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.