This was a nice weekend project that came out of a result of me doing
some spring cleaning in my components parts box. By the time I had moved
all my component to the new parts box I found some old LEDs that had a
strange width to their anode and cathode leads. The leads measured 0.80mm
as opposed to the typical 0.25mm found on most generic 5mm LEDs. Strange,
and not too helpful since the increased width does not fit in the protoboards
I have. As I was about to throw these basket case LEDs out I noticed…1,2,3,4,5,6,7,8!
There were exactly eight weird LEDs! A perfect byte. I couldn’t get rid of these now.
This had to be something fun involving a byte. I decided that a binary counter would
be a fun and quick weekend project that I could keep on my desk as a novelty. The idea
would be that the LED display would count from 0 to 255 and then reset to 0 and the
cycle continues. Simple!
Next came thinking of components. A microcontroller could solve
this problem in an instant, but what fun is that? Also, that comes at such a great cost
for such a simple function. In the spirit of this project everything was going to come
from the component box, no programming or intense processing behind it.
I first had to think of something to set the time for counting.
A clock frequency for the circuit would be a 555-timer set-up in astable mode. The resistors,
capacitor combo I used of 100Kohm, 10Kohm, and 10uF giving a clock frequency of about 6.5Hz.
Given by the equation from the datasheet (see pic). The clock would send a signal to a 74
series counter chip to continually add to the binary output of these 74xx counter chips.
I chose two of the 7493 counter chips to drive all eight LEDs. Divide by 16 counters were
the highest I had on hand although I am aware there are counter chips that give an 8-bit
output. It didn’t matter the counter chips are essentially an array of J-K flip flop memory
blocks. The output of one chip could easily be put to the input of the other to easily make
an 8-bit counter from two 4-bit ones.
Once a schematic was drawn up, it came time to prototype. I started with a breadboard prototype and it worked! Lovely and no real issues. Time to move on to the protoboard prototype. The protoboard version would be the one on my desk since a full PCB seems like a bit much for something so tiny and rather personal. The protoboard was put together and now has a happy spot on my desk! This was plenty of fun and didn’t cost me a thing since I was able to use parts just lying around. I had so much fun in fact that I got a bit carried away and CADed a design for a printed circuit board of this circuit. I haven’t produced any boards yet but the Gerber files are ready so…maybe one day.
Here's the loop of the new desk trinket. As you can see, counts up to 255 then resets to zero. Such fun!
Article last edited: August 2021