1Welcome

This manual explains what the Kepler Watch F91 does and how to use it.

The Kepler Watch 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.

This manual describes firmware 3.3.0. A watch running older firmware may lack some of the functions described here.

Please read Section 3 (Important safety information) first. It covers the coin cell and water — both behave differently on a Kepler Watch than on the original watch.

2What’s in the box

The package should contain:

• Kepler Watch F91, with battery fitted
• A spare CR2016 cell and a thank-you card
• 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.

4Your watch at a glance

Kepler Watch 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.

Short press, hold, and the display

• A short press is a quick tap. A hold means keeping the button down for about two seconds. The hold acts while the button is still down; letting go afterwards does nothing.
• To save power, the display turns off after about 3 seconds on the clock screen. Any button wakes it again; that first press only wakes the display and does nothing else.
• On every other screen (Timer, Stopwatch, etc.) the display stays on longer — about 30 seconds — and then also turns off. The flashlight is the exception: it switches off after two minutes.
• On the clock you can hold the top-left button 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 day/month (24/10). With the weekday switched on in the app, the top row shows the weekday and the day of the month instead (SA 24; in German DO 24); the month is left out then. Date and weekday stay blank until the watch has been set.
• Weather: a small icon, if your phone sends it — plus the temperature in 24-hour mode.
• Notification icons: a small row for waiting email, text, voicemail or missed-call alerts.
• Bell (alarm): a small bell left of the date while at least one alarm is on (or snoozed).

Whether your phone is connected is shown on the Bluetooth screen (Section 8); the clock face has no connection mark.

5Getting started

Three short steps: wake it, learn the buttons, connect your phone.

1. Wake the watch

The watch ships with the battery fitted, in shipping mode (deep sleep: display dark, radio off). To wake it, hold the top-left button for about 2–3 seconds, then let go; the display stays dark until you press a button, and from then on any button wakes it. If it stays dark, check the battery (Section 11) and contact support; see Section 14.

2. Get to know the buttons

Take a quick look at Section 4. The idea: the top-left button 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 Watch connects to an Android phone using a free app. Once paired, your watch automatically shows the correct time and your phone’s notifications.

Step 1: Install the app

Scan to download

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, Kepler Gadgetbridge, 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

1. Make sure Bluetooth is on at the phone. The display doesn’t need to be on, but the watch must not be in shipping mode (Section 5).
2. In the app, tap add device (the + button) to scan.
3. Choose “F91 Kepler” from the list of found devices.
4. 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. Because there is no PIN or identity check, do the first pairing somewhere you trust.
5. Within a few seconds the watch takes the correct time, date and time zone.

The first time the watch pairs with a phone, it greets it with the Kepler Watch logo: a small moon circles the planet twice. On the first lap it draws its orbit and the planet appears; on the second the logo moves to the left and KEPLER WATCH appears beside it. After about three seconds the watch shows the clock face. It only plays for a phone the watch has not been paired with before, never on an everyday reconnect. Any button ends it early and goes straight to the clock face; that press does nothing else.

7Setting the time, date & alarm

The easiest way to set the time is from your phone: when connected, the watch is set to the second and regularly corrected automatically — for your time zone and daylight saving. You can also set the time on the watch itself when no phone is to hand.

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

You can change the time format on the watch itself. On the clock face, press the right button to switch between 24-hour and 12-hour (with PM in the afternoon and evening). The watch keeps your choice while it runs; on the next connection your phone applies its own 12/24-hour setting again, so change it in the app to make it permanent.

Set the time on the watch

You can also set the time and date without a phone. On the clock face, hold the right button to open the SET TIME editor. It shows the time (HH:MM) and the date (DD/MO), with the field you are changing underlined.

• The top-left button moves to the next field (hours → minutes → day → month).
• The right button counts the underlined value up; the lower-left button counts it down. Values wrap around, and the day always fits the month you pick.
• Hold the top-left button to save. The watch takes the new time and the seconds start from zero.

To leave without saving, simply let the display switch off on its own. A time you set this way is saved; after a battery change the watch restarts from that saved moment (it cannot keep time without power), so set it again or let your phone resync. The next time your phone connects, it takes over again.

Alarms

Set up to five alarms in the app, each with its own time and repeat days. They are stored on the watch and ring without the phone. Section 10 explains how they behave and how to snooze one.

