If you’ve been lusting after your own glowing display we’re here to help by sharing some simple building techniques that will result in an interesting project like the one you see above. This is a super-accurate clock That uses ping-pong balls as diffusers for LEDs, but with a little know-how you can turn this into a full marquee display. Join me after break where I’ll share the details of the project and give you everything you need to know to build your own.
Take some time to sit down and figure out how many pixels you need in your display. Above you can see the sketch that I drew on the back of some junk mail. Since I need a clock for my shop I’m only going to include LEDs for a twelve-hour time display. But I did plan to populate the entire grid with diffusers (ping-pong-balls) because it will look a bit cooler that way. I also planned to include a ring round the entire display so my final pixel area will be 15×7. Note the numbers below each digit, which are an LED count.
I also did quite a bit of planning at this point for the electronics. I need to make sure that I’m handling the current properly so that I don’t burn out any of the LEDs or chips that drive them. For now, let’s skip over the electrical issues and build the actual display.
First you need a box to host the project. I chose to use pegboard for the clock face because it already has accurately spaced holes that fit a 5mm LED quite snugly. The pegboard will be wrapped in a plywood frame which gives it strength, protection, and a place to mount the buttons and other hardware. You’ll also need all of the LEDs for the display (46 in my case) and ping-pong balls for the grid (105 in total).
From the start I wanted to complete this project with parts on hand. I already had the scrap of pegboard, and the plywood left over from building a wall-mounted desk for my office. Most of the electronic hardware was salvaged from another project (more on that later) and in the end I only had to buy ping-pong balls, hot glue, and some mounting hardware for the buttons. Here’s a list for the display itself, excluding the controller board and buttons:
- Pegboard for the face
- Plywood for the frame
- Ribbon cable
- 22 gauge hookup wire
- IDC connectors
- KK connectors or another 0.1″ pitch connector
- Woodworking tools: table saw, circular saw, straight edge
- Pocket screw jig and pocket screws
- Hot glue gun and hot glue
- Soldering supplies
Assembling the case
I first started by taping out the digits on a piece of pegboard, then clamped a straight edge along the cut lines and used a circular saw to trim it to size.
Pegboard is usually a bit flimsy so you should build a frame around it to make it rigid. I’ve just got some rudimentary woodworking tools so I’ve ripped 3″ plywood pieces, cut them to length with butt joints, and then cut a dado the width of the saw blade to receive the pegboard.
I bought this Kreg pocket screw jig when I was building my desk and I love it. Here I’ve cut pocket screw holes into the ends of the long rails. They butt up to the side rails and two screws will hold them nice and tight. You can get this jig for around $20, but if you don’t want to spend the money just pre-drill and countersink some holes from the outside and use wood screws to hold your frame together.
Here’s the outside of the finished box. All-in-all I’ve put about 90 minutes into the project. The majority of the time was spent measuring carefully, which you should take seriously. One wrong cut could cause an impromptu field trip for more supplies.
There’s plenty of room inside for the wiring and controller board. Now to start inserting the LEDs.
I had a great time building the LED Jack-o-lantern but that hardware has gone unused since Halloween night. Here I’ve desoldered all of the components from the protoboard and clipped apart all of the LEDs, separating them by color. I’ll reuse all of the green LEDs, the transistors, pin headers, some resistors, and many of the longer wires in this project.
Start by pressing the LEDs into place on the back of the pegboard. They’ll be quite tight and I found this process made my fingers hurt after a while. I checked each LED using a battery and resistor to make sure I had the polarity right and that they all worked and were the same color. Above I’ve started soldering all of the cathodes for each LED using ribbon wire. The cathodes are grouped by digit, and will be connected to ground by way of an NPN transistor.
Here all of the cathode connections have been made. I’ve used hook-up wire to run each of the four buses to one side, and inserted them together into a 4-channel KK connector. This will make it easy to plug the low end of the digits into a pin header on the control board. Don’t forget to hot-glue these wires to the pegboard as a form of strain relief.
Now it’s time to solder the anodes for the same pixel in each digit together. I used the waste pieces of ribbon cable from the last step in the process to do this. In the upper right you can see two ribbon cables which have IDC connectors hanging over the side. The top bundle drives seven of the 13 LEDs in each digit. The bottom bundle drives the remaining six. These will plug into double pin headers on the driver board, connecting the pixels through a resistor to a pair of shift register. The colon in between hours and minutes has been grouped with the hour-tens digit which only has pixels for the numeral 1.
