Traffic lights, an unpleasent thematic.
Controlling traffic lights usually takes part at beginner classes,
to teach the basics of programming or digital design.
It is known to have bored generations of pupils/students
out of their minds.
Unfortunately, I don't have a better example.
To start simple:
One road, a narrow spot (maybe roadworks), two traffic lights
(each one with three different colours).
Obviously, two cars driving in different directions won't fit
through at the same time.
Only one traffic light can be green, while the other is red.
Now to define, what one traffic light should do.
green: drive through. yellow: prepare to stop, or hurry up. deceide ! red: stop. red and yellow: prepare to start racing.
We could imagine one complete 360 degree cylinder rotation to happen
within 6 minutes (360 seconds),
what would make one degree per second.
To be independend of rotating speed, the axis is labeled with graduations.
(Phases are not to scale with a "real" traffic light, but drawings look
nicer that way.)
After 360 degrees of rotation, the cylinder will start again at 0 degrees.
We can imagine to take the blue axis, and to bend it to form a circle.
Then we draw our colours into that circle...
We imagine a disc, rotating counterclockwise.
For the three lightbulbs of one traffic light, we place the switches
at 0 degrees.
Switches for the tree light bulbs of the other traffic light are placed
180 degrees away.
Note the red arrows, marking the positions of the switches.
And now, we know what to do.
We just build tree metal discs (one per colour), and cut away anything
that is not marked blue (ignore the lines used for construction).
Since we have a big hole in the center of every disc, we turn them
90 degrees to the left, and stick trough our cylinder.
Now to add switches, wires, light bulbs, motor and gear.
Coming to mind, that the sequencer of the washing machine had two rows
of switches, 180 degrees apart.
All we have to do, is to replace the gear (for a faster rotation speed),
and the cylinder (for another "program").
But what, if we have two roads ?
For a crossing, we first connect two traffic lights (blue marked casting)
with our sequencer.
The left and the right traffic light show always the same color.
Same thing for the upper and lower traffic light.
Looks like we can use our sequencer without modifications,
when connecting two light bulbs in parallel.
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(c) Dieter Mueller 2005