays quiet overnight.
Check that you granted the app notification access (Section 6) and that it is exempt from battery optimisation. See Section 13.
10The alarm
Kepler has a daily alarm. Set the time in the app; turn it on and off on the watch.
- Set the time in the app on your phone.
- Turn it on or off on the watch from the Info screen with the right button. When armed, an “A” appears on the clock face.
- When it goes off, a full-screen “ALARM” flashes. Any button dismisses it.
This model has no buzzer and no vibration. The alarm only draws attention by flashing the display. It works well as a visual reminder while the watch is in view, but it is not suitable for waking you from sleep — please don’t rely on it for that.
11Battery & replacement
Kepler runs on a single CR2016 3-volt lithium coin cell that you can replace yourself.
How long the battery lasts
Battery life depends a lot on how you use the watch, mainly how often the display lights up and how many notifications arrive. As a guide, expect several weeks to several months on a fresh cell. Frequent display use, lots of notifications and cold weather shorten it.
Checking the level
See the percentage on the Info screen or in the app. Replace the cell when it weakens. A weak battery can cause restarts or a dimmer display.
Replacing the cell
Keep the coin cell away from children at all times (Section 3). Work on a clean, dry surface. Treat the watch as not water resistant and keep it dry; opening it can affect the seal further. If you are unsure, ask a watch repairer or contact us.
- Turn the watch display-down. Using a small Phillips screwdriver, remove the four screws at the corners of the metal back and lift the back off.
- Note how the old cell sits, then gently release it from its holder.
- Fit a new CR2016 cell the same way up. The “+” side (flat, marked) faces up, toward the back of the watch. Don’t touch the contacts or press on the display.
- Refit the back and screws. Don’t over-tighten.
- Press MODE or the right button to wake the watch, then connect your phone to resync the time (Section 7).
Your settings (time format, time zone, alarm and the pairing with your phone) survive a battery change. Only the current time is resynced, and that happens automatically as soon as your phone connects.
12Care & cleaning
- Keep it dry. Treat the watch as not water resistant; its water resistance is no longer guaranteed (Section 3). Take it off before washing, showering or swimming, and protect it from rain.
- Cleaning. Wipe the case and strap with a soft, slightly damp cloth and dry immediately. No solvents, no alcohol, no abrasives, no compressed air.
- The display. Clean it gently with a dry microfibre cloth. Avoid scratches and don’t press hard.
- Temperature. Don’t leave the watch in very hot places (a car in summer) or very cold ones; both shorten battery life and can affect the display.
- Storage. For long-term storage, consider removing the battery (to prevent leakage) and keeping the watch somewhere dry.
13Troubleshooting
Most issues are quick to fix. Find your symptom below.
| Symptom | What to try |
|---|---|
| Display stays dark / won’t wake | Press MODE or the right button. If it stays dark, the battery is probably flat or removed. Replace the CR2016 (Section 11) and wait a moment after the change. |
| Watch not responding / frozen | Press all three buttons at the same time to restart (Section 14). Settings and pairing are kept. |
| Time or date wrong | Open the app with the watch nearby; the time then syncs automatically. Check the time format with the right button on the clock. |
| Won’t connect / not found when scanning | Make sure Bluetooth is on. In the app, remove the device and add it again. Toggle the phone’s Bluetooth off and on. Move closer. |
| Connected, but no notifications | Grant the app notification access and exempt it from battery optimisation (Section 6). Check your per-app choices and quiet-hours in the app. |
| Keeps disconnecting | Usually battery optimisation. Set the app to Unrestricted. A weak watch battery can also drop the connection. |
| Battery drains quickly | Lots of screen time and many notifications use more power. Don’t leave the display latched on (the “keep display on” hold). Cold also shortens life. |
| The alarm didn’t wake me | The alarm is visual only: it flashes and makes no sound (Section 10). Don’t rely on it as a wake-up. |
Still stuck? Section 18 explains how to reach us.
14Reset & recovery
Restart the watch
If the watch ever stops responding, press all three buttons at the same time. The watch restarts. This is safe: settings and pairing are kept, and the time resyncs on its own the next time your phone connects.
Automatic recovery
The firmware watches itself. If it ever gets stuck, a built-in safety timer (watchdog) restarts the watch on its own. Often you only notice the display went briefly dark. MODE or the right button wakes it.
15Specifications
| Display | 0.83-inch OLED, 96 × 39 pixels, white on black |
| Connectivity | Bluetooth Low Energy (Texas Instruments CC2640R2F) |
| Phone compatibility | Android 6.0 or newer, with the Gadgetbridge app (Android 12+ recommended). iOS / iPhone not supported. |
| Battery | CR2016 3 V lithium coin cell, user-replaceable |
| Battery life | Several weeks to several months, depending on use |
| Timekeeping | Quartz accuracy (about ±30 seconds per month); kept exact by automatic sync |
| Controls | Three buttons (short press & hold) |
| Functions | Clock (12/24h), notifications, calls, timer, stopwatch, music remote, weather, visual alarm, flashlight, find my phone |
| Water resistance | Not guaranteed; treat as not water resistant (see Section 3) |
| Case & strap | Original classic resin case and strap |
| Dimensions & weight | Approx. 38 × 35 × 9 mm, approx. 21 g |
| Firmware | v2.9.0 |
Specifications are subject to change as the firmware and app are updated.
16Safety, compliance & disposal
Battery disposal
Don’t put coin cells in household waste. Take used batteries to a battery-recycling collection point (many shops that sell batteries provide one). The crossed-out wheelie-bin symbol means collect separately, not in household waste. Keep used cells away from children until disposal.
Disposing of the watch
At end of life the watch is electronic waste (WEEE). Please take it to an electronics take-back or recycling point rather than putting it in general waste. Remove the battery first and dispose of it separately.
Regulatory information
Frequency band: 2.4 GHz (Bluetooth Low Energy). This product is a small-batch, independently converted device; it is not affiliated with or endorsed by the original watch’s manufacturer.
17Open-source software
Kepler is built on open-source software, and we pass those parts on openly.
- The watch firmware is based on the open-source F91 Kepler project, used under the MIT licence.
- The companion app is our fork of Gadgetbridge, a free, privacy-respecting app for connecting watches and fitness trackers to Android, used under the GNU AGPL v3.
You can obtain the corresponding source code, including our changes, and the full licence texts at kepler.watch/opensource.
“Kepler” and the Kepler logo are trademarks of the seller. The original case design and any markings on it are the property of their respective owners; Kepler is an independent conversion and is not affiliated with, sponsored by, or endorsed by that manufacturer.
18Support & warranty
Getting help
Most questions are answered in Section 13 (Troubleshooting). Beyond that, reach us here:
- Web: kepler.watch
- Email: support@kepler.watch
Warranty & right of withdrawal
In the EU you have statutory warranty rights independent of any voluntary manufacturer warranty. Online purchases also carry a 14-day right of withdrawal; see our withdrawal notice and terms in the shop for details.
Kepler F91 User Manual · Firmware v2.9.0 · Edition 2 · kepler.watch
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