Here's a puzzle that requires a slight knowledge of electronics, and some verbal skills:

physics/two.boxes.two.lamps New

New entry, edited from material collected for the archive and not yet in a released version.

Problem

Here's a puzzle that requires a slight knowledge of electronics, and
some verbal skills:

A black box has two single [Editor: "sling" in the original] wires
coming out of it and both wires entering a red box.  The black box also
has a line cord coming out of it so that it can be plugged into a wall
electrical outlet.  The black box contains two single-pole light
switches, the cheapest you can buy at the hardware store.  It also
contains two other simple electronic components.  The red box contains
two lightbulbs, and also contains two other simple electronic
components.

When both switches are closed both lights go on.  When both are open, no
lights go on.  When A is closed, only light a goes on.  When B is
closed, light b [Editor: "light B" in the original] goes on.  What is
the simplest circuit that can do this.  Explain without drawing a
diagram.

-- David Roche <droche@innocent.com>  (March 1998)

Show solution

The black box has one wire from the line cord directly connected to one
of the "sling" (I think you meant "single", didn't you?) wires leaving
the box.

Call the switches SW1 and SW2 and call the two contacts of each switch A
and B.

The other line cord wire connects to SW1-A and SW2-A.  SW1-B connects to
the anode of a diode (rectifier), and SW2-B connects to the cathode of a
second diode.  The other ends of both the diodes are connected to the
second "sling" wire.

In the red box, each of the two "sling" wires connects to one end of
each of the two lightbulbs, the other ends of the lightbulbs are
connected together.  Each lightbulb has a diode connected across it,
with the diodes "pointing" so that both their anodes (or both cathodes)
connect to the "sling" wire ends of the two lightbulbs.

If you want the lightbulbs to light to "full" brilliancy, you'd better
use 120V bulbs and plug the cord into a 230VAC outlet.  60V bulbs would
work too but are sort of hard to find these days!

-- Jeff Wisnia <wkrp@tiac.net>

The wiring in the red box is a bit confusing and I don't think will
work.  The first box was fine although there is an alternative solution
where the switches are placed in series with the diodes across the
switch contacts, with opposite polarity

The red box should have one sling wire connected to the base of each
bulb.  the second sling wire should be connected, through a diode to the
thread of the first bulb and also connected through a reversed polarity
diode to the thread of the second bulb.

-- Joseph Vankay <vankayj@candu.aecl.ca>

Don't bet your farm that *my* wiring for the red box won't work, Joe.
It is the dual of your suggested bulb/diode wiring just like the
"alterntive" solution you gave for the black box is the dual of my
suggested wiring for that box.

It's nice we could *both* be correct!

-- Jeff Wisnia <wkrp@tiac.net>

The four simple electronic components are half-wave rectifiers.  One of
the wires from the line cord goes straight through both boxes and
connects to one of the terminals on each light bulb.  The other wires
connects to the common terminal on each switch.  The output of the each
switch goes through a half-wave rectifier, but to opposite terminals on
it.  The other terminals of the rectifiers are connected together and
the common wire goes to the other box.  In the other box, two half-wave
rectifiers are attached to that wire (again, to opposite terminals).
One of the light bulbs has its remaining terminal connected to the
remaining terminal on one of the rectifiers, and the other light bulb
has its remaining terminal connected to the remaining terminal on the
other rectifier.

-- Matthew T. Russotto <russotto@pond.com>