Wanting to add some sparkle to a LEGO project its time to add some lights.

Bricks with integrated lights do exist but they have leads on.

What if you could include inductive lights and do away with lots of connecting wires and the batteries within the project.

If you have an inductive charging mat there no reason why this could not be realised.

Read on to find out more.

Supplies

Clear Lego Brick (2 x 2 bump)

PLA Filament - Yellow (although any colour will be suitable).

Surface Mounted LED's Red

Enamelled copper wire (ECW) 35AWG/0.15mm

May prove more cost effective to buy a range of values rather than individual values unless you already have them available. Some components may also have a MOL greater than the quantity specified in the component list.

Tools

3D Printer

Pliers

Wire cutters

Soldering Iron

Solder

Sanding paper

Needle files

Lacquer or Varnish

Know your tools and follow the recommended operational procedures and be sure to wear the appropriate PPE.

No affiliation to any of the suppliers used in this project, feel free to use your preferred suppliers and substitute the elements were appropriate to your own preference or subject to supply.

Links valid at the time of publication.

Step 1: Methodology

The process works on the principle of magnetic induction.

If a first coil of wire is energised with a changing voltage it produces a changing magnetic field which induces a changing voltage in a second coil which is placed in close proximity to the energised coil.

The coils are created using multiple turns of insulated copper wire as uninsulated coils would create short circuits which would reduce the effective number of coils and the magnetic flux.

Applying static DC to the above configuration will couple a magnetic field between the coils that will only produce a voltage when the voltage changes (when switched on or off), and therefore for practical purposes AC is used instead of DC. As the coil needs to be driven on an off repeatedly to generate a usable voltage.

The magnitude of the induce voltage is influenced by the rate of change of the magnetic flux the tighness of the coil windings, the number of windings, the proximity of the coils and coil orientation.

V = -N(d!B/dt). where d!B = change in magnetic flux, dt is the rate of change of the magnetic flux and N = turns of the coil. Referencing: Faraday's Law and Lenz's Law

Adding a ferrite core in the receiver to concentrate the field as can be found in commercially available units can also be added but in this application an air core coil will be used which simplifies the project, reducing size, weight and cost at the loss of some efficiency. However, the loss of efficiency in this case is not sufficient to seriously compromise the performance.

The coupling coefficient (k), between the transmitter and receiver coils is expressed as a number between 0 and 1.

k for air core coils is 0.4 to 0.8 subject to spacing compared to ferrous based cores at 0.99

For commercial units In addition to a ferrite former a capacitor can be found which forms a parallel resonant circuit, optimising efficiency at high frequencies. However, in this case the frequency is low whereby the use of a capacitor is impractical due to the large size required that would affect practical use.

The wireless charger contains a horizontally coil which is driven by the electronics to produce a changing magnetic field. Examination of the field generated by the charger reveals that this is not energised continously but with rapid bursts of an AC signal. Assessed with a coil across a 1kR load this consists of a 80Hz-400Hz signal in 100mS bursts active every 500mS.

The approximation for resonance can be found from Rf = 1/((2*PI)*SQRT(L*C))

Tuning the circuit at a frequency of 80Hz-400Hz with a 270uH inductor would require a 15mF-0.6mF capacitor which is impractical and not strictly required in this implementation.

The energy captured will not be used to charge a battery or converted to a static DC for other uses but used directly.

Maximum energy transfer takes place when the receiver sits in the middle of the charger with rapid falloff at the edges.

The receiver will simply consist of a multiturn coil connected in parallel with a high efficiency LED fitted in each brick.

High efficiency red LED's will make best use of the energy being more visible at lower energy levels compared to other colours.

The orientation of the receiver coil aligned horizontally and in contact with the surface of the charger will be energised switching on the associated LED's. Vertically, positioned coils will not be sufficient energised due to poor coupling to switch on the associated LED's.

Step 2: Design

The coil former was designed using BlocksCAD, to fit a 2 x 2 brick, a square panel with hollow cylinder that slides over the internal central column.

This encapsulates both a wire coil and a single LED whilst allowing sufficient clearance to enable stacking of the brick as normal.

4x4lego_coil_form.stl
Download View in 3D

Step 3: Printing

Elements were printed in yellow but any colour would be suitable.

Filament: PLA+ Yellow

Layer Height: 0.15mm

Infill Density: 100%

Base Adhesion: Skirt

No supports.

Size: 12.5(L) x 12.5(W) x 6(H) mm

Print Time: 5 mins

Step 4: Post Processing

Some post processing in the form of filing and sanding may be required subject to print quality to remove strings or blobs that may be obscuring holes or result in uneven surfaces.

Test fit the coil former to ensure that it will fit the brick.

Ideally they should fit snuggly and hold in place. In any event a loose fit is prefereable to not fitting at all as a loose fit can be remedied with glue.

Ensure the edges of the elements are square and smooth.

Do a dry assembly of the coil former to ensure it fits within the brick without issues as the design is a close tolerance fit.

Step 5: Circuit

The basic circuit design consists of an air core coil and parallel wired LED.

The red LED is a surface mount 2mm x 0.775 mm.

The coil (~270uH), is made up of ~200 turns of 0.15mm (dia), ECW wrapped on the central form.

An estimation of the coil inductance can be derived from Brooks formula: L (mH) = 1.6994x10e-6 x (R*(N^2))

Where: R = mean radius of winding (mm) , N = number of turns.

Step 6: Coil Winding

Create a free end of ~60mm of wire and wrap 200 turns of wire around the central former of the side element, trying to ensure that the windings are tight and evenly spaces. Due to hand wiring there will be some variation in the coil due to turns, wire spacing and overlapping but not enough to significantly effect operation.

Slide a 6mm tube/dowel wrapped with tape into the centre of the former wedging it in place makes it easier to hold and wind the coil.

Once complete secure the free end and cut the wire leaving 60mm and tape to the back of the side element.

The free end lengths of 60mm are to allow easy manipulation during assembly and test and can be trimmed as required as the side pairs are fitted. Better at this stage for wires to be too long rather than too short.

Apply clear lacquer or varnish to the coil to hold the winding in place.

Step 7: LED Wiring

Solder the two wires from the coil to the LED.

Separately or inaddition to testing with a meter verify operation with a coil.

Test the LED and coil by placing the coil in the centre of the wireless charger the LED should flash. If it does not flash check for shorts and/or opens with a meter.

If everything is fine at this stage apply clear lacquer or varnish to hold the wiring in place.