Since the key fobs are powered by a rechargeable battery, one design challenge is to make the charging as user-friendly as possible.  I didn’t want to have deal with wires or replacing batteries.  Therefore, I implemented a wireless charging system similar to what’s available for mobile phones and electric toothbrushes. Energy is transferred through inductive coupling.  An alternating current is run through a coil in a cradle which creates a fluctuating magnetic field and thanks to Faraday’s law of induction this generates an alternating current in a secondary coil which will be inside the fob.

The charging cradle was designed using the free version of SketchUp.

The cradle is 3D printed using a 2 color scheme. The first 11 layers are green PLA and the rest is a black PLA with a bespeckled finish.

The front panel holds the key fob. It covers the main prism shaped piece which holds the transmitting coil and a magnet designed to activate a reed switch. The transmitting coil is powered by a 9 VDC power supply.

The charging coil generates heat which gets hot enough to deform the PLA. Therefore, the base of the cradle holds a quiet 60 mm fan which sucks air in from the vents on the bottom and blows it over a ferrite plate (38 mm x 38 mm x 2 mm) which is adhered behind the coil. Beside acting as a heatsink, the plate should also improve charging efficiency by concentrating and directing magnetic flux between the transmitter and receiver.

Multiple charging cradles can be snapped together and powered by the same 9 VDC power supply.

This guide was first published on Jul 22, 2020. It was last updated on Jul 22, 2020.
This page (Charging Cradle) was last updated on Sep 16, 2020.