Analog Computers

Although I was brought up on digital computers I have quite a soft spot for analog computers. I used to have an old web page that had some notes and pictures plus some rare analog construction notes that I copied from a 1978 article I had. I thought I’d rejuvenate the page and notes here.

The digital computer is a sequential device, in general, operating on data one step at a time, in addition the digital computer represents data internally using a verbose but very robust form of representation called binary. Thus a single transistor in a digital computer can only store two states, on and off. Obviously to store a number to any sensible degree of precision, many transistors are required.

An analog computer operates in a quite different way. For a start, all operations in an analog computer are performed in parallel. Secondly, data are represented in an analog computer as voltages, a very compact but not necessarily robust form of storage (prone to noise corruption). A single capacitor (equivalent to the digital’s computer use of a transistor) in an analog computer can represent one entire continuous variable.

EC-1 Educational Analog Computer, introduced in 1960 by Heathkit.

The Heathkit Educational Analog Computer is completely self-contained and contains nine DC operational amplifiers with provision for balancing without removing problem setup. It also features three initial condition power supplies, five coefficient potentiometers, four sets of relay contacts, an electronically regulated power supply and a built-in repetitive oscillator for automatic operation. The complete EC-1 kit also contains an assortment of precision resistors, capacitors, special silicon diodes and patch cords for setting up scores of complex computer problems easily and accurately.

Vannevar Bush’s differential analyzer

(Image from

The first mechanical/electrical analog computers were developed in the early part of the 20th century. One of the most well known of these was the Vannevar Bush’s differential analyzer that filled a room at the Massachusetts Institute of Technology. Vannevar Bush’s differential analyzer crunched through calculus in seconds, although technicians often spent hours setting it up to solve an equation. A multitude of wheels, discs, shafts and gears handled the computations with precision unmatched by any contemporaneous machine. With the advent of the digital computer after World War II, the development of the analog computer slowed and by the early 1970s it was being rapidly supplanted by digital machines. One wonders whether with today’s electronic technology the analog computer might reemerge as a general purpose computer.

Designs for Analog Computers

Practical Electronics (UK), 1978

Part 1
Part 2
Part 3
Part 4

Practical Electronics (UK), 1968

Update June 16th 2012

I came across this description of a small home-brew analog computer designed by a Dr F Vogel. This machine has 2 integrators, 3 summers, one inverting amplifier, 4 coefficient potentiometers, one multiplier and one diode function generator for the sine function. It would serve as an excellent teaching computer.

In addition it has +10 and -10V voltage reference source, override indicator, saw tooth voltage generator, a single meter readout, overdrive indicator among other facilities. The image below shows the finished computer:

Details can be found in the documents: Description and Schematics. A description that highlights its construction is also given at the analogmuseum page by Bernd Ulmann.

Another recent page I came across which describes building an analog computer which boasts four summer/integrators, two summers, four inverters, a sine/cosine generator, voltage dividers, two AD633 multipliers and a selection of diodes. An image of the front panel is shown below:

Finally here is an analog computer designed to simulate a bouncing ball built by Eric Archer. Unfortunately at this time (June, 2012) a detailed component schematic is not available although a basic circuit diagram is given. The video below shows the circuit in operation:

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