My niece has a toy that I constantly find myself playing with when I visit. It's a simple wooden box with some switches and LEDs on the front. Electronics wise, the switches are just wired to toggle certain LEDs but it's very fun to fiddle with and I find the overall design with the wooden enclosure very satisfying. For a long time I thought it would be cool to make something with a similar wooden enclosure that does something a bit more involved. This fall, I finally got around to following through with this idea and made something in the same spirit as my nieces toy except the LEDs and switches are connected to a binary adder.
I made it with a scrap piece of wood from my yard and a 74LS83 chip. The chip has all the logic for adding two binary numbers in a nice little breadboard friendly package.
How it works
The top row of LEDs is divided into three numbers. Starting from the left, the first four LEDs are red and represent the first number to be added. The next four LEDs are yellow and represent the second number to be added. The next five LEDs are blue and represent the resulting sum. The bottom row of switches control the numbers to be added, so the first four switches from the left control the red LEDs, and the next four switches control the yellow LEDs.
Let's look at some examples.
The following image shows the adder set to 1 + 1. In binary this is 0001 + 0001 = 00010, the LEDs that are on represent 1's and the LEDs that are off represent 0's.
The following image shows the addtion of 7 + 3 or 0111 + 0011 = 01010 in binary.
The final example shows the fifth bit of the sum being used. This illustrates how you need five bits to add two four-bit numbers together due to overflow.
Making it
The circuit is straightfoward, I followed the data sheet for the 74LS83 chip to connect LEDs to the inputs and outputs. Then I added switches to control the inputs. The circuit is powered with a mini-USB breakout board. The breadboard circuit is what is used in the enclosure because I estimated it would be small enough to fit and I didn't want to go through the trouble of soldering a protytype board.
For the enclosure I took a scrap piece of 2x6 from the backyard, cut off a portion for the back, and hollowed it out with a drill press.
Then holes were drilled for the LEDs and switches on the front. A hole was drilled into the side for a power switch and another hole was drilled into the back for the usb connector.
After some sanding and a thin coat of finish, it's ready to be assembled.
Then it was just a matter of mounting the LEDs and switches. Panel mount LED holders made this a breeze.
Conclusion
Being a software developper, it's fun to show off a physical project to people. Originally, I wanted it to be battery powered like my nieces toy but space inside the enclosure ended up being too tight so I stuck with the USB connector. This worked out for the best since now I like to have it sitting on my desk turned on as a decoration.
