DIY Large LED Lit 7 Segment Display

A friend of mine who teaches high school science commissioned me to make a Jeopardy style quiz game controller with a large timer display and buttons for players. The best sort of timer I could think of was a large 7 segment display with 3 digits plus a colon. Since these things can cost quite a bit to buy, I decided to make one out of an array of LEDs. The complete project description is available on my website!

This is nothing new - I have seem numerous designs of big segment displays that incorporate large arrays of parallel LEDs to serve as each segment, but using so many LEDs is such a waste! Not only do you have to use a lot of LEDs, it can take quite a bit of current to light up that many LEDs at a time.

For these reasons, I decided to come up with a way to diffuse the light from one or two LEDs over a large thin area to serve as each segment. This would reduce the need for so many LEDs and also reduce the total current consumption.

The reason the LEDs appear to flash in the video is because the camera has a faster capture rate than the human eye. The LEDs are flashing on and off - that is how the multiplexed control works, but the naked human eye cannot see it because the pulse is too fast - around 100Hz.

My final display uses 54 LEDs to form three 4" x 2.5" digits with a colon to separate the minutes and seconds. Each digit in multiplexed, so they share the control lines. The total current through each segment is 6mA, but because each digit is only on 1/3 of the time and each segment is comprised of two parallel LEDS, this equates to about 1mA per illuminated LED at any given moment. More current could certainly be used, but my LEDs are controlled by a low current sourcing register. If more current is provided, then display could be much larger than the one I created.

Step 1: Required Materials

Materials

Foam Board - As with most all of my LED array projects, I like to use crafting foam board as the base instead of a solderable perf board or PCB. The primary reason for this is cost. A full sheet of foam board can be found at many dollar stores or at any craft store. 

LEDs - This design uses 2 LEDs per segment, so 14 per digit plus 2 for the colon (dot). Pretty much any color can be used. More on the selection will be discussed in Step 2 - Picking Out LEDs.

Black Paint - This isn't necesary, especially if you get black foam board, but I like the look of the black background behind the LEDs.

Wire - You will need at least a few feet, depending upon how long of leads you need. The wire gauge for the power lines should be just big enough to handle a half of an amp max, so 25 gauge or bigger wire is good. Some lengths of a much smaller gauge would be fine for connecting LEDs together in strings.

Sheer Black Pantyhose - For filtering the display output (see Step 10)

Aluminum Foil

Tools

Sharp Pick - I use a dental pick, but an ice pick would also work; it just needs to be thin and sharp.

Hot Glue Gun - Hot Glue works very well to dissipate LED light. It's also a great way to protect and insulate the soldered leads.

Helping Hands - Not entirely necessary, but definitely very helpful.

Exacto Knife

Needle Nose Pliers

Soldering Iron and Solder

Classroom Glue Stick

Comments

[samaddon]

Great! Great ! pure genius man you solved my problem of buying screens !
Thanks for sharing!

[Kurt E. Clothier]

Ha, thanks. I don't know about "pure genius," but it's a pretty good alternative to buying large LCD displays.

[samaddon]

Great! Great ! pure genius man you solved my problem of buying screens !
Thanks for sharing!

[Gordon Shumway]

For light guide ideas, search "angle eyes" or "halo eyes" and dyi. That is the light rings you see around the BMW head lights. Most use clear window shade rods, the kind you turn to tilt your home shade or blinds up and down. They heat them in the oven to soften them for molding. P.S. I like your chronometer, I was thinking of making a large clock with LED strips.
Kurt, I like your idea of just a few leds, now I have to ponder how big I want it and see what would look the best.

[Kurt E. Clothier]

I found that this same technique will work for a much larger display by increasing the current in each LED segment. Right now, only about 1mA is being consumed by each LED when it is illuminated. I try to make things as power efficient as possible, and only a tiny amount of current was needed for the display to be as bright as I wanted it to be.

[CatalinRO]

Well done, I wondered if there is some material which can be used as a light guide. What else can be used as well as a light guide?

[Kurt E. Clothier]

Thanks. What do you mean by "light guide?" The hot glue works to dissipate the light, and the aluminum foil reflects it in the upwards direction. I am sure there are other ways to do this, this is just the first thing that I came up with that actually worked!

[CatalinRO]

You put the hot glue over the LEDS to dissipate the light - so it is a guide for the light :). I realized that If I'll bent the hot glue, I may drive the light around the corner :) (like here). For some time I was looking for a material which I can mold at home, to be cheap and can drive the light from LEDs, I thought that you tried several stuff until you get to the hot glue.  Thanks!

[Kurt E. Clothier]

Oh, OK. Yes, the glue is acting as the light guide. It does work around a corner, but the light quickly becomes too dim to notice unless there is something at the end of the initial straight path to reflect it around the corner. I tried that for fun while I was messing with this other process. I didn't really need to try much, the hot glue was my first thought, and it worked exactly as I wanted!

I just saw your Huge 7 Segment Instructible. Checking it out now. Nice work!

[amandaghassaei]

nice idea! I recently bought some large 7-segs off evil mad scientist, but they were ~$50 apiece! this is a much smarter solution.

[Kurt E. Clothier]

I know! That's exactly why i was determined to make something instead. This did take a bit of time though, but I don't think it was $150 worth of time.