Analogmachine Blog: Analog Computer

Sunday, January 27, 2013

Building an Analog Integrator

January 27, 2013 10:22 pm

Something I've been wanting to do for a while was is to build a simple analog integrator. Here is my first attempt. I bread-boarded the circuit first to get it working then make it more permanent and stable by soldering the components onto a vero board. The picture below shows the result:

The three cables coming into the top left of the board are the +12, 0 and -12 volt power lines. The two cables coming out from the right-hand edge of the circuit board are the input and output lines (from the top) respectively. The input is fed from a function generator and the output fed to an oscilloscope. The figure below shows the result of the feeding a 50 Hz sine wave. Integration results in a cosine wave on the output. Note the two decoupling capacitors in the top left corner that despike the power lines. These are 0.1 microF capacitors.

A final test shows the result of feeding in a square wave with the resulting triangle wave as output.

In the circuit I used a LF356 as the op amp which has a JFET input stage resulting in a very high input impedance and is typically used for precision high speed integrators. It is also quite cheap at about a dollar or less. The 1 megohm resistor across the integrator capacitor is there to avoid any unnecessary integration as a result of slight biases in the inputs when the input is supposed to be at zero.

There are a number of improvements that could be made to this circuit. The first is that any voltage bias as the inputs when both are set to zero should be removed by placing a potentiometer across the +voltage rail and the pot adjusted as appropriate. The second improvement is to add a reset button (mechanical or electronic) across the integrating capacity to reset the circuit when necessary.

Friday, October 17, 2008

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 http://archive.computerhistory.org/)

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.