7-segment LED H:M:S Countdown Timer advice needed

Thread Starter


Joined Oct 14, 2009
I'm looking to build a display timer, 7 segment discreet LED's (DigiKey 160-1678-ND). Each segment will consist of 5 LEDs for a digit size of 2 1/2"x5".
There will be 5 digits (hour, minutes, seconds