A 3D-Printed Framework and Some Copper Wire Turns Your Smartwatch Into a Field-Expedient Ammeter

Pseudonymous maker "jp3141" has come up with an unusual trick for the smartwatch owner needing to make power measurements in a pinch: turning its built-in magnetometer into an ammeter, as demonstrated with an Apple Watch placed in a 3D-printed coil assembly.

"Apple Watches newer than Series 5 which have a compass also contain a magnetometer. The magnetometer is also sensitive to magnetic fields from nearby currents," jp3141 explains of the project. "This demonstration uses a coil of wire around the watch to alter the magnetic field sensed."

In smartwatches, a magnetometer is typically used to find magnetic north as a means of improving navigation capabilities and gesture tracking. As it measures magnetic fields, though, it can be put to work on other tasks — such as acting like a simple metal detector or, in this case, an ammeter, aided by nothing more than some copper wire.

"A circular coil of wire with N turns and a diameter D will generate a magnetic field of B = u0.I/D (u0 is defined to be 4.π.10^-7)," jp3141 explains, "so for a watch with a diameter of about 48mm (approximately Apple Watch 5), 5 turns with 1 A will generate a field of 5 * 4.π.10^-7 * 1/0.048 = 131 uT (or 1.3 gauss). Because the magnetometer is not centered in the coil (it's usually in the very bottom right corner of the watch), the actual sensitivity is a bit lower; 100 uT/A is a reasonable approximation that can be improved by calibrating."

Even without calibration, though, it's possible to use an app that provides access to the raw magnetometer readings to measure current changes down to about 10mA — using a five-turn coil of copper wire around a 3D-printed framework to place the watch at its center. While the same trick could also be used on other devices with a magnetometer, it becomes more challenging: the hack works best with the magnetometer at the center of the coil, while larger devices like smartphones and tablets are more likely to have it further off-center than a small smartwatch.

The project is documented in full on GitHub, where an STL file to print the coil framework for an Apple Watch Series 5 is provided under the reciprocal GNU General Public License 3.