# Design: 3 Button/1 LED Circuit

• robh
In summary, the task is to design a circuit with three momentary, normally open, button inputs and one LED output. The LED should light up when exactly two of the buttons are pressed simultaneously. The LED has a Vf of 0.83 and If of 14.2. The approach to this problem would be to use 2-input NAND gates, with the inputs coming from the switches. The switches should be configured with pulldown or pullup resistors to generate a logical "1" when closed. The next step would be to create a truth table to determine the logic for the circuit.

## Homework Statement

Design a circuit with three momentary, normally open, button inputs and one LED output. The LED must light anytime exactly two (2) of the buttons are pressed simultaneously. The LED has the following characteristics:
Vf = 0.83
If = 14.2

- Draw a neat schematic diagram for the entire circuit, including input buttons, logic , etc.

V = IR
Logic GAtes

## The Attempt at a Solution

I know there are several ways of doing this but i am just not sure how to approach this. I was going to attempt to put the switches in parallel with pull up resistors to ground any voltage floating when the buttons arent pressed but as far as the logic i am not sure how to "attack" this problem. Thanks in advance

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robh said:

## Homework Statement

Design a circuit with three momentary, normally open, button inputs and one LED output. The LED must light anytime exactly two (2) of the buttons are pressed simultaneously. The LED has the following characteristics:
Vf = 0.83
If = 14.2

- Draw a neat schematic diagram for the entire circuit, including input buttons, logic , etc.

V = IR
Logic GAtes

## The Attempt at a Solution

I know there are several ways of doing this but i am just not sure how to approach this. I was going to attempt to put the switches in parallel with pull up resistors to ground any voltage floating when the buttons arent pressed but as far as the logic i am not sure how to "attack" this problem. Thanks in advance

Welcome to the PF.

Since the combination of each pair of things is simplest with 2-input NAND gates in this case, start with some number of NAND gates that have their inputs coming from the switches. Configure the switches and their pulldown/pullup resistors (one or the other) and access to Vcc or GND (one or the other) so that closing a switch generates a logical "1". Please take it from there, and post your thoughts.

Oh, and you will draw a truth table as part of your next post...