Inductive_Dice

Wireless chargers are widely available in may shapes sizes and power ratings for changing phones and other portable appliances. Versions are available which include inductive LED's for experimentation.

It was whilst looking at these various chargers it occured to me that I could make use of this technology to create a practical application in the form of a dice housed in a 3D printed cube.

The simplicity in this electronic dice is that it contains no microcontroller, accelerometers, gyroscopes, compass or batteries whilst at the same time only requiring 21 LED's and wire.

Orientation and the required connectivity of existing inductive LED receivers precluded their use directly in this project.

Therefore, this would require making the inductive LED receivers by hand.

The size of the housed die is 26 x 26 x 26 mm and weights 16g.

Read on to find out more.

Surface Mounted LED's Red - Qty 21

Enamelled copper wire (ECW) 35AWG/0.15mm

Tinned copper wire 24AWG/0.58mm

Wireless Charging Pad (types between 5W to 15W will work.), ~100mm (dia) x 10(H) mm

PLA Filament - Yellow (internal parts, although these could be any colour)

PLA Filament - Green (dice body, works well with red LED's)

Clear Lacquer

Clear Tape

Glue

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

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.

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.

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

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.

Adding a ferrous core in the receiver to concentrate the field can also be added but in this application air core coils will be used which simplifies the project, reducing size, weight and cost at the loss of some efficiency.

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

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 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 receivers will simply consist of a multiturn coil connected in parallel with high efficiency LED(s) with one fitted to each side of the cube.

Therefore as the position of the receiver coil will vary in use, high efficiency red LED's will make best use of the variable energy being more visible at lower energy levels compared to other colours.

The orientation of the receiver coil will play a key role in ensuring that only receivers 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.

Due to the large variety of chargers with different power specifications it seemed worthwhile to evaluate a cross section to determine if there were differences that would impact the operation of the Inductive die.

Wireless charging of devices is covered by the Qi specification

Three different devices were selected:

All single coil with a round body of ~100mm in diameter.

Non of the devices indicated the specific QI version.

1: 5W - Micro USB

2: 5W, 7.5W & 10W - USB-C, FOD (Foreign Object Detection)

3: 5W, 7.5W, 10W & 15W - USB-C, FOD

FOD was not triggered by the presence of the dice however, FOD was verified to work with a metal can.

There is normally a communication protocol between the charger and the appliance to be charged to determine if and how much power is to deliver.

However, in this particular application there is no communication and the charger is effectively in a stand by mode, but this is sufficient for this project.

The wireless dice operated as expected with all three charger types.

The physical structure for the dice is 3D printed.

It will be made up of six identical sides, each containing seven holes to allow the LED's to be positioned to form the dots in the dice and a cylindrical section to support the coil and mount the side on a central support cube.

The central support is a hollow cube with holes passing through each face to accomodate the sides of the dice.

Once the sides have been assembled on the central support the completed elements are housed in a cube consisting of two parts, a main body and lid. No markings are made on the cube surface to identify the numbered sides as this is indicated by the illuminated LED's.

The purpose of the hollow cube is to help support and strengthen the elements of the die, diffuse the light from the LED's and enhance the randomisation.

Dimensions of each element.

Side: 19.2(W) x 19.2(L) x 5.8(H) mm - Qty 6

Central support cube: 12(W) x 12(L) x 12(H) mm

Hollow cube: 26(W) x 26(L) x 26(H) mm

Lid: 22.5(W) x 22.5(L) x 2.8(H) mm

The elements are all printed using the same settings.

Filament: PLA+ Yellow and Green

Internal elements were printed in yellow but any colour would be suitable.

However, for the external cube when using red LED's a green filament gives good contrast without reducing the light intensity too much.

Other colours were tried, Yellow - resulted in too bright a light with poor contrast, Blue - resulted in too dark a light.

Performed some experiments with infill pattern and density but settled on 100% density in this case.

Layer Height: 0.15mm

Infill Density: 100%

Base Adhesion: Skirt

No supports.

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.

Apply special attention to the holes in the central cube to ensure they are smooth and round.

Likewise, to the top rim of the coil formers on each side element that they are smooth and round.

Test fit the rim of the coil formers to ensure that they fit the holes in the central cube.

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 side elements are square and smooth.

Remove any blobs or high areas on the inner surfaces of the hollow cube.

Do a dry assembly of the central cube and side elements and ensure it fits within the hollow cube without issues as the design is a close tolerance fit.

Ensured any abberations with the internal surface of the hollow cube and/or the assembled side elements have been addressed and that it slides in and out easily prior to fitting the coils and LED's.

The basic circuit design is the same for each side, an air core coil and parallel wired LED's the quantity depending on the side associated with the die pattern required for the represented number.

The red LED's are surface mount 2mm x 0.775 mm.

The sides are arranged in pairs such that the sum of the LED's on each side adds up to seven.

