# Logic Design

1. Sep 25, 2013

### robh

1. The problem statement, all variables and given/known data

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.

2. Relevant equations
V = IR
Logic GAtes

3. 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

Last edited by a moderator: Sep 25, 2013
2. Sep 25, 2013

### Staff: Mentor

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.

3. Sep 25, 2013

### Staff: Mentor

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