# Creating a circuit with 3 lights and 4 switches (1 master switch)

I'm a high-school junior in AP Physics 1. My class was assigned an optional, grade-boosting project where we could make a circuit under certain guidelines. I figured I'd give it a shot, but I'm really struggling at the design. Here are the guidelines:

• You can only use wires, 4 switches, 3 lights, and one power source, and apparently now diodes
• Switch 1 is a master switch, and must turn on all three lights while the other 3 switches are open
• Switch 2, 3, & 4 must turn on an individual light while switch 1 remains open.

I have been using the Circuit Construction Kit from PhET found here, and I have really struggled. No other components are allowed. I eventually have to physically build the circuit and get the calculations of the current and voltages for each scenario, but I wanted to just get the design down. Any solutions or tips would be greatly appreciated!

EDIT: Here is the original assignment page given. I am going to try to clarify with my professor about what components are allowed.

EDIT 2: I have just talked with my teacher, and it seems that diodes are allowed, but NOT multi-way switches. He was very explicit about that.

• Yup. The folks on Phy.SE would definitely send this to EE.SE. ;) – jonk May 13 '17 at 20:14
• Hint: You are going to need a multiple pole master switch or add some diodes if the power source is DC. – kiwiron May 13 '17 at 20:30
• Have you come across LEDs in your physics class? – Tom Carpenter May 13 '17 at 21:08
• Nope, we spent months on kinematics and forces but spent about a week altogether on circuits. But I'm sure that LEDs would likely be more "allowed" than multi-way switches. – TheBricktator May 13 '17 at 21:11
• With all the answers, it's lucky this is for extra credit! – StainlessSteelRat May 13 '17 at 21:15

So something like this then:

simulate this circuit – Schematic created using CircuitLab

When SW1 is closed all the lights turn on.
When SW2 is closed lamp1 turns on but D1 prevents the other lamps from getting power.

The downside is that the lamps will get a slightly different voltage depending which switch has turned them on.

• Thank you so much! It's much more straight forward with diodes, I appreciate it :) – TheBricktator May 15 '17 at 13:40

I'm assuming that 3-way switches are allowed. Currents through individual lamps will be additive. Voltage across any lamp should be equal to the voltage across any other lamp. Currents and voltages can be calculated using V=IR.

simulate this circuit – Schematic created using CircuitLab

• The software he's using only has SPST. – tjbtech May 13 '17 at 20:40
• This wouldn't work anyway. If you consider SW1 to currently be "closed", then if you change SW3 from it's current position, LAMP2 would turn off, which is not what is required – Tom Carpenter May 13 '17 at 20:42
• I'm going to ask my teacher if anything other than one-way switches are allowed, but he's generally pretty strict, and we've never worked with anything else. I appreciate the answer, however. – TheBricktator May 13 '17 at 20:43
• The master is supposed to only turn on all three when SW2-4 are closed, so this is right if SPDT is allowed. It looks like the assignment doesn't specify that the software must be used, nor that a specific switch type is required, even though the schematic symbol shown in instructions is SPST. – tjbtech May 13 '17 at 20:54

OK, my original answer was made in haste, and I overlooked a very obvious problem. This can't be done without multi-throw or multi-pole switches or diodes. Are you sure the master switch was meant to turn all three ON and not OFF?

• Might be a small problem there. At least in the schematic itself, if not otherwise. – jonk May 13 '17 at 20:14
• Yeah, a big one - I'm an idiot. But thanks for the heads-up. Will fix now. – tjbtech May 13 '17 at 20:24
• I've updated the OP with the exact information I have. I either am terribly misreading it or my professor is expecting something impossible (I'm in high school, so the latter isn't unlikely). – TheBricktator May 13 '17 at 20:49

There is an answer to this question, and it doesn't require resistors, diodes, SPDT switch etc.

You can do it using 3 Lamps, 4 Switches, Some wire, and 4 Batteries. Yes, 4 batteries. You will notice that it doesn't say you can only use a single battery.

I'm not going to give you the solution because this is a homework question afterall, but I will give you a big hint:

• First think about each lamp individually. Using a battery, a switch, and a lamp, fulfil the requirements for switches 2, 3 and 4.

• Now using another battery and a switch, connect up your sub circuits to fulfil the requirements for switch 1.

In fact, the approach I have in mind could be argued to be one power source. All of the batteries are placed in series in a ring, but with switches between them. Technically it is still one power source.

Essentially you have 4 batteries in a circle with a switch between each one. Then attach the lamps. Just never close all 4 switches at the same time (boom).

• That's what had crossed my mind, as well. But.. the question then turns on the interpretation of "Only one power source is allowed." – jonk May 13 '17 at 21:04
• @jonk hmm, missed that bit. – Tom Carpenter May 13 '17 at 21:05
• I'll add another thought that had crossed my mind, as well. It says "Devices" and these might be "LEDs." I can't see anything in the text that specifically excludes their use. Nor even implies they cannot be used. – jonk May 13 '17 at 21:06
• @jonk playing hard and fast with the rules, is four batteries in series but with switches in between (nothing else) really more than one power source? ;) – Tom Carpenter May 13 '17 at 21:27
• It's a multi-tap power supply. About the same thing as a multi-pole rotary switch in this problem, I'd imagine. ;) Oh, well. – jonk May 13 '17 at 21:41