A short diagnostic for when one key refuses to type
Difficulty
Beginner
Time
20 minutes
Steps
5 stages
Every file for this build is up on GitHub, the plates, the case and the firmware
Our GitHubGather everything below before you start and the whole build goes a lot smoother. The printable parts are on GitHub, so you only need to buy the hardware.
| Component | Qty | Specification |
|---|---|---|
| Multimeter with continuity mode | 1 | The cheapest one will do. This is the tool that finds the fault |
| Soldering iron | 1 | For whatever you find |
01
Multimeter
02
Soldering iron
03
Good light
04
A keyboard tester in the browser
Work through these in order. Anything you need to download along the way, the plates, the case and the firmware, is already waiting for you on GitHub.
01
Step 1 of 5
Before you open anything, press every key and write down which ones fail. One dead key is a joint or a diode. A whole dead row or column is one wire or one MCU pin. Everything dead is power or firmware. Five minutes here tells you which of the next steps to jump to, and saves you inspecting sixty joints for no reason.
02
Step 2 of 5
Open the sandwich and put a light on the suspect switch. You are looking for a joint that is dull rather than shiny, a wire that is resting against a pad without being soldered to it, or a strand that has wandered across to a neighbour. A surprising number of dead keys are visible before you ever pick up a meter.
Pro Tip
Reheating a suspect joint and adding a touch of fresh solder takes ten seconds and fixes this more often than it has any right to.
03
Step 3 of 5
If your board has diodes, put the meter in continuity mode across one. It should beep in one direction only. A diode in backwards gives you exactly this symptom, one key that never reports while its neighbours are fine, and it is easy to do when you are placing dozens of them. The band on the diode body is the cathode and it faces the row wire.
Pro Tip
Compare against a key that works rather than trying to remember which way round is correct. The working one is your reference.
04
Step 4 of 5
Still nothing? Put one probe on the MCU pin for that key's row and the other on the switch leg, and press the key. No beep means the break is somewhere along that wire. Repeat for the column. This finds breaks hiding under insulation, which you cannot see, and it is why the meter is worth owning.
05
Step 5 of 5
If the hardware tests clean, the key may simply not be mapped. Open your sketch and check that position actually has a keycode rather than NONE, and that your rowPins and colPins match the pins you soldered to. Rebuilding the layout in the Keymap Editor and reflashing is the quickest way to rule this out.
Pro Tip
Still stuck after all five? Send us a photo through the comment form. We have almost certainly made the same mistake ourselves.
Stuck on a step, got a tip, or just finished your build? Drop it in our form. We read every single one and it helps us make the guide better.
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