# Simple Circuit. Three N/O switches and 3 bulbs. Shouldn't be complicated but is.

#### therig2005

Joined Sep 1, 2019
8
Hello,

First time posting here, hopefully I can be pointed in the correct direction. I work in heavy industry. I am constantly building panels, working with / programming PLC's and designing control interfaces for various pieces of equipment, so it embarrass me that I can't figure out this simple circuit. I am currently enrolled in an entry level Voice and Data cabling class and the first lab for the class involves setting up a series of lamps and switches to get the desired control. I am stuck on this one scenario:

3 Normally open switches, and 2 bulbs. Switch 1 turns on Bulb 1, Switch 2 turns on Bulb 2, and Switch 3 ONLY turns on Bulbs 1 and 2.

I am sure I will be even more embarrassed when I see the simple solution, but at this point I am stuck.

Thanks

#### WBahn

Joined Mar 31, 2012
30,229
Hello,

First time posting here, hopefully I can be pointed in the correct direction. I work in heavy industry. I am constantly building panels, working with / programming PLC's and designing control interfaces for various pieces of equipment, so it embarrass me that I can't figure out this simple circuit. I am currently enrolled in an entry level Voice and Data cabling class and the first lab for the class involves setting up a series of lamps and switches to get the desired control. I am stuck on this one scenario:

3 Normally open switches, and 2 bulbs. Switch 1 turns on Bulb 1, Switch 2 turns on Bulb 2, and Switch 3 ONLY turns on Bulbs 1 and 2.

I am sure I will be even more embarrassed when I see the simple solution, but at this point I am stuck.

Thanks
You haven't fully specified the problem. With three switches there are eight possible combinations. Are you saying that when Switch 1 is on that ONLY Bulb 1 is on (even if Switch 2 is also on)? Similarly for Switch 2 and Bulb 2. What does the emphasized ONLY mean? That Switch three turn on use Bulbs 1 and 2 and nothing else (what else is there), or that only Switch 3 turns on both Bulbs 1 and 2? Or that if Switch 3 is the only switch turned on that Bulbs 1 and 2 are on? What if Switch 1 is off but Switch 2 and 3 or on? What if all three switches are on?

Are these single-pole, single-throw switches? Or can they be multiple-pole and/or multiple-throw switches?

Depending on what the full requirements are, this is either trivial or very complicated.

Since this is homework, you need to also show your best attempt at working it. That will give us a starting point for discussion to help you move forward.

#### therig2005

Joined Sep 1, 2019
8
The conditions I had listed were the verbatim criteria I was provided with by the instructor.

All three switches are SPST.

When SW1 is closed, L1 will be energized
When SW2 is closed, L2 will be energized
When SW3 is the only switch closed, L1 and L2 will be energized

No other conditions for the states of the switches are listed.

#### iimagine

Joined Dec 20, 2010
511
When SW3 is the only switch closed, L1 and L2 will be energized
Using diodes:

#### iimagine

Joined Dec 20, 2010
511
I forgot this is homework, and hope you didnt see the no diodes solution. (I deleted it)
Hint: Series connection

#### crutschow

Joined Mar 14, 2008
34,669
Guess I'm dense, but I don't see how that can be done with only 3 SPST switches.

#### iimagine

Joined Dec 20, 2010
511
It can be done if the third switch is configured to connect both L1 and L2 in series

#### RBR1317

Joined Nov 13, 2010
715
Is this an exercise to consider what is stated versus what is implied?

Suppose the requirements imply the following:

When SW1 is closed, L1 will be energized (L2 is a Don't Care)
When SW2 is closed, L2 will be energized (L1 is a Don't Care)
When SW3 is the only switch closed, L1 and L2 will be energized

Then a solution where any switch closed energizes both lamps will meet the stated requirements.

#### MrAl

Joined Jun 17, 2014
11,559
Is this an exercise to consider what is stated versus what is implied?

Suppose the requirements imply the following:

When SW1 is closed, L1 will be energized (L2 is a Don't Care)
When SW2 is closed, L2 will be energized (L1 is a Don't Care)
When SW3 is the only switch closed, L1 and L2 will be energized

Then a solution where any switch closed energizes both lamps will meet the stated requirements.
he he

What bothers me is the statement that when SW3 is the ONLY switch closed the two light up, but that implies that when either SW1 and/or SW2 is/are closed along with SW3 something else is supposed to happen like one or both turn off, or maybe they blink to the rhythm of the Star Spangled Banner

Ok on the more serious side, and may not work either, we have 3 switches and 2 bulbs and presumably 1 battery for a total disconnected node count of 12. Number each node 1 to 12 and figure out ALL possible combinations of connecting them to the battery. Some will be ridiculous so scrap them right away and see if any of the other combinations work.
It's called a brute force connection search circuit design
The trick is to figure out how to generate valid circuits, or at least figure out how to generate all of the circuit configurations (even if ridiculous) and then figure out how to dismiss ones that violate practical circuit connections (such as a shorted battery).

Actually now that i think about it that will in fact work if there really is a solution but may take a lot of combinations to find the one or ones that work.

Last edited:

#### iimagine

Joined Dec 20, 2010
511
Actually now that i think about it that will in fact work if there really is a solution but may take a lot of combinations to find the one or ones that work
As I said earlier, it can be done and I have solved it, just didnt want to post the solution because its a homework section, i gave hint thought

#### jpanhalt

Joined Jan 18, 2008
11,087
As I said earlier, it can be done and I have solved it, just didnt want to post the solution because its a homework section, i gave hint thought
Ditto.
There is of course a trivial solution in which each switch turns on both lights, but it is unlikely that is the best interpretation of the question. I interpret the questions to mean that SW1 only turns on L1, SW2 only turns on L2, and SW3 turns on both lights only when SW1 and SW2 are both open.

#### MrAl

Joined Jun 17, 2014
11,559
As I said earlier, it can be done and I have solved it, just didnt want to post the solution because its a homework section, i gave hint thought
I found a solution also but dont want to say anything yet.
Wouldnt it be funny if we all found a DIFFERENT solution that worked? ha ha.

But i replied for the sake of the OP.

#### WBahn

Joined Mar 31, 2012
30,229
I found a solution also but dont want to say anything yet.
Wouldnt it be funny if we all found a DIFFERENT solution that worked? ha ha.

But i replied for the sake of the OP.
There are several solutions but I think they are, to some degree, homomorphic.

#### therig2005

Joined Sep 1, 2019
8
Unfortunately I am still at a loss as to where to head.

#### jpanhalt

Joined Jan 18, 2008
11,087
You need to present your efforts so we can help you in the right direction. The Homework section is designed as such.

#### therig2005

Joined Sep 1, 2019
8
My efforts consist of scratch paper and a generous amount of rubber eraser debris, but if need be I would be more than happy to upload photos of my failed drawings so far.

#### therig2005

Joined Sep 1, 2019
8
Every solution I find inevitably winds out causing both lights to be energized simultaneously, without the use of of diodes or various other switches and push buttons.

#### iimagine

Joined Dec 20, 2010
511
I gave you a hint: Series connection. You do know that you can light 2 or more bulbs or leds in series right?

#### WBahn

Joined Mar 31, 2012
30,229
iimagine gave you a really bit hint when he suggested looking at how to use the switches to reconfigure the bulbs from series to parallel.

So start with the bulbs in series with a switch to turn them both on. Can you figure out a way to position the switch so that when it is open the two bulb are disconnected from each other?