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# Simple Analog Computer (Electronic Slide Rule)

An electronic slide rule, which was often called a simple analog computer in the 50s and 60s.

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This is my version of an electronic slide rule, which was often called a simple analog computer in the 50s and 60s. I also want to make a hub for information about these because it's really spread out. They make a nice introduction to the point of analog computation, but don't seem to have been made since the 1960s. I tried to give mine a 70s computer vibe.

Goals:
- To recreate a simple computer as a learning tool
- To raise awareness/educate others to its history
- To get a better understanding of the history of the circuit used
- To share designs with others.
- At some point I'd like to make one with high-precision pots

BACKGROUND

The simple analog computer is essentially a comparator circuit where balancing the circuit gives you an approximate answer to several mathematical functions. It functions similar to a slide rule. I don't know what the earliest version of this is, but they were common in the late 50s and early 60s as kits.

The ones I am aware of:

In December of 1961, the Edmund Analog Computer and G.E. EF-140 were described in an overview of this type of circuit in an article in Popular Electronics (https://worldradiohistory.com/Archive-Poptronics/60s/61/Pop-1961-12.pdf).

More recently, similar instructions were given to explore this circuit as a school activity in https://schoolbag.info/science/laboratory/8.html. It was also recreated here: http://www.arthropodsystems.com/AnalogComputer/AnalogComputer1.html. The circuit was also implemented with high-precision potentiometers in 2008 as listed above.

I would be very interested in any other versions of this or more information about these. Specifically, more information about the Mac-1 and copies of the manual for the Edmund Analog Computer and the Science Fair one. Here's a summarized version of the Edmund manual in Czech (I think): http://dexovo.cz/analogove-vypocetnictvi-edmund-cast-i.php.

### diydials.png

Blank dial areas for custom gauge values/spacing which may be different depending on the potentiometers chosen.

Portable Network Graphics (PNG) - 912.80 kB - 02/07/2021 at 08:29

### AnalogC.png

The graphics for the Analog Computer with some 70s design flair.

Portable Network Graphics (PNG) - 814.94 kB - 02/07/2021 at 08:27

• 2 × 50 Ohm potentiometers
• 1 × 1K Ohm potentiometer
• 1 × SPDT Switch
• 1 × 2 1/8" headphone jacks
• 3 × Knobs
• ### Construction

The pieces used to build the analog computer.

For space, I overlapped the 3 potentiometer gauges. Rather than 3d print needles, which might hit each other, I glued the plastic circles to the knobs and drew a line on it for the needle.

The face of the computer with the gauges was laminated and glued to wood. I thin added side walls to mount in the pencilbox. I also 3d printed a battery holder for AA batteries. Again this is wired for either the voltage or the audio input. The finished bottom looks like this.

And that's it! There's a lot to be said about the actual potentiometers used. The tolerance should be less than 5% in order to get fairly accurate results. Using 20% tolerance means that between the 3 pots, your error could be too large to be useful.

The article in Popular Electronics discusses how there are two ways to make this circuit. You can test whether the comparator is zero by using a galvanometer (or potentiometer) if you give the circuit a voltage. However, if you feed it an audio signal, you can use headphones to hear when the sound is silent. Most of them used the meter, but some like the G.E. kit used headphones.

I am going to use both. I want it to be relatively cheap overall, so I don't want to embed a meter into the computer. I'm going to have two posts in order to attach a potentiometer, but I'll also have audio input/output jacks. A simple switch will select which to use- so the audio option is essentially off.

Here's the version with the meter.

You can see that they're essentially the same circuit, so both can be wired with a single switch. These images come from the Popular Electronics article.

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## Discussions

Mark wrote 03/09/2023 at 14:57 point

Love this. Question - is there a straightforward way to design (PhotoShop or other) custom dials. I.e. what is the process to create calibrated dials based on a start and end point (and interior angle) for a given pot?

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milesvw wrote 04/01/2022 at 14:24 point

The 2008 article was down for me, here is the archive.org copy: https://web.archive.org/web/20200404180656/http://www.nicolamarras.it/calcolatoria/calcolatrice_analogica_en.html

You can source 3590S 10 turn precision potentiometers and knobs here.  Buying 3 pots and 3 knobs got me to \$9.89 with tax and shipping.

https://m.aliexpress.com/item/32909220084.html

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davepet wrote 04/02/2021 at 13:08 point

I had one of these in 1961.

Boy, I was rockin'.

Thanks for the memory.

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Dan Maloney wrote 02/12/2021 at 18:52 point

You really nailed the 70s look on that front panel. Glad that these otherwise forgotten bits of computer history are getting some love. Nice job!

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deftcoyote wrote 02/12/2021 at 19:11 point

Thanks!

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