I redesigned CVM Scanner, an Android app that reads live data from emergency-vehicle equipment over Bluetooth. Then I critiqued my own redesign and built a second pass you can try below.
Second-pass CVM Scanner screens: a live scan finding devices, and a device that has lost signal, flagged in red.
Overview
A small tool with a lot of live data
A German company that builds radio technology for fire and rescue vehicles fits its equipment with CVM modules: small sensors that show whether a device, like a radio, is plugged in and charging. CVM Scanner finds these modules over Bluetooth and lists their signal and live voltage, current and power. It worked, but every screen asked users to decode it.
Constraints: keep every existing feature, a German-language UI on Android, and people who check the phone at a glance, standing next to a vehicle.
What changed: a clear scan state, signal shown with a word and not only a color, and readings that line up. A second pass adds charging status, honest "last seen" times and the missing error states.
The problem
What the old app asked users to guess
I reviewed the old screens one by one and marked every place where users had to stop and interpret.
01
Signal was shown by color alone
A colored icon and a raw number with no unit: unreadable without color vision or knowing the scale.
Design response
A word (Good, Fair, Weak) plus the value in dBm.
02
Was it scanning or not?
A small spinning arc next to "Scan", easy to miss.
Design response
A clear "Scanning…" state with a radar and a way to stop.
03
Finding one device took scrolling
No search, and sorting hid behind a top-bar icon.
Design response
Search and filter side by side above the list.
04
Icons and layout sent mixed signals
A gear opened renaming, readings had no hierarchy, and the empty state was one small line.
Design response
An edit icon, a card hierarchy from name to readings, and an empty state that explains itself.
Before → after
Every feature kept, every screen rethought
Every feature is still there. Each screen now says what is happening and what to do next.
Empty state
The scan is now visible: a large radar while searching, a scan bar that can stop it, and a plain "no devices found" when nothing is nearby.
BeforeAfter
Device list
Each device is a card: name, then signal with a word and dBm, then readings in monospace so the numbers line up.
BeforeAfter
Renaming a device
Renaming starts from an edit icon instead of a gear, and opens a focused dialog.
BeforeAfter
Settings
The checkbox became a toggle with a short description, and the signal slider shows its value.
BeforeAfter
Device cards
Two ways to show a device
The card carries most of the app, so I designed two variations: same order (name, signal, readings), different weight. The second pass builds on them.
Variation 1 · while scanningVariation 2 · scan stopped
These are early variations, so the device names and numbers are placeholders.
Design system
A small system for live data
Two typefaces, two jobs. Inter for text, IBM Plex Mono for live values, so changing digits don't jump.
A calm dark palette. Navy surfaces, one blue accent, and green, yellow and red kept for signal quality, always with a word.
I tried gray and black themes and more colorful cards. I dropped them: the navy felt calmer, and extra colors competed with green, yellow and red, the one place where color has to mean something.
Draft explorations, not part of the final design
Second pass
Critiquing my own redesign
I then reviewed my redesign the way a crew member would use it: next to a vehicle, glancing at the phone to check every device is in place and charging. Eight questions were still open. This second pass answers them.
Interactive prototype
Try the second pass
Pick a scenario to jump straight into a state, or just use the app.
Scenarios
Try this
Watch devices appear while it scans, then tap × in the scan bar to stop.
Open Filter and show only devices that are not plugged in.
Tap the pencil on the unnamed device and give it a name.
In Connection lost, tap the red banner to jump to the problem.
Open settings and raise the minimum signal strength.
The UI is in German, just like the real app. The data is simulated.
The eight changes
01
Volts don't say "charging"
Users had to read voltage and current to work out a device's state.
Every card leads with a status chip: Lädt (charging), Voll geladen (full) or Nicht eingesteckt (not plugged in).
02
IDs aren't names
The technical ID competed with the name the crew gave the device.
The nickname is the title, the ID sits below, and unnamed devices are one tap from a name.
03
"Nothing found" too early
The empty state appeared the moment a scan started.
While scanning it says Suche nach Geräten… (searching). Only after a timeout comes an empty state with Erneut suchen (search again).
04
No obvious way to stop
Scanning drains the battery, but stopping was not clearly offered.
A labeled stop button (×) sits inside the scan bar.
05
Old data looked live
A device that went out of range kept showing its last values as if nothing happened.
Each card shows vor X s (X s ago). After 30 s it turns into Verbindung verloren (connection lost) and a banner points to it.
06
Problems hid in the list
There was no device count and the default order was arbitrary.
A device count, problems first by default, and a sheet for sorting and filters.
07
Small targets, quiet icons
Some icons had no label, some targets were too small for gloves, and logout sat next to everyday actions.
48dp targets, labeled icon buttons, higher contrast, and logout moved to a header icon.
08
Missing system states
Bluetooth off, missing permission and a lost connection were not designed at all.
Each gets its own state that explains what happened and offers the one fix.
Outcome
Every screen, plus a clickable second pass
Every screen and state of the app, the components and styles behind them, and a second-pass proposal as a clickable prototype.
What I learned
Color alone wasn't enough for the signal. Once I paired it with a word, everyone could read it. Setting the values in a monospace font made the whole device list calmer, because the numbers finally lined up.
I also learned to design the error states early. Stale data or Bluetooth switched off are exactly the moments when people need the app most.