8Using your watch: the screens

Press the top-left button to move through the screens in order. Hold it at any time to jump straight back to the clock.

Clock → Notifications → Timer → Music → Stopwatch → Info → Flashlight → Find Phone → Bluetooth → Image → (back to Clock)

These are the ten screens of the factory cycle. Four more can be added in the app under Watch modes: Forecast and up to three Counter pages. In the same list you change the order and hide screens; every screen can be in the cycle at once.

Button reference

Clock

Your home screen: time, date (or weekday and day) and, when your phone sends it, the weather. Notification icons and the alarm bell appear here too (Section 4). The screen sleeps after a few seconds to save power.

Hourly chime. Switch on “Hourly chime” in Kepler Gadgetbridge: the display flashes briefly and the watch beeps at the start of every hour. It stays dark during the app’s sleep times (“Sleep times”), and an alarm due in the same minute takes precedence.

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. The newest is shown first; the right button pages to older entries, the lower-left button back to newer ones (both wrap around). To remove the entry you are looking at from the watch, hold the right button. That clears it on the watch only — your phone can send it again.

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, “TIMER” flashes full-screen and the watch beeps. Any button dismisses it; unattended it stops by itself after a minute.

• Presets: while the timer is stopped, hold the right button to load a preset. Each hold steps on to the next one: 1, 2, 3, 5, 10, 15, 20, 30, 45 and 60 minutes, then round again.
• Interval mode: hold the lower-left button to switch it on or off; an R in the top-right corner shows it is on. When the time runs out, the timer flashes and starts the same countdown again straight away, without drifting, until you reset it.

Music

A remote for the music on your phone. Lower-left button = play/pause (hold for previous track); right button = next track. The screen always shows both symbols, ▶ (play) and ❙❙ (pause), because the watch does not know whether your phone is playing. It controls the music app on your phone. Holding the right button (2 seconds) switches the watch’s own sound guide: metronome, then breathing guide, then off again.

Stopwatch

Counts up. The right button starts and stops. While it runs, the lower-left button records a lap: the lap number and the time of that lap appear under the running time (for example L3 01:12.40). While it is stopped, the lower-left button resets it to zero, laps included; holding it resets at any time.

Info

An at-a-glance summary: the temperature the watch itself measures (shown as “IN” — an approximate reading from a sensor inside the watch, so a temperature is shown even with no phone connected), the watch’s estimated battery level (BAT) and an alarm line (AL). When your phone has sent them today, the right-hand column shows sunrise (e.g. ^06:58) and sunset (e.g. v18:41); the app sends them along with the weather. In polar day or night a time reads --:--.

The right button on Info switches all alarms off or on, including alarms set in the app. AL OFF means alarms are disabled; AL -- means no alarm is scheduled. Otherwise the line shows the next alarm time. The switch is kept across a restart; alarms due while it is off pass silently. The alarm label is beside the right button, and SND ON / MUTE is beside the lower-left button.

Sound. The lower-left button on Info switches between SND ON and MUTE. Mute silences automatic sounds except enabled alarms. A metronome or breathing pattern you start yourself can still play. To turn all alarms off, use the right button and check for AL OFF. The app's Mute setting does the same as the lower-left button.

Flashlight

The flashlight lights the whole display. The right button changes its brightness. It switches off after two minutes; a brightness press starts the two minutes again. The lower-left button switches it off immediately and returns to the lit clock face. The top-left button moves to the next screen. The flashlight uses considerably more power than the clock.

Find Phone

Makes your phone ring so you can find it. The lower-left button makes the phone ring; the right button stops it. The app can also find the watch: it flashes FIND full-screen until you press a button, or for a minute, and beeps; any button stops it. This feature needs the paired app running on your phone.

Bluetooth

Shows the radio status: CONNECTED (linked to the phone), ON (radio on, waiting for the phone), OFF (radio disabled) or SLEEP (the radio is off during the app’s sleep times, “Sleep times” → “Radio off overnight”; it comes back on by itself when the window ends). Use the right button to turn the Bluetooth radio on or off; during sleep times this switches it back on until the window ends. With the radio off, no notifications arrive and the time is not synced until you switch it back on; after a restart or a battery change the radio is on again. Hold the right button for 2 seconds to open CLEAR ALL PAIRINGS?. Press right to confirm or lower-left to cancel. Leaving the screen or waiting 10 seconds cancels too. This works with the radio off. See Section 14.

