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

• Engineering
• Sahil#
In summary, the student is trying to build a logic gate using only LEDs, but is having difficulty because of the different voltage requirements between the input and output signals.
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.

## 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.

What logic levels did you use?

CWatters said:
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 equivelent.

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.

Sahil# said:
I just need a simple level 1 logic. Like simple AND or OR.
And I can't use that switch equivelent.

I meant what voltages :-)

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

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.

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

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).]

Last edited:

## 1. How do I calculate the resistor value for each LED in my circuit?

The resistor value needed for each LED can be calculated using Ohm's Law, which states that V = IR, where V is the voltage (3V in this case), I is the current, and R is the resistance. The current for each LED can be found by dividing the total current (in this case, 3V) by the number of LEDs (5 in this case).

## 2. Can I use any type of LED in this circuit?

Yes, as long as the voltage and current requirements of the LED match the specifications of the circuit. It is important to check the datasheet of the LED to ensure compatibility.

## 3. How do I connect the LEDs to the battery in the circuit?

The LEDs should be connected in parallel to the battery, with each LED having its own resistor. The positive terminal of the battery should be connected to the positive (Anode) side of the LED, and the negative terminal of the battery should be connected to the negative (Cathode) side of the LED.

## 4. Can I use a different power source instead of a 3V battery?

Yes, as long as the voltage of the power source matches the specifications of the circuit. However, the current requirements may vary and therefore the resistor values may need to be recalculated.

## 5. How do I test the circuit to make sure it is working properly?

You can use a multimeter to measure the voltage and current at different points in the circuit. You can also physically check each LED to make sure it is lighting up. If the circuit is not working, double check all connections and make sure the LED polarity is correct.

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