Can someone please help me create a simple 7-segment number display? [see below for details]

Background info :
My name is Simon. I'm a member of the Classic Celica Club of Victoria (Australia) that competes in an anual event called the Winton 6hr Regularity Relay.
It's a 6 hour long track event with about 40 teams and 6 cars per team.
Each team has one car on the track for the 6 hours.
Prior to the race, each car nominates a time that they believe they can consistently do for their share of the race (usually approx. an hour each).
Points are awarded for each lap achieved that is within 1 second of the nominated time. Points are deducted for any lap that is faster than the nominated time.

The aim :
To build a board that clearly displays a 3 digit time. (eg: 1min 56sec).  Team members will time the car on the track and then use the board to display the time for the driver to read as they pass by the pit wall.  (note: this practice is not against the rules).

The requirements :
The board must be big enough, and clear enough, to be read from >75m away.
The adjustment of the board must be very quick and intuitive. (the task of timing is shared around and there's no time to train people to use the device).
The board must be light enough to be held by outstretched arms.
No timing device is required in the board itself. The time displayed on the board isn't always the time that is recorded. Adjustments are sometimes made for one reason or another.
Scrolling through the numbers is fine, so long as you can scroll both up and down.
Each digit should be completely individual (ie: not linked).

Layout :
[see attached images]
I was thinking of using a typical 7-segment display layout made up of 79 x 10mm LEDs.

The actual physical construction of the board is well within my ability. It's all the circuitry that i am clueless on.  I am a total newbie so please be gentle when describing electronics.

I'm new to instructables but i have spent quite a few hours searching this site for answers. The thing i've found is that my device is far simpler than anything posted on here but i'm still too much of a newb to work out how to do it.

ps: I hope i have posted in the correct section.


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You missed the point. "If its the right sort of size, I'll sort out the PCB layouts for you." meant I'm highlighting the large digits. Are they visible enough ? You'd drive them from a very simple circuit, with a set of thumb-wheel switches or suchlike.

Obviously you don't need the rest of the electronics.

Si Mo (author)  steveastrouk6 years ago
Oh, sorry Steve. I didn't understand.  I can't see any dimensions on the digits. It looks pretty freakin' huge though and at 4feet square, the numbers must be large.  Far bigger than i require. Someone has to be able to hold out the board (comfortably) for ~5 seconds every 1min45sec on average, for 6 hrs.

A digit size of around 200mm x 90mm should be okay i think.  I really should do some kind of visibilty test to make sure.
The larger the better but i'm also concerned with cost. 200 x 90 should be the minimum though.
I haven't yet found a good source of cheap LED's so the fewer i used, the better.

I also decided that having the decimal point lit up is unnecessary.

Thanks for your interest and offer to help. :)
Hi Si,

These are about 250 X 150.  -ish

You can buy 100mm high LEDS off the shelf, but I dont' think they meet your viewing spec, from 75 metres !! Nor will a single row of LEDs be visible in sunlight.

There are rules from our department of transport for the kinds of signs that you are supposed to be able to read, at highway speeds, and at specific distances - all based on careful research - I used to design such signs at one point. Basically, you need a big character !

You can pick up large quantities of LEDs on EBay.

Si Mo (author)  steveastrouk6 years ago

You're right... ebay is a great source for LEDs.
These work out to be 7.6 cents each. :O

So, assuming i use your digit size of 250 x 150mm. How do we proceed with designing a board?
What kind of input device would you recommend?

Tonight, my friend suggested a single keypad whereby the first time a button is pressed it changes the first digit, the second time a button is pressed it changes the second digit and the third time one is pressed it does the third digit.

I don't know how easy this would be to do but would simplify the way the board is used and reduce the amount of hardware required.

Hi Si,

Your friend's idea needs some sort of electronics, which you specifically excluded in the original spec....

See my related 'ible again, where I have put the PDF for the boards up for you to check sizes. www.instructables.com/id/A-countdown-clock-with-LEDs/

I think the easiest way to do it is with multiple switches and some simple logic.

Si Mo (author)  steveastrouk6 years ago
I know my friend's idea requires electronics. I wasn't against the idea of electronics. I just can't design or understand them myself.

The PDF isn't downloadable by non-pro members like me. :'(  so i've sent you my email address by PM.

No PM yet Simon ?

Si Mo (author)  steveastrouk6 years ago

I sent another one about 90minutes ago.


I've put the original segment PCBs in my 'ible on the clock

Take a look and see if they're too big/ small whatever.
...Then I'll show you how to wire 'em.

Re-design6 years ago
OKay, so what I get from your description is a board that will display 3 digits that someone dials in.  They set the digits, there is no timing function in the circuit.

Is this the simple definition of your needs?
Si Mo (author)  Re-design6 years ago
That is exactly right. 

The 10-position rotary switch idea was my initial thought. But since i'm not at all familiar with electronics, i was just going to have the various digit combinations wires wired to each position of the switch.  It works out to be around 70 wires. That's when i was told about those BCD encoder things. Which is why i'm here asking for help because i wouldn't know how to implement one.

Thanks Steve, but that giant Y2K clock is far more complicated than it needs to be and i don't actually want it to count down (or up) by itself. It's meant to be purely a display device.
Does this help  ?

If its the right sort of size, I'll sort out the PCB layouts for you.

Re-design6 years ago
If all you need are 3 dials to set the numerals to the chosen one, then start with this diagram at the bottom of the page.

In place of the push buttons you might be able to use a rotary switch that has 10 positions.

You will need a complete circuit for each digit you want to display.

You might need a way to isolate your out put from you actual display, since your display might draw more current than your decoder can sink.
Si Mo (author) 6 years ago

I forgot to add the images.

Pit board concept 1.jpgPit board concept 2.jpgPit board concept 3.jpg