ARWadsworth wrote:

Ronald Raygun wrote:
harry wrote:

On Mar 24, 5:41 pm, Ronald Raygun
wrote:

So here's a wee exercise for you. You have two lamps up a pole.
Call them lamp A and lamp B. You never want both of them on at
once, i.e. you want either both off, or A on with B off, or A off
with B on.

There is only a 2 core cable going up the pole, but that's OK
because they are on a DC supply and there are a couple of diodes
up there, wired so that the polarity of the supply determines
which bulb comes on.

Question 1 (easy): You are given a two pole changeover switch
with a centre off position. How do you wire the battery supply,
the switch's 6 terminals, and the cable to the pole to get what
you need?

Question 2 (more difficult): You don't like the changeover switch
and decide you would rather have two separate switches, one for
lamp A and one for lamp B. What types of switch do you need and
how do you wire them? The obvious solution involving two double
pole one way switches will short-circuit the battery if you are
stupid enough to turn both switches on at once. So you need a
safer solution which avoids that.

I never heard such drivel. Where would you find DC lighting up a pole
these days?

Firstly, I said this is an exercise. You can't dismiss exercises on
the basis of them "being drivel" because they are often contrived.

Secondly, it isn't drivel, and in this instance isn't contrived. As
I have explained elsewhere, it's on a boat.

Now just answer the question, if you can.

Well I used 2g 2way light switch.

Is there a prize?

There is no prize other than your own satisfaction, but if there were
a prize, you would be disqualified on grounds of being too expert. I
was interested in whether harry could do it. :-)

Presuming that the "g" in your "2g 2way" stands for "gang", i.e. that the
switch has two poles, each of which can be switched 2 ways, in what is
commonly referred to as a DPDT switch, then yes, that's the answer to Q1.
Indeed I have already given wiring solution for that one.

But Q2 explicitly specifies that you must use two switches, one to operate
each lamp. I find I need two DPDT switches for that (I don't know if there
is a simpler solution), and the puzzle lies in working out how to wire them
up so that when switch A is on and B is off, then the two cores X and Y of
the cable are connected to + and -, respectively, whereas when switch B is
on and A is off, you want X and Y to be - and +, respectively, instead.

When you turn both switches on, you don't want to short out the battery, or
blow your fuse if there is one, as you would if your solution were based on
two 2g 1way (DPST) switches.

Turning both switches on simultaneously is an operator error condition,
since you can't turn both lamps on together. What gets connected to X and Y
in those circumstances may be undefined if you wish, but preferably it
should be that X and Y are both disconnected from the battery, same as when
both switches are off.