The “Pegboard 4 by 4”

Learning to build state machines with the Pegboard 4 by 4.

To set the scene: it is fall of 2015 and I have decided to “throw my hat over the wall” – commit to demonstrating the AndOrBit device ion a few months even though it does not yet exist.

The commitment is to teach three workshops as part of the one-day annual “Expanding Your Horizons” event for girls of grade 6 – 12 on March 12, 2016. This, I thought, would force me to get through the process of making a first production run of my long-awaited “AndOrBit” digital logic training device.

Of course, this strategy is also a good way to lose one’s hat, or to force one to make drastic changes in plans so as to meet the commitment.

The outcome of this decision was that six devices were built consisting of drastically scaled-back versions of the original device concept. They were larger on the outside (24 inches square) and smaller on the inside (4 bits of latch with 4 logic terms per bit as versus 8 bits with 8 terms per).

It wouldn’t perform many of the functions planned for the larger device (no external connections, for example) and the matrix interconnections were made with the bare hands manipulating pairs of miniature “grabber clips”connected by an axial-lead diode.

But they did work well enough to allow students to create a 4-bit binary counter, and I began to learn how to teach using them. As of this writing I’ve brought them to about six “Hack the Future” one-day events for students aged 10 – 18 and tried both group courses and more chaotic individual instruction.

I call them “Pegboard 4 by 4” units as opposed to the intended “8 by 8” units. They take a lot of time to build, with a grid of 16 bare wires hand-strung in the vertical and horizontal directions (like a tennis racket) between 64 solder lugs bolted to the 2-foot-square pieces of pegboard. While there is a small (3 by 6 inch) circuit board that could be machine-assembled, the interconnections are made with two multi-wire “ribbon cables” that make 32 and 16 soldered connections each.

It is therefore well suited for assembly by the purchaser, as a kit, and the age range of the target market would start at the point where the student was willing to do several hours worth of hard, exacting work over several days and learn to use a soldering iron while retaining interest in the project.

The advantage of this approach was cost reduction – I was able to purchase the parts for a run of 10 (4 are as yet unbuilt) with about $2300 raised by a one-time crowd-funding effort. That, plus several days’ labor by Elaine Sweeney and more by myself, managed to produce two working units by the morning of the event (I “went on” with 30 minutes of sleep in the previous 24 hours and lasted the whole day, fueled by adrenaline – the video showing me in the classroom was taken that day).

I have not done a cost analysis of the units in reasonable quantity (100) lots, but it should be able to come in at less than $100 each for the parts.

I am reluctant to consider making any more of the existing design because it needs some improvement. The matrix connectors consisting of two miniature “gripper clips”, each of which must be handled with two fingers and a thumb are not only difficult to attach (requiring practice) but result in a connection that appears in two places at once – what is needed is a single element that contains the connection and rests exactly at the intersection of the intended wires – this would faithfully replicate the connection diagrams I’ve made. This could be realized with a specially-molded plastic cap which would make a fractional-turn lock over the wire intersection and contain the diode component inside.

I have a conceptual sketch of this part but there remains mechanical engineering to be done by someone with experience in these kind of contact materials. The jumpers must work not only the first time but after thousands of removals and replacements – mechanical connections are the most unreliable type of element in electronics.

The electronics will require a review and possible redesign – at present it uses a lot of components to perform a logic function which could be simplified by using logic ICs assuming the economics work out. A redesign of the ribbon cables will be necessary to reduce their cost and there will be some better labelling.

The Pegboard 4 by 4 was a temporary solution to a temporary problem but it has become the version that currently exists and which can fill a niche in the ecology we have to build. It’s an introductory device sized closer to the size of a user’s hands that is simple, relatively easy to use, has no options and can be resold once the user moves beyond it. I should have planned for it in advance.

Elaine Sweeney building one of the early Pegboards – 2016
Creating a 4-bit counter at Hack the Future – 2017

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