Build a Retro Calculator

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Build Your Own
Retro Calculator

Would you like to own a retro LED calculator which you have built from a kit?

Paul Scmidt was taken with the readily available calculator kit from Spikenzielabs, and here shows the stages in its construction. He also passes on valuable tips to achieve the best results.

© 2015 Paul Schmidt

This is a new article which was not originally in "The International Calculator Collector".



Calculator kits were readily available in the first half of the 1970s from several companies, most notably Heathkit and Sinclair.  This was at a time when many calculators were manufactured in the U.S.A. and U.K. where labour costs were high and so there could be some cost saving in buying a kit of parts and assembling it yourself.

Sinclair Cambridge Calculator Kit

A Sinclair Cambridge calculator kit from the 1970s.

With the great transfer of calculator manufacture to Asia, where the labour costs were lower, calculator kits practically disappeared from the market from the late 1970s.

However, with the rise of interest in early electronics there is now an enthusiasm to own modern items that have a retro look.  Such is the basic 4-function calculator with LED display and clear casing (showing the components inside) from Spikenzielabs.

Paul Scmidt assembled a Spikenzielabs calculator kit and passes on his experiences with it:

Building the Spikenzielabs calculator kit

I bought a Christmas gift in the form of a snazzy calculator kit.  Looking at the kit, I liked it so much that I bought another for myself and put it together last week.  It is a basic 4-function machine with floating point, but limited number size (max 6 digits positive, 5 digits negative, all numbers rounded to four decimal places - that is digits to the right of the decimal point – six decimal places may be entered but they are automatically rounded to four decimal places – and gives an error if either number of a calculation is zero, e.g. 5 + 0 = error.


Great clicky tactile keyboard action, brilliant large red LED display, beautiful laser-cut clear case.


Very few electronic components; the main Arduino-programmed microcontroller IC, two capacitors (for internal clock?) and six resistors that are for the keyboard-display-shared matrix scan lines (but resistors only used for the keyboard…they are not current limits for the display).


Powered from one watch battery, powers down after 10 seconds of non-use to save the battery.


The other unique thing about this calculator (the manufacturer claims that they are the only ones doing this) is the laser cut keys.  Each key is comprised of two pieces; a clear plastic piece that fits between the actual PCB key switch and the key button, riding within a chamber formed by cutouts in the layers of clear plastic that comprise the case, and the black plastic key itself.  The clear part and the black part are bonded together with instant adhesive. The unique part is that the black key starts out as a piece of shiny-finish acrylic plastic, with a protective film on one side.  A laser cuts or vaporizes the unwanted film, leaving only the part that protects the desired image on the key surface (like etch resist on a PCB).  Then the laser scans the top of the key to cause a roughed up matte surface, except where the protective film remains.  Peel off the film and you get a cool looking black key with high contrast between glossy and matte regions.

Calculator kit box

The box with the calculator kit.

Calculator kit components

The components of the kit removed from their bags.

Atmel 8-bit micro-controller

Atmel 8-bit micro-controller with in-system programmable Flash program memory.  For protection from damage due to any electro-static discharge during handling, the pins of the integrated circuit are inserted in anti-static foam.

Calculator kit PCB assembled

The circuit board with the components soldered in place.
From the top: battery holder, LED display, integrated circuit, resistors & capacitors, keyboard switches.

Trying red filter over calculator display

Checking to see if the display visibility could be improved by using a red filter (not supplied with the kit) over the LED modules.

Red filter over calculator display

It was decided that the display was improved with a red filter, and so this was super-glued to the LED modules.

Peeling the protective film from the front of a keyboard button

Peeling the protective film from the front of a keyboard button.  It was found that it was very easy to damage the surface using a sharp, hard tool and so this operation was largely performed using a thumb nail.

Calculator kit keyboard buttons

The buttons after removing the protective films.

Assembling the two parts of the buttons of the calculator kit

The keyboard buttons have a separate clear bottom part.  The top  part of each button was accurately attached to its bottom part using casing sections as an alignment jig.

Side view of the assembled calculator kit

Side view after assembly was completed.

Testing the assembled calculator kit

A final test after assembly by performing a simple calculation and pressing the '=' button.

The major challenges of building this kit:


Copyright 2015, Paul Schmidt.

Vintage Calculators

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