Saturday, July 16, 2011

Circuits 101: What is a multivibrator?

I mean, in a certain sense physics-based or mechanical engineering-based metaphors for electrical engineering problems are not metaphors, because the mathematics representing these problems are not merely similar enough to draw a useful comparison, but they are actually identical down to every last number. And since numbers are the only objects that exist, any work that can be done by the devices described in these EE problems can be done equally well by the devices described in these ME and physics problems.

[After clicking that link try this one, it's pretty fun.]

Anyway, the point of my last post that I didn't mention to you is that I'm not going to be able to explain what an LM555 Timer Integrated Circuit does without first explaining what a multivibrator is. [And I won't be able to explain how an #LM555 Timer IC works without first explaining what a comparator and a flip-flop is.]

So, a multivibrator is a circuit that has multiple states, with rules that describe when the state changes.

Anyway, here are some pictorial examples of one mode of multivibrator, an astable multivibrator.

EXAMPLE ONE: Electrical engineering

EXAMPLE TWO: Physics

astable

As you can see, if the ball is at point A, it will roll to point B. If it is at point B, it will roll to point A. Likewise, if capacitor C1 is charged, it will discharge and charge capacitor C2, if capacitor C2 is charged, it will discharge and charge C1. The amount of charging that has to be done depends on the size of capacitors C1 and C2, and the rate at which they charge depends on R1 through R4. Likewise, the height of point A and B determine the amount of work that gravity must perform to move the ball down from slope A and back up slope B. The rate at which the ball actually moves is determined by the slope of each slope, just like how the size of the resistors determined the rate of charging.

So, the ball moves back and forth, the electrons on the capacitors move back and forth. If you put the same numbers in for the slopes as you did for the resistors, and the heights that you did for the capacitors, and assume that friction doesn't exist, then it's the same damn equation that describes the ball and the electrons.

Likewise for the other two modes.

Monostable:

=monostable

If the ball is at point B, it moves to point A. If the ball is at point A, it stays there. The parameters affect how long it takes to get from B to A and that's it.

Bistable:

=bistable

If the ball is at point A, it stays there, if the ball is at point B, it stays there. You can add energy to the system to move it from A to B or vice versa, but once you do it'll stay wherever you put it.

There you go, three types of circuits that do things exactly like balls on funny shaped hills. Even a caveman could understand it!

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