Image

Shows a picture full-screen: the built-in Kepler Watch mark, or an image of your own sent from the app. In the app open the watch’s settings and choose Image → Upload image: pick a photo, pinch and drag to frame it, choose dither (better for photos) or a brightness threshold, then Send to watch. The preview is exactly what the watch will show — 96 × 39 pixels, black and white. If the watch is not connected, the picture is sent on the next connection. The picture is kept only while the watch has power: after a battery change the watch shows the built-in mark again, so upload your picture again from the app. This screen has no buttons of its own.

Forecast (optional)

A 7-day weather forecast, three days per page: the weekday, a weather icon, and the day’s high and low temperature. The first page starts with today; the right button pages on to the next three days, then to day seven, then back. The app sends the forecast together with the weather, so a weather source must be active in the app. A day the forecast does not cover shows --. The weekdays are in English or German, following “Weekday language” in the app. Add the screen under Watch modes (“Forecast mode”).

Counters (optional)

Up to three tally counters, each on its own page, for counting anything: walking laps, coffees, sets. Give a counter a name in the app (“Counter 1 name” and so on, up to ten characters; umlauts show as ae, oe, ue) and tick its page under Watch modes. The page shows the name and the count. The right button adds one, the lower-left button takes one off, and holding the lower-left button resets the count to zero. The count is saved on the watch a few seconds after you stop tapping, or when you leave the page. A counter without a name is skipped in the cycle; clearing its name in the app removes it.

9Notifications & calls

When your phone is connected, Kepler Watch 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 Watch 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

Calls and messages appear as a full-screen popup with the caller or sender and, for messages, the app name. The message body stays on your phone. Any button dismisses the popup; otherwise it expires and the display goes dark or returns to the previous lit screen. Any button also closes pairing PIN, pairing-failed and pairing-cleared notices.

Names use plain letters: ä shows as ae, ö as oe, ü as ue, ß as ss, and most other accents are left off (é shows as e). Letters from other alphabets, such as Cyrillic, Greek or Chinese, show as a small box. Emoji show as a short text such as :-D or as a small box.

Quiet text notifications

If the full-screen pop-up is too much, switch on “Quiet text notifications” in the app (it needs “Show sender as full-screen popup” on). A message then no longer takes over the display: the clock face lights up and the sender’s name appears in the icon row for about three seconds, while time and date stay visible. Names longer than nine characters scroll, and the row stays up until the last letter has been shown. Then the icons return and the display goes dark again if it was dark before. Calls still use the full pop-up and still ring.

If the watch had gone dark on another screen, it switches to the clock face to show the name. A screen you are using at that moment (display on) is left alone; the icon appears on the clock face when you next return to it.

Sounds

Message beeps and call ringing are on by default. Quiet text notifications turn off message beeps, while calls can still ring. A call ring ends after about 20–22 seconds, on a button press or when Bluetooth disconnects. On Android it does not stop by itself when you answer or end the call on the phone. MUTE on Info silences automatic sounds except enabled alarms.

In Kepler Gadgetbridge the watch’s settings have a Sounds group (it appears once the app has seen the watch report firmware 3.2):

• Mute — silences automatic sounds except enabled alarms. Manually started metronome and breathing patterns can still play. Off by default.
• Call ring — the watch beeps when a call comes in. On from the start.
• Text beeps — the watch beeps when a message arrives. On from the start.
• Low-battery sound — the watch beeps once when the battery runs low, so you know a new coin cell is due soon (Section 11). Off from the start.
• Lost-phone alert — the watch beeps if your phone goes out of reach: after about one to two minutes, or after about a minute if the phone’s Bluetooth is switched off. Off from the start; the switch appears once the app has seen firmware 3.3 (needs Kepler Gadgetbridge v0.94.0-kepler-3 or newer).
• Snooze length — how long a snoozed alarm waits before it rings again: No snooze, 5, 9 or 15 minutes. One setting for all five alarms; 9 minutes from the start. With “No snooze” the right button stops a ringing alarm.
• Test sound on watch — plays one beep, to check the sound works.

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 on the phone, so the display does not light up from notifications at night.

10Alarms

Kepler Watch has five alarms. You set them in the app; they are stored on the watch and ring even when the phone is not nearby.

