LED Pinball Display For Early Bally/Stern Games
Author: Wayne Eggert
Date: 08/14/2011
Introduction
I've been working on prototyping an LED display for early Bally/Stern pinball machines. There are commercially available pinball displays for these machines readily available, but at a pretty steep price point. So since I'm interested in learning how the displays work, I decided to make it a project =)
There won't be much here for right now, just some pictures but I'll be updating it as I complete the project. Once I have a solid working prototype I plan on having some professional PCB boards created and will most likely offer some for sale and hopefully fund some projects that way.
Initial Prototype
This has been several months in the making, originally I was prototyping
entirely on a breadboard and only hooked up two digits due to a MESS of
wires. I was using common anode LED digits with the digit enable transistors hooked up in an emitter-follower circucit that would have in theory minimized the amount of components neede. It was not working correctly -- as the number of segments lit changed, the segments would either dim slightly or be slightly brighter. I'm thinking the emitter-follower circuit was too dependent on the current gain (that's what a transistor does, it takes a small current at its input and amplifies it at the output). Anyway, I ordered some common cathode LED digits so I could try prototyping a different circuit.
Digit Display Board
I worked on the digit display board first. As the name suggests, this is just a board with the display digits. On the old Bally/Stern games the displays were hooked into the display boards at a 90 degree angle and the display board then slid into a metal chassis. Anyway, this board was an important first step because it will be the first time I will have had all 6 digits wired up and ready to test.
Wiring it was a mess -- each digit needed to have its segments connected to the next digit. But such is the nature of prototyping. The end result is a pinout for the 7 segments and a decimal point and separate pins to enable each digit. This board will connect into the component board that will have all the ICs, transistors, resistors, etc needed to interpret the signals from the pinball MPU board.
Picture: Hot Glued over all solder points and then put a thin sheet of plastic over entire back
That's it for now.. I'll be working on the component board next, most likely on the breadboard for a while since that circuit will most likely get a lot of tweaking before I'd want to solder the circuit together permanently.
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