# Need help in designing a logic circuit

1. Feb 1, 2015

### Bhavin

Design a circuit that will light the same number of LEDs as there are active switches. You will utilize three switches as input. The LEDs should be lit as a bar-magnitude display. The output pattern should look as follows: Active Input switch count LED bar-magnitude display (vertical) no switches active 0 - 0 - 0 any 1 switch active 1 - 0 - 0 any 2 switches active 1 - 1 - 0 all 3 switches active 1 - 1 – 1

I have tried the above design but didnt work and now I am back to square one.
There are three switches and three LEDs so total 8 combinations and LED is on when any switch is active...
Thanks

2. Feb 1, 2015

### phinds

I suggest that you draw a K-map or a truth table to illustrate the logic of the desired outcome. If you can get that right, turning it into a circuit will be trivial

3. Feb 1, 2015

4. Feb 1, 2015

5. Feb 1, 2015

### Staff: Mentor

Re your circuit, I don't think you want to be shorting your Vcc to ground via the switches. Use some pull-down or pull-up resistors.

What types of gates are you allowed? I see ANDs and ORs (even three-input ORs), but I don't see any NOT gates.

6. Feb 1, 2015

### Bhavin

Yeah I wanted to use pull-up resistors and connect my switches to ground and any types of gates are allowed.
I am not sure how can design because LED one is on when any one switch is on, LED2 turns on when two switches are on and LED turns on when all three switches are on.??

7. Feb 1, 2015

### Staff: Mentor

Start with a pure Logic circuit, and table the switch implementation for now. Draw three vertical lines representing inputs A, B, and C. They take on the logic values according to the switch values. You can add in the electronics (switches, resistors, power supply) to set the logic states on those lines later.
Okay, this is why you should take @phinds advice and start with a Karnaugh map approach. The state of each LED can be represented as a function of three inputs, so three separate 3-input Karnaugh maps can describe the three functions required. You can probably design circuits for LEDs 1 and 3 without use of the maps, since "at least one" and "all three" are pretty easy to deduce and implement with a pair of gates each. So concentrate on the "any two are on" issue for the middle LED.

8. Feb 1, 2015

### Bhavin

Thank you so much for the help.
Much appreciated.

9. Feb 1, 2015

### Staff: Mentor

Add the 3 resistors to Vcc as gneill suggested, then test your circuit again.

10. Feb 3, 2015

### LCKurtz

Have you labeled your LED's correctly? Or mis-connected them? Looks to me like LED 3 turns on when any two switches are on.

Last edited: Feb 3, 2015