How to make a Logic Circuit with 5 LEDs and a 3V battery? Nothing else allow

[Sahil#]

Homework Statement

We can't use anything else, and just have to make a logic circuit (either OR, AND or NOT) with just this material. We can use less or more diodes but I have only 5.

Homework Equations

The Attempt at a Solution

I tried setting it up like doides but no help at all. I thought LEDs would function same as diodes but it didn't.

 

[CWatters]

What logic levels did you use?

 

[Sahil#]

What logic levels did you use?

I just need a simple level 1 logic. Like simple AND or OR.
And I can't use that switch equivalent.

 

[vk6kro]

If you were asked to make a 2 input OR gate, could you make one with 2 diodes and a LED?

You can specify which colour LEDs you are going to use.

 

[CWatters]

I just need a simple level 1 logic. Like simple AND or OR.
And I can't use that switch equivalent.

I meant what voltages :-)

Have a look at the I/V curve of various LEDs and compare with diodes.

 

[vk6kro]

The question specifies a 3 volt battery, but two RED LEDs can run in series off a 3 volt battery. One could function as a diode and the other as a LED showing the output state.

Remember the truth table for an OR gate.

 

[CWatters]

Anything to stop you defining a logic 1 as a 10mA current rather than a voltage :-)

 

[collinsmark]

If the assignment is flexible regarding the current capability on the input signal(s), then I suggest researching a little about "open-collector" gates, as a starting point.

Normally, we're accustomed to logic gates having voltage rails that are separate from the inputs and outputs; and normally inputs have very high input impedance, and outputs have low impedance. But you might want to ignore that for this exercise. If impedance/current requirements are laxed, than this is separation is not always necessary.

As a matter of fact, it is possible to build at least one of your logic gates with a single LED*, using this open-collector [like] scheme. (Following the same idea with the emitter will create yet a different type of gate.) You may not be able to build all types of logic gates with just LEDs alone, but you can build at least a couple of them.

*(If the output state is indicated by the LED lighting up or not)

I'll let you figure out how many types can be built.

[Edit: It also can make a big difference depending on what your inputs representations are. For example, if input state '1' is represented by Vcc; and input '0' by "no-connection" allows for different opportunities than '1' being Vcc and '0' being GND (0 V).]