• Set an alarm in the app’s alarm area: a time and the days it repeats on (for example Monday to Friday). An alarm without repeat days rings once, at the next occurrence of its time.
• Turn alarms on or off in the app. While at least one alarm is on, a small bell appears on the clock face, left of the date.
• When it goes off, “ALARM” flashes full-screen and the watch sounds.
• Snooze: press the right button. The alarm rings again after the snooze time, 9 minutes unless you change it in the app (“Snooze length”, Section 9), and you can snooze it again then. With “No snooze” set, the right button stops it instead. The bell stays on while a snooze is pending.
• Stop: press the lower-left button. This also cancels a pending snooze. Unattended, the alarm stops on its own after one minute.
• To silence all alarms at once, for example on a day off, press the right button on the watch’s Info screen (Section 8); press it again to switch them back on.

An alarm without repeat days switches itself off after ringing, on the watch and in the app.

Let the phone ring too. For extra certainty, the app can also alert you on the phone: under “Alerts on this phone” in the watch’s settings, “Alert me when the alarm fires” and “Alert me when the timer ends” make the phone ring or vibrate (“How to alert”: Ring or Vibrate only) while it is connected to the watch.

The timer’s “TIMER” and the “FIND” signal cannot be snoozed: any button ends them.

11Battery & replacement

Kepler Watch 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, a fresh cell lasts about 6 months. Lots of display time and many notifications make it shorter, light use makes it longer. Cold weather shortens it too.

Checking the level

See the percentage on the Info screen or in the app; it is derived from the battery voltage and, for a coin cell, only approximate. Replace the cell when it weakens. A weak battery can cause restarts or a dimmer display.

Replacing the cell

1. 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.
2. Note how the old cell sits, then gently release it from its holder.
3. 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.
4. Refit the back and screws. Don’t over-tighten.
5. Wake the display. At NEW CELL?, hold the right button for 2 seconds beside the tick and HOLD label to confirm a new cell and reset the battery estimate. The lower-left button beside the cross cancels. Other buttons do nothing. After confirming, cancelling or the 30-second timeout, the lit clock face returns. Only confirm if you fitted a fresh cell.
6. Connect your phone to resync the time (Section 7).

Your settings (time format, time zone, alarms, counters, screen order 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, put the watch into shipping mode (Section 8) or remove the battery (to prevent leakage), and keep it somewhere dry.

13Troubleshooting

Most issues are quick to fix. Find your symptom below.

Still stuck? Section 18 explains how to reach us.

14Reset & recovery

Restart the watch

If the watch ever stops responding, hold all three buttons together for about 3 seconds. The watch restarts while you are still holding them. This is safe: settings and pairing are kept, and the time resyncs on its own the next time your phone connects.

If the three-button restart does not work, especially after a battery change: contact support. A known power-up limitation can leave the watch unresponsive before the firmware starts; a programming-jig reset may be needed. Do not treat a failed restart as proof that the battery is empty.

Removing the pairing & passing the watch on

On Bluetooth, hold the right button for 2 seconds. At CLEAR ALL PAIRINGS?, press right beside the tick to erase every saved phone, or lower-left beside the cross to cancel. Waiting 10 seconds or leaving the screen also cancels. The confirmation appears before anything is erased, even with the radio off. Afterwards remove the watch in the app and in the phone’s Bluetooth settings, then pair again. This clears pairings, not all watch settings.

If you want to pass the watch on or sell it, do the above first. No message contents are stored on the watch; the stored settings (time zone, alarms, brightness) remain — the new owner’s phone sets the time and time zone afresh on its first connection. Counter names you chose are stored on the watch as well: clear them in the app before you hand the watch over.

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. Any button wakes it.

15Specifications

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.

Manufacturer: ZAM Labs UG (haftungsbeschränkt), Schellingstraße 87, 80799 München, Germany, support@kepler.watch. Kepler Watch is a brand of ZAM Labs UG (haftungsbeschränkt).

17Open-source software

Kepler Watch 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 Kepler Gadgetbridge 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.

The original case design and any remaining third-party marks belong to their respective owners. Kepler Watch is an independent project, not affiliated with, endorsed or sponsored by the original watch 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
• Contact form: kepler.watch/pages/contact

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 Watch F91 User Manual · Firmware 3.3.0 · Edition 8 · kepler.watch
© 2026 ZAM Labs UG (haftungsbeschränkt). All rights reserved.