Heathkit EC-1 Electric Analog Computer

The word ‘analog’ has come to describe things that aren’t ‘digital.’ This includes vinyl records and cassette tapes, old music amplifiers and radios, some music synthesizers—any device where the output is derived non-digitally. The term ‘analog computer’ fits the description, too; an analog computer performs all of its processing non-digitally. Interestingly, the term ‘analog’ used to mean something else, too. It was short for ‘analogous,’ and described systems that could faithfully represent others.

A roadmap is an example of an analogous system. You can trace out a path on a map, and you can calculate the distance to be traveled on a map. But, you don’t travel on the map. The map is an analog for the real world. In the same vein, an analog computer can model the real world. It does this by utilizing electronic components that behave analogously to the real world systems. For example, a capacitor discharged through a resistor will exhibit a voltage curve that mimics the behavior of a rock dropped from a cliff, showing the velocity due to gravity over time. That voltage can be further fed into a second simple circuit that integrates the velocity over time to show the distance traveled. Taken together, this simple combination of circuitry can compute the analog of a body falling under the influence of gravity.

Physical problems involving summation, multiplication, integration and differentiation are programmed into an analog computer by plugging components together with jumpers.

Passive components, like resistors and capacitors, are plugged together with active components called ‘operational amplifiers.’ The combination of passive and active components describes one portion of a real-world problem—an ‘operation.’ The outputs from the operational amplifiers vary in response to their input, to feedback and to the way they are cobbled together with capacitors and resistors. Two such combinations give us the two operations of the falling body program, described above. Op amps also buffer portions of the analog computer from each other to prevent the computer from internally dissipating the electrical charges that are meant for calculations.

The Heathkit EC-1 is an educational analog computer kit from the early 1960s, built by the buyer. It features nine electron tube-based operational amplifiers, five variable resistors (for adjustments), a variety of high-tolerance resistors and capacitors for operations, a multivibrator for repeatedly resetting and re-running the computation, plus a meter and other panel adjustments. The operational amplifiers are powered at +300 volts and -150 volts, and have outputs ranging between -60 and +60 volts. The operational amplifier is an ‘ideal’ amplifier offering very high impedance at its inputs and low impedance at its output, protecting the physical analog from the electrical requirements of the circuitry.

How do you read the output of an analog computer? Typically, one uses an oscilloscope. The horizontal sweep might represent time with the vertical trace displaying the output of the analog computer. When the analog computer models your real-world problem, you take a picture!