Creating a Simple Logic Device

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Joined Oct 15, 2015
For my electronics course at my University, I have to create a simple logic device to show my Professor (this may seem very elementary to some of you, so I apologize in advance, but I am just getting started in EE).

This circuit is supposed to have two buttons and an LED. When either one of the buttons is pressed, the LED turns on. If neither or both of the buttons are pressed simultaneously, the LED stays off.

The components I can use are: SPST or SPDT buttons, resistors, transistors, diodes and an LED. Also, I can supply any sort of power I need to the circuit.

I am having trouble designing a circuit diagram for this because I am confused as to how the SPST / SPDT buttons function; namely, which prongs direct the flow of current when pressed or not pressed. I have to be able to recreate this design on a breadboard with wiring clear enough for troubleshooting.

I appreciate any sort of help on how to get started on this design. Thanks all!
The SPDT switch has a common (on the left in blocco's image above) and two other contacts. In the case of a momentary pushbutton, they'd be labeled normally-closed or "NC" for the top right terminal, and normally-open or "NO" on the bottom-right terminal. When you push the button, the diagonal line switches from having the Common connect to the Normally Closed terminal on the top-right to having the Common connect to the Normally-Open terminal on the bottom-right.

You can do the exercise you described entirely with two SPDT pushbuttons. It's like a so-called "3-way light circuit" in the home where you have two different wall switches at either end of a hallway both operating the light in the hall. It's a basic exclusive-or function.


Joined Aug 7, 2008
Draw two SPDT switches, Connect + 5V to one common & a 100 ohm resistor in series with a white LED to other common & - (supply return). LED cathode toward - V supply. Now see how to connect remaining terminals to accomplish the task.


Joined Mar 31, 2012
As suggested already, look up what a three-way light circuit is in house wiring and be sure that you understand how it does what it does. Then apply that understanding to your problem.