Now it’s time to make sure everything works. We do need to add ping-pong balls as diffusers before the display will be finished but if there’s a problem you don’t want to have to remove the balls to fix it. Let’s build and test the control circuitry now.
I wanted to use parts on hand for this project. I’ve got plenty of 595 shift registers but there’s one problem with those; the supply pin has a 70 mA absolute maximum rating. I can get around that limitation if I run my LEDs no higher than 10 mA each and split the 13 total pixels between two shift registers.
That takes care of the high side. To switch the low side of each digit I’ve sourced 2N3904 NPN transistors. They have a collector current limit of 200 mA which will have no issue sinking the 130 mA max coming off of a digit when the numeral 8 is displayed.
The multiplexing is handled by an ATmega168 running on the internal RC oscillator at 8 MHz. This makes it a breeze to drive the display without any visible artifacts, but it’s lousy at precision time keeping. I had a Maxim DS3232 real time clock on hand that will keep very accurate track of time. It has a backup battery which will keep time when power to the display is lost. This is perfect since I intend to power this from the bench outlets in my shop. I turn them off when I’m not working and that means the clock will only be illuminated when someone’s there to see it.
This how-to is intended to focus on the physical build and not the electronic design. If you make your own it would be much better to choose shift registers that offer constant current on each pin. This way the LEDs can be brighter and there’s no need to worry about pushing up against the current ratings of the shift registers. Most constant current drivers are low side, which means you would then use P-channel MOSFETS, PNP transistors, or similar to switch the high side of each digit. Basically the opposite of what I’ve done here.
Check out the LED pumpkin matrix for more on designing your own multiplexed displays. As for constant current drivers, there’s some nice hardware used in this whiteboard/LED marquee project. Just don’t feel locked into Maxim parts as they can often be difficult to source.
Building the controller
Having designed the circuit it’s just a matter of wiring it up and writing some firmware.
Here’s the breadboarded circuit. You can see the two IDC connectors jumpered with resistors to the shift registers on the breadboard. The yellow wires to the right connect the digit cathodes to their respective transistors. In the foreground is the DS3232 on a breakout board. You can see the coin cell in its hacked holder. I’m using a multimeter to measure the frequency of the 1 Hz square wave this chip provides, and a Bus Pirate to see what’s going on with the i2c communications. Now that it’s working, I just needed to find a more permanent solution.
Voila! The top of the finished controller board. Note the two pin sockets to received the DS3232 breakout board.
And the point-to-point soldering on the bottom. This took perhaps four hours to complete. It’s winter right now and I don’t like using Cupric Chloride inside to etch circuit boards so I went this route.
The last piece of the puzzle is adding buttons. I knew I had this old circuit board from a Sony shelf stereo system.
I needed four buttons so I used a Dremel to separate this segment from the larger board. I added two holes to use for mounting and soldered wires (reused from the pumpkin) terminating in another KK connector.
Here’s a view of the underside of the button board. I had to remove the resistors that connected the buttons into a matrix and I used hot glue for strain relief.
After a trip to Ace Hardware I was able to install the button board. I started by tracing the location of the buttons on a piece of paper, then using that as a template to drill holes through the top of the plywood case. You can just make out the ragged hole above each button.
While and the hardware store I picked up a dowel as well. Here I’ve cut it to length, eased the top edge with some sand paper (I spun the dowel in a power drill for that), and added a hole for a retaining pin.
Once installed I glued a finish nail into the hole of each button dowel so they won’t fall out.
Here’s the finished product. I love it!
Everything’s working, time to add the diffusers. I originally planned to buy ping-pong balls from the dollar store but they only had six 9-packs. I ended up ordering a gross online. I drilled a hole in each ball for the LED to stick through, then used hot glue to attach them. Make sure the drill bits you use for this are nice and sharp.
Here’s a test with the lights on.
And another with the lights off.
I’m quite happy with the way things turned out. If you build one yourself take into consideration the use of constant current LED drivers as I mentioned before. Also, I had crystal clear LED packages, you may want to experiment with diffuse packages. You can see that there is a bright point on the top of each ping-pong ball because of this. On the other hand, in bright light you can still make out the time because of those bright spots, so test this out before you purchase all of your parts.
When all is said and done the display portion of this was easy and quick to build. It took much longer to solder the control board and to finish writing the firmware. A link the git repository is included in the resources section below.