Coils are connected in such a way that they drive the LED's on the opposite side of the cube.

E.g The coil on the six side lights the LED on the one side and the coil on the one side lights the LED's on the six side.

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

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

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

The oscilloscope trace shows where the LED is switched on and where it switches off (ringing).

The ringing is were the collapsing magnetic field in the inductor charges the parasitic capacitance, the discharge of the capacitor creates an increasing magnetic field in the inductor. However, losses in the circuit result in the oscilliation deminishing over time until the next pulse of energy is applied to restart the process.

The basic assembly process is the same for each side.

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 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.

The LED's are held together on a wire frame made up of 2 U shaped sections formed from tinned copper wire.

Using one of the sides as a template and using pliers to form the wires.

One section follows the outer perimeter of the side aligned to the holes. Try to keep this as close to the holes as possible, to reduce the likelyhood of contact with neighbouring sides when fully assembled.

The other section aligns to the perimeter of the side but on the opposite location of the holes closest to the coil former.

In order not to damage the 3D printed side whilst soldering a template was created by tracing around the edge of the side element and the holes on card.

Decide on the polarity orientation of the LED's and maintain this for all the LED's on all of the sides for consistency and to aid troubleshooting in the event of issues.

Numbers 2, 4 and 6 are the simplest configurations as they do not use the central hole.

Position and tape one of the U shaped wires on the card aligned with the hole positions and sit the LED's to be soldered in the hole positions and solder in place.

Position and tape the other U shaped wire on the card aligned with the LED's and solder to complete the attachment.

Cut the excess wire leaving two parallel 5mm stubs, these will be used to connect the flexible wire from the appropriate coil.

Test with a meter in diode/continuity mode, the LED's should illuminate (verify correct lead orienation from the meter). If they all do not light check for shorts and/or opens with a meter.

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

Test the LED's on the frame work by clipping the ends to the coil and place the coil in the centre of the wireless charger all the LED's should flash. If they all do not flash check for shorts and/or opens with a meter.

A dry assembly of side elements prior to finally connecting the coils can be tested with a meter to check for shorts with neighbouring sides as it is easier to make adjustment before the coils are wired.

The same process applies for the numbers 3 and 5 with the exception of the centre LED which is attached by flexible wires which are looped via the channel under the coil.

For number one the LED is soldered directly to the associated coil wires.

If everything is fine at this stage apply clear lacquer or varnish to the framework to insulate the wiring.

The side elements are attached to holes in a central cube by the top rim of the coil formers.

Assembly is conducted in pairs.

Attach the number 6 side to the central cube and directly opposite to this attach the number 1 side.

Trim the free ends of the coil wires to reduce the wire length to minimise stray coupling.

Strip and tin the free ends with solder.

Solder the number 6 side coil to the number 1 side LED.

Solder the number 1 side coil to the number 6 side LED frame work.

Test the two sides operate independently by placing each side in turn on the wireless charger.

When the number 6 side is placed on the charger, the number 1 side LED should flash.

When the number 1 side is placed on the charger, the number 6 side LED's should flash.

If there are no issues secure the sides in place with glue, ensuring these are square and parallel so as to prevent issues when fitting in the hollow cube to protect the assembly.

Test fit with the hollow cube to ensure a smooth fit and file any rough or high areas.

Next select another number pair 5 and 2 and repeat the assembly and test process.

As more sides are added ensure that the proximity of the frame work outer wire does not short with neighbouring sides.

This will be evident during the testing process if sides not horizontally aligned and in contact with the charger are illuminated during testing.

If necessary reapply the lacquer/varnish and/or insulate the frame work with a strip of clear tape.

Repeat the process for number pair 4 and 3.

Once all the sides are in place and the cube is complete, test to ensure all the sides behave independently and remedy any issues if found.

The size of this assembled is: 20(L) x 20(W) x 20(H) mm

The purpose of the hollow cube is to protect, support and strengthen the elements of the die, diffuse the light from the LED's and enhance the randomisation as no pips are visible on the exterior.

It just remains to finally put it in place in the hollow cube and fit the lid. The lid is a push fit and should stay in place.

But for added security, subject to use apply a little glue around the rim if required. The downside is that this will prevent easy removal should this be required.

The inductive dice is thrown in the same way as a standard dice. However, it must land on a wireless charger within the central area encompassing a radius of ~26mm for effective iilumination.

This may be difficult to achieve without an enclosure to contain the dice or simply placing the dice on the charger.

Simply rolling the dice picking it up and placing it on the pad will work unless the sides are marked allowing the numbers to be known removing the random element.

However, adopting the enclosure method removes any intermediate intervention that may affect the randomness, I took this one stage further and added a dice tower.

Full details of the Dice Tower can be found in another Instructable.

Hope you found this of interest.

Let the games commence.