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#### Travis Cannon

Mar 24, 2014
3
I have a mystery circuit I have to build and I just cannot get it, I would really appreciate it if anyone could help me out

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• 1395682365669.jpg
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#### kpatz

Feb 24, 2014
329
I'd start by drawing out a circuit for each of the given bulb combinations, leaving out the switches. Note that some bulbs need to be in series and some in parallel to get the indicated voltages.

Then compare the resulting circuits and see where you would insert the switches to achieve the specified combinations by closing the switches.

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#### Travis Cannon

Mar 24, 2014
3
What really gets me, is how all the switches can be closed but still have one of the lights off, how is that possible?

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
12,516
Travis,
I moved this threaad to the homework section.

Before we start guiding you along, show us what you have donw so far to tackel the problem yourself and where you have failed.

Tip: You can deal with the lamps as ordinary resistors (you do not have to know their value!) each lamp has the same resistance (not completely realistic but good enough for this homework). Resistors can be connected in series and in parallel, lamps too, dividing the battery voltage in various ways depending on the connections.
Connections can be opened and closed by the switches.

#### kpatz

Feb 24, 2014
329
There's two ways to stop current flowing through a bulb. One way is by breaking the circuit to the bulb. The other way is... ??? hint #1: 5 letters starting with "S".... hint #2, no current flows through the bulb but current still flows somewhere.

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Moderator
Nov 17, 2011
12,516
Travis,

#### Eerik1111

Mar 26, 2014
7
do you have ICs or is this a wire only problem

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
12,516
What really gets me, is how all the switches can be closed but still have one of the lights off, how is that possible?
A switch can be connected in series to a lamp or in parallel ...

#### Travis Cannon

Mar 24, 2014
3
The circuit on the right is the first half of the circuit, the one on the left was juat me brainstorming about how to get one bulb turned off. Since electricity takes the path of least resistance the buld will turn off. This is what I have come up with so far

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• 20140327_131441.jpg
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#### Laplace

Apr 4, 2010
1,252
What does it mean that all voltages shown are approximate? How accurate is approximate?

For instance, take the 'all switches closed' configuration. It would seem that there would be two lamps in series with two lamps in parallel. But that would mean that the two lamps in series would each have an individual resistance much greater than the individual resistance of the lamps in parallel, and the difference is too great to be labeled 'approximate'. But in the real world the resistance of a lamp filament varies with the brightness of the lamp. A cold filament has much lower resistance than a hot filament.

If the temperature effect is taken into account, then the given voltages make make much more sense.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
What does it mean that all voltages shown are approximate? How accurate is approximate?

I think it means "assume the lamps have a constant and equal resistance"

#### Laplace

Apr 4, 2010
1,252
Assumption of equality will certainly lead one to the same circuit configurations (because what else is there?), but it irks me that calculated values are so far from the given values yet still deemed close enough, 'approximate'. Makes me wonder whether the given voltages were calculated, or measured from a working model of the device.

#### Laplace

Apr 4, 2010
1,252
Since the switch/lamp-voltage table looks a lot like a truth table, what design hints could a Karnaugh mapping provide?

The first thing that stands out is that L1 and L5 are identical. This can only happen if they are in series or in parallel. But parallel won't drop enough voltage so they must be in series and connected as a single bulb.

Now look for groups of four. L2 is always off when S1 is open. So L2 must be in series with S1. Both L3 and L4 are always off when S2 is open. So S2 must be in series with L3 and L4.

L4 is always off when S3 is closed. So S3 must be in parallel with L4.

The voltage drops give the series/parallel configuration of the lamps, while the K-map gives the configuration of the switches. It's not a complete design but it should be enough to get to the point where the solution is obvious.

#### Attachments

• Karnaugh-Map_3-input_lamps.pdf
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