# How do I describe this circuit?

simulate this circuit – Schematic created using CircuitLab

Yes, this circuit diagram is correct. No, I did not design it.

How do I describe this circuit in words? I thought about "a switch and light bulb in parallel, instead of series" but the circuit is so bizarre that I want to make sure what I am saying is unambiguous.

I cannot insert a schematic/picture/diagram/drawing, and I have a fairly tight word limit.

If anyone knows the tags for this question, add them.

EDIT (due to interest from the comments): The circuit was made by my 3rd grade teacher for an electricity test (when she thought the questions from the curriculum were too hard for us...)

EDIT 2: No, this is not a fancy NOT gate. The question was "What will happen when the switch closes" and the teacher-accepted answer being "Lightbulb turns on"

– W5VO
Commented Sep 16, 2016 at 18:19

How do I describe this circuit?

Do you want subjective opinion? Or technical description?

Subjective opinion would be: this circuit is badly designed, no matter what problem the designer tried to solve.

Technical description would be: it is a battery killer with stand-by indicator.

– W5VO
Commented Sep 18, 2016 at 1:52

The best description for this circuit is "How to NOT control a light with a switch." This was clearly not designed by anyone with even basic knowledge of electricity. Or else it was incorrectly "interpreted" or reproduced by someone with absolutely no understanding at all.

Obviously, closing the switch will put a DEAD SHORT across the power supply. That is NEVER a good idea, and can be quite dangerous in many cases.

Clearly, the light bulb (or LED, it makes no difference!) turns OFF when the switch is closed. Anyone who thinks otherwise has zero understanding of electricity and should not be attempting to teach others. In any grade.

"Battery short circuit stress tester".

This circuit will allow you to observe how the supply source reacts when short circuited. Also tests the switch under extreme fault conditions as well. An expendable person should be deployed to operate the switch.

Current tends to select line with lower resistance. Short circuit resistance is 0 and open circuit resistance is unlimited. The current of lamp when switch is closed = zero then you have no light but when switch is open the current of lamp is (1.5v/100ohm = 0.015A) then you have light like this picture :

and something about the battery :

I'd describe it as: Switch and bulb in parallel connected to a voltage source.

I think the voltage source should be mentioned (without it you cannot reproduce the circuit, although most would think implicitly that a voltage source is there).

And I don't think a description should contain any hint that this is a quite useless contraption, stuff in school is often not useful or practical but only a test to see if the concepts are understood. An explanation of how it works can contain all the comments on how dangerous and useless this might be. However the solution is wrong - like pointed out by everyone else, if the switch is closed the light will go out.

I've seen something similar while restoring my old Vespa. This answer is more of a try to explain why the teacher might ended up with a wrong circuit.

The generator of a Vespa works like a Dynamo on a bike, so it acts like some sort of current source. And the horn was controlled by a switch which was hooked up in a similar way.

simulate this circuit – Schematic created using CircuitLab

If you press the button (break the short), the current goes through the horn and you get a nice sound.

In the old schematics, there was no nice current source and the switch was also not depicted like in this schematic but as a normal switch. While analyzing the whole thing I was baffled at first until I drew a complete schematic and measured the switch to realize it's normally closed and pressing it does not close it but open it.

So maybe your teacher came across some circuit like this and in an attempt to make it easier, the circuit was changed in a way to make it wrong and the switch being normally closed was not clear. Most people think of pressing a button or flicking a switch will make a contact.

• Nice answer. Most accurate description regarding to electronics without joking on it. And a good real-world example which might explain the source of such a circuit. It's true: the light of an old Vespa gets darker when honking. Never thought about it. Commented Sep 17, 2016 at 20:29

I would describe it as a:

## "Inverted bulb short ckt w' switch."

Or if you want to describe the circuit construction through words:

## "Bulb+switch in parallel"

Though, you could also describe it as:

## "Simplest bulb NOT gate."

If you short an alkaline AA 1.5v battery, it's probably not going to heat up as much as you think, especially that it's a small BATTERY which is current limited.

See here.

• No, I think you misunderstood how little my teacher knew (see edit 2 on question) Also, it's a lightbulb, not an LED (there is no lightbulb symbol in the editor).
– dpdt
Commented Sep 14, 2016 at 0:58
• @dpdt Whoops. Also there is a bulb. Scroll down and you should see a circle with a curved wire or an X inside it. Commented Sep 14, 2016 at 1:00
• Having shorted a couple of D cells together before, they certainly get hot enough to give you a burn, although they didn't explode. Commented Sep 14, 2016 at 1:19
• I use 4 x AA Alkaline batteries in a relatively high powered camera flash for 'events' - allows disposable and fast reload when needed. They supply about the same number of flash cycles as high capacity NimHs in rapid recycle mode. | If used at near maximum recycle rate they come out of the flash so hot that they cannot be handled and probably hot enough to cause burns say 80 C or more. And that's not on short circuit but just heavily loaded. | Shorting a few NimH in a pocket with coins and keys raises them to temperatures that DEMAND immediate depocketisation. Ask me how I know :-). Commented Sep 14, 2016 at 1:27
• @RussellMcMahon +1 for "depocketisation" (without the 'z') Commented Sep 14, 2016 at 10:15