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Light Panel

A project log for DEC H-500 Computer Lab Reproduction

Reproduce the Digital Computer Lab H-500, a training tool from the late 60's aimed at teaching people the basics of logic circuits.

michael-gardiMichael Gardi 04/28/2020 at 14:450 Comments

This piece separates the Switch Panel from the Plug Panel, holds the eight output lamps and the fine tuning knob for the clock speed, and also transitions from the higher switch level to the lower plug level within the wooden frame. 

It's impossible to know from just pictures what kind of bulbs they used in the original, but if I had to guess based on the estimated size (about 4 mm) I would say grain of wheat bulbs.  I could have ordered some but given the current lead times for delivery of non-essential items I decided to go with what I had on hand, LEDs.  I modified my Panel Mounted LED Socket by adding a 4 mm diffuser printed with transparent filament.

When put together it looks like this.

These bulb assemblies were slotted into holes on the back of the Light Panel. The panel itself was printed in two pieces and glued together with the aid of a reinforcing strip. You can see the clock fine adjustment potentiometer mounted on the back as well.

From the front here is the fine tuning knob that I modeled...

... based on the original.

The "bulbs" ended up looking pretty good. I found that I will have to add a 4K limiting resister to each LED to get the brightness down to incandescent levels.

The light panel is ready to go.

Updated May 7, 2020: Last week Bob Roswell, curator of the System Source Computer Museum, was kind enough to spend some time on the phone with me talking about the H-500 that they have on display. He also provided me with some invaluable first hand measurements and some close-up pictures. While my light panel was pretty close to the original measurement wise, I had learned that the potentiometer on the original was also the power switch so I redid the panel to fine tune the measurements and accommodate the new pot with switch. The photo of the tuning knob Bob provided also allowed me to produce a more accurate replica. Thanks Bob. I have update some of the pictures above with the new parts.

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