What we learned shipping EZ Fuse Tester, the app that turns your iPhone into a fuse tester

EZ Fuse Tester started on a Tuesday evening in late October 2023, with a half-untangled string of Halloween lights on the kitchen floor and the realization that we had no idea which of the tiny glass fuses inside the plug had blown. The cardboard packaging said something about replacement fuses being included for exactly this purpose. They were, of course, long gone. The nearest hardware store was closed. Testing each suspect fuse with a multimeter meant unscrewing the plug, prying out each one, and probing it with the leads in good light, which is a slow and grumpy way to spend the half hour before kids show up at the door.

The iPhone was sitting on the counter, screen up. The screen on a modern iPhone is essentially a giant capacitive sensor. The thought arrived as one of those middle-of-the-task questions: could you tell whether a fuse is good by laying it across the screen and seeing if the touch sensor registers the resistance change?

The answer, after a weekend of prototyping, was yes. EZ Fuse Tester shipped to the App Store free, without ads, and without collecting any data from its users, a few weeks later.

How it actually works

A standard glass cartridge fuse is a thin filament inside a glass tube with metal end caps. When the fuse is good, the filament is continuous and the metal end caps are connected through it. When the fuse is blown, the filament has melted and the end caps are no longer electrically connected.

The capacitive touch sensor in an iPhone screen detects changes in the local electric field. A continuous piece of conductive material laid across the screen registers as a long, connected touch zone. A broken piece of conductive material registers as two separate touch points. The app uses the multitouch detection capabilities of UIKit to figure out which case it is looking at.

The user opens the app, sees a guide image showing where to place the fuse, lays the fuse across the marked area on the screen, and the app immediately shows either PASS in green or FAIL in red. There is no calibration, no setup, no account, no in-app purchase. It is a single screen that does one thing.

The whole app is a few hundred lines of Swift, weighs less than three megabytes, and has no networking code at all. The privacy policy is one sentence long because there is nothing to disclose.

Why we shipped it free

A paid version would have made sense. The app saves people a multimeter and a trip to the hardware store. We chose free anyway, for two reasons.

The first reason is that the app is for a moment. You need it when you have a string of broken lights or a dead toaster, and you need it right now. Adding a paywall would add friction to a use case that should be one tap from search result to working app. Free, no account, no data collection means a user can install it, fix their lights, delete it, and move on with their evening, which is exactly the relationship we wanted with the people using it.

The second reason is that EZ Fuse Tester turned out to be the best advertising we ever made for WildTech. People in electrical trades and hobby electronics communities started sharing the app on forums. App Store reviews started showing up from people who fixed Christmas lights, classic car wiring, soldering iron stands, and old radio chassis. Each one of those reviews is a small piece of social proof for the larger business that we never could have bought directly.

What App Store users have done with it

The most common reviews are some variation of "I was about to throw out my Halloween lights and the app saved them," because of course that is how it started. The second most common review is about cars. Old British cars in particular have a lot of small glass fuses and a fanbase that likes to fix things themselves at home. We get reviews from people who used the app on Land Rovers, MG Midgets, and old Volvos. One reviewer used it to test the fuses in a vintage Sony reel-to-reel tape deck and reported back that the app saved his weekend.

The reviews that always make us smile are from people who were not technical at all. They downloaded the app on the recommendation of someone in their family, used it to fix a lamp or a string of lights, and wrote a review that was more about the relief of not having to drive to a store than anything else.

What we would do differently

If we built EZ Fuse Tester today we would add support for blade fuses, which are the larger flat fuses found in modern cars. The current version only works with the small glass cartridge fuses, which is mentioned clearly in the App Store description but does generate the occasional one-star review from someone who tried to test the wrong kind of fuse on it.

We would also add an optional dark mode UI. The current PASS and FAIL screens are bright and high contrast, which works well in the daylight and terrible at three in the morning when your refrigerator just made a strange noise and you are trying to find the bad fuse without waking anyone up.

We probably would not change anything about the business model. Free, no ads, no data collection is the right answer for an app whose job is to save someone twenty minutes of frustration. The goodwill from doing that for free is worth more than the dollar or two we could have charged.

The bigger lesson

The whole project took about a weekend of focused work plus a couple of evenings of polish before submission. The total cost was the time. It earns no money. It runs entirely on the phone. It generates no support tickets because there is nothing to support. And it has produced more inbound interest in WildTech than any of our paid marketing experiments combined.

There is a lesson buried in that for any developer or small studio: the projects that pay off are not always the ones you build to make money. Sometimes the right thing to build is the tool you wish existed at the moment you needed it, and then to ship it the way you wish it had been shipped. Free, fast, no nonsense. That is what EZ Fuse Tester is, and it is one of the things we are most proud of having put into the world.