Turning the iPhone 6s Into a Digital Scale

Ryan McLeod
Oct 27, 2015 · 7 min read

September 9th, 2015

September 12th, 2015

My friend Chase McBride calls with a few tech questions (rough translation: startup idea). Chase and his friend Brice Tuttle have been talking over this idea of a scale app using 3D Touch. It’s the first I’ve heard of the idea. A quick peek through the APIs showed that we wouldn’t be able to simply place anything on the screen and get a force value. The only App Store friendly way to get a force value would be by touching the screen with a finger and even then the force returned would be the force directly under the finger.

We’re on the phone talking through this initial hurdle, when my excitement turns to concern. I start recalling other times Apple hasn’t exactly been stoked on novel uses of their devices’ sensors, but then I remember that despite this the App Store is speckled with creative workarounds including a panorama app that uses vibration to rotate the phone, magnetometer-based stud finders, camera/flash-based heart-rate monitors, and even Square’s ubiquitous headphone-jack card reader. A creative solution is waiting to be found. Creating a scale turns from impossible to a challenge.

September 21–24th, 2015

I have a 5s I love, but decide if this is going to happen I need to preorder a 6s. We have roughly five days until the phone is in my hands, so what can we do in the meantime? We need to build a barebones testing app to hit the ground running and we need to come up with a way to weigh objects without touching the screen. Without that, we’ll be dead in the water.

With Chase and Brice in San Francisco, and me in San Luis Obispo we hop on FaceTime to channel our inner MacGyver and brainstorm a solution. We tried foil, watch batteries, apples, carrots, coins, even salami— at one point we had built a meat basket out of a piece of salami stuck to a jar lid with a needle as a conductive handle (pictured below). None of these solutions fit the trifecta of being convenient, common, and non-perishable.

Meat basket

September 24th, 2015

iPhone due to arrive tomorrow

We thought through the problem again: we needed an object that was conductive, had finger-like capacitance, formed a single finger-like touch point, was a household item, and could hold items to be weighed…

Conductive, capacitive, common, and curved to a single-point of contact. A spoon was the perfect solution we had been looking for.

Gravity’s calibration screen

September 25th, 2015

0–10 nickels and then 10 nickels + 100g weight (on a metal spoon)

“Dude. It’s perfectly linear”

With the force values linearly correlated to weight, turning any force into a weight was going to be as simple as recording the force of known weights and creating a linear regression. It’d even be possible to use some statistics to predict how well the calibration went (there are many factors that can throw off a calibration). We opted to use coins for calibration, with a framework that made it easy to internationalize in the future.

September 29th, 2015

Waiting for Review

October 7th, 2015


Screenshot from an App Store search for “digital scale”

The Appeal

The Demo Video

October 15th, 2015

No Scale Apps

We were—and still are—bummed to say the least, but we understand some of the reasons Apple might not be allowing scale apps at this time.

Damage to the phone?

Misuse of the API?

Drug use?

So now what?

“An app store with over one-and-a-half million of the best apps available. That’s over one-and-a-half million hand-picked, awe-inspiring, just-plain-surprising, who-knew-a-phone-could-do-that apps.” —Apple’s Amazing Apps Ad

We have a strong respect for the subjective process Apple uses to maintain a selection of high quality apps and look forward to seeing other creative uses of 3D Touch, but do hope for a day when Gravity can be one of the hand-picked, who-knew-a-phone-could-do-that-apps anyone can download on the App Store and have in their pocket.

Want to know more? Sign up for Gravity updates at gravity-scale.com.

Like this? You’ll probably love my other app: Blackbox

Published in #SWLH (Startups, Wanderlust, and Life Hacking)

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Ryan McLeod

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Making @blackboxpuzzles – Apple Design Award cube carrier – was expelled from preschool for “defiance”.

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