Lamps and Switches.

Return with me now to those incandescent days of yesteryear, before compact fluorescent lamps and LED lamps, when most artificial lighting was from incandescent lamps. There are still some around. Our intrepid inventor, Goufhoff Pfumbler, an admirer of Rube Goldberg, had some in his closet that he was reluctant to trash. He also had a floor lamp in his living room, that he sometimes used as a reading lamp, but usually just left on for mood lighting. It had two incandescent light bulbs.

Why not save energy when lower light levels aren't needed by adding a switch to the lamp that would change the lamps from series to parallel configuation, thereby reducing the light output by about half? That would be less expensive than buying a light dimmer. All he'd need was a switch, and he figured he could do the job with a DPDT (Double Pole Double Throw) switch. The illustration shows an open form of this as a knife switch, but small enclosed versions can be bought at hardware stores, called "three way" switches, for reasons we needn't go into here.

So Goufhoff needed to design a circuit to do the job. We have included a picture below to illustrate the situation and serve as a worksheet.

The power source (illustrated schematically here by a 9v battery) is at the left, the DPDT switch in the center and the two lamps at the right.

Design the simplest wiring diagram that will accomplish the result of switching the lamps betwen series and parallel. Be careful to avoid any circuit that might short out the power source, since we didn't explicitly include a fuse.

When you find a solution, now try to find another distinctly different circuit to do the same thing. If you are into topology, you might prove how many unique circuits there are.

I suspect all of these solutions would violate electrical house wiring codes of most countries, though none would pose a safety issue with a floor lamp.


This is the sort of thing that there's no recipe for, and nothing to help you in books or even on the web. You just have to tinker with it, using your understanding of circuits and switches. One circuit that will accomplish the task looks like this.

Here's another solution. This one doesn't use one switch terminal, so in that sense, it is simpler.

And another solution..

I first solved this problem when I wanted to modify a stereo slide projector. It had two 750 watt lamps, for 3d slide shows presented on large screens. For previewing shows at home on a smaller screen, that much light wasn't necessary, and it does over time fade the dyes in color films. It also shortens the lifetime of the expensive lamps. So I devised this circuit to switch the lamps from series to parallel as needed.

Surprisingly the same problem with three lamps is even easier, and it only requires a double pole single throw (DPST) switch.

  • Donald Simanek, 2017.

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