Pinball electronics

 

I finally started on the hardware design. This will make it easer to start testing playfield parts.

The pinball will be driven by the following parts:

- A custom designed pinball IO board. This controls 8 coils and reads 16 inputs.It can control 2 led chains.  It is connected with an 100baseT Ethernet port. A firepower based pinball will use 3 or 4 boards, depending on design.

- A Raspberry Pie as central pinball controller. Alternative the custom IO board can play the pinball controller.

- Ethernet switch to connect it all.

- A 12V power supply to feed the LEDs and electronics

- A 24v or 48v  power supply for the coils.

- 7 segments units for score and ball counters.

Except for the IO board are all other of the shelf parts.

The first version will have simple 7 segment number displays for scores. Later on I like to add a LED matrix display.

 

Pinball IO controller

The pinball controller will have the following interfaces:

- 100 BaseT Ethernet interface

- 8x high current coil driver outputs with overcurrent protection.

- 16 inputs, working both at 5v and 12v

- 2x ws2812B led chain outputs

- Some digital IO to control stepper motor driver or other electronics.

- 2x servo outputs.

- Cpu program header.

- power connectors.

- if PCB space allows: 2 analog outputs

- Also pcb space depending : mini usb-b connector and micro Sd card connector.

To feed the ws2812b LEDs the IO board will need to have a switch mode power supply to generate 5v from 12v. The board will have a glass fuse for the coil , and PTC fuses for the rest.

The board is quite generic, and can be used for other purposes besides driving a pinball machine.

Design choices

CPU

There are not many low end ARM processors that have an Ethernet interface.  Initially the NXP LPC1759  was a good candidate, but it already some years old.  The ST  STM32F407VGT6  is a more modern chip with more ram and faster cpu. The 100QFP package has enough IO pins, so can interface with all the IO’s directly.

The issue is that all the IO pins of the CPU have about 5 functions. So it will be a puzzle how to connect them up.

Because of the Ethernet phy clock requirement, the cpu has to run on 25Mhz.  To reduce any risk I selected a crystal oscillator for this. They are the size of a grain of salt in:3225metric or 3mm long and 2mm wide.

Ethernet PHY

I was searching for an Ethernet phy with a PLL. It seems that the only types available all have a QFN package. This leadless package is a bit tricky to use. Even worse it has an exposed pad that also need soldering.  You really need to  work with a paste stencil and reflow oven to use them. Settled for a LAN8720 .  Both mechanical as electrical. IT is supplied with 25,hz from the CPU, and the phy PLL creates the 50mhz needed for the RMII interface. This part will be the most critical part of the whole design.

The rj45 connector has an integrated common mode coil and transformer.

PSU

Used here a super standard switch mode chip: LM2596S-ADJ. This device is used on 1000’s of designs. It will create 5v at max 3A. The 3.3v rails for cpu and phy are using a LM1117 LDO linear regulator from 5v rail.

Coil driver

Tons of fet available for this. Selected the same transistor as used the Medieval Madness Remade (MMR) electronics only in SMD version: the IRF3710ZS. These N channel Fets have impressive characterises for their price: 100V rating at 59A with a resistance of 18mohm.  You can get lower resistance versions, but the price rises. Also the transistor is not switched on all the time. At 10A continues they only dissipate 1.8watt.

The n channel fet needs to be driven to saturation. To do this the gate has to be above 8v. A dedicated driver is easier to use, and I selected a dual channel low end driver IR4427S. This so8 device can drive 2 fets, so there will be 4 present to drive the 8 fets.

A 0.01 resistor is used to sense the current through the transistor, monitored by the CPU.

Connectors

For all low current connections the family of Kf2510 connectors is used. A 3 pins version for each of the digital switch input. This connectors has a latch function that prevents the connector falling out. Sadly it is a through hole part, so can’t be reflow soldered.

For the higher current connections the family of  Vh3.96 connectors is selected. also these are lockable. The coil outputs are 2 pins.

Sourcing

Most parts are sourced from Aliexpress. I really hope that there will be no fake chips send, particularly for the CPU.

PCB

I am using Kicad as PCB design tool. A very nice tool with extensive PCB footprint library. Sadly the BOM part of the program is not implemented very will, but no complains. It is free.

A common and thus cost effective size for the PCB is 10 by 10 cm for double sided board. I would have preferred a longer and smaller board, but that would increase the costs a quite lot. To much for a prototype.

On this moment still drawing the schematics and need to finalise the cpu pin selection.

Playfield construction

 

What would be the best way to make a playfield? There are so many ways to do this.

Playfield constructions

In general there seems to be two approaches

1. Plywood with print on top and a thin protective coating op top. Inserts are glued in the playfield for lights
2. Plywood or mdf with thick plastic plastic sheet that is printed on the back.

Opt. 1) plywood with inserts and coating

Approach 1) plywood with inserts and coating is the most common way to make playfields.

The playfield is first drilled/milled. Then the inserts are all glued in and everything is sanded, sealed and printed. Finally it is covered with some car grade clear top coat. 

Topcoats

The biggest issues for a DIY using 2 component clearcoat are that this stuff is poisonous, so you need a spraying boot with good dust masks etc. Also you need a good sprayer and compressor. And there is lot of sanding involved between every clearcoat layer. But this can be outsourced to a car painter.

An alternative clearcoat approach for DIY is using self leveling compound. This is normally used by artist to make there artworks shiny. This can applied in a single thick coat. Because it is self leveling property it creates a very smooth surface. For this you don’t seed sprayboots or sprayers so it done with less investments. I know one diy that has used this on his playfield successfully, but have not seen reports how good this holds up.

A other diy only way is to cover the whole playfield with mylar or thin polyester sheet.

Prints

The prints can be done in various ways:

- Silkscreen printing. This is the main way playfield where made in the past. You need to make a lot of silkscreens: one for every color printed.

- Direct print on wood/plastic with dye’s. This seems to be the common way most playfield are produced nowadays. For DIY is this also nowadays a usable approach, because the dye printers are quite common now. If the dye printer can not print white, the playfield first has to be sprayed white. Any insert need to be masked to prevent it being painted.

- Vinyl overlay glued on wood/plastic substrate. Typical only used for the outside of a pinball cabinet. Some people have used this approach also for there one diy playfields.

- Paper poster glued down: have not see anyone using this way.

- Decals: Typical only used for small area’s like inserts or small repairs. Possible worth a try if you can cover larger area’s without distorting the very thin decal

 

Opt. 2) Playfield with thick plastic sheet

The playfield with thick plastic is only used in a couple of pinball machines For example it is used in Elektra, Bushido, Canasta.

The playfield looks like this:

(picture from pinside user Star_Grazer)

The wood is either plywood or MDF.  MDF is more difficult to screw in, but it surfaces are normally very smooth. The wood has only holes, and maybe a sealer, but not other paints.

The plastic sheet has the graphics on the bottom/reverse side, so the graphics are fully protect against damage. The surface of the plastic is fully smooth, so no sanding or coating is needed. Likey a wax coat can be used to protect  the plastic better against scratches.

No inserts need to be glued. The shape the hole in the wood defines the insert outline. So this gies a huge flexibility and allows to choose any ‘insert’ shape and size as you like. There will likely be light bleed from the hole to the surrounding playfield. The bleeding can be reduced by making wide black area’s around the “inserts”.

Because you need to have access to a cnc to mill the wood, the plastic sheets can also be milled on the same machine . Instead a cnc a laser cutter can also be used. A laser cutter has the advantage that it create much nicer edges  in the plastic then what a mill can make, but cannot cut all types of plastic. If you have a powerfull enough lasercutter, you could even cut the wooden playfield with it. Any charring on the wood in fine and not visible. Because the holes are black, it is possible that that even helps reducing light bleed.

I looked around , but I could not find information on the web what the actual thicknesses used for the wood and plastics. If standard pinball mechs are used, then the total thickness of both wood and plastic need to be 1/2 inch or about 12mm.  Thus you could use 9mm wood and 3mm plastic.  If the thickness is less critical, then use of 12mm ply or mdf with 3mm plastic to make a stiffer playfield.

The type of plastic. This is also unclear what the excising machines are using for this. The most likely options are:

-  Plexiglass (PMMA, acrylic glass). This is very transparent and reasonable stiff material.

- PETG is a more flexible material. Lot of times used for transparent playfield parts and ramps.

- Polycarbonate. This is the strongest material (and the most expensive). Can not be lasercut due to poisonous vapours escaping. It scratches easy. 

print

The print can use the same approaches as for the plywood print. If the decal or vinyl approach is used, then these need to have holes cut in the same places as the plastic sheet to pass though any pinball mechs.

After the colors & black are added to the plastic, it has to be coated with white to create better colors. Possible the wood can also be painted white to help reflacting the light.

The places where the ‘insert’ are present can either be coated white too, or covered with a diffusor layer. This will help spreading the light from LED , so the insert is lighted evenly.  The diffusor layer could be a layer of thin sime (transparent) white plastic. Not to thick, otherwise it creates more light bleed.

 

The firepower playfield

After looking though all the options I think I go for the wood/plastic sheet sandwich approach. No messing with paints and sanding. No cupping of inserts. Always a very smooth playfield. Drawback is that it needs 2 milling sessions.

I will do some test to see if the 9mm wood and 3mm plastic sandwich is this is stiff enough for a playfield. Otherwise I will use 12mm wood and 2 or 3mm plastic. Because I print my own playfield mechs, the thicker playfield is no issue. I just have to correct my drawings.

For the first test will use plexiglass with wax.

For printing: I will get some quotes for dye printing on plexiglass and vinyl prints and view some examples how the colors will look. I will also try how decals work on plexiglass. Further some testing what type of diffusor layer is the nicest for the ‘inserts’

So plenty of things that need experimenting. I will keep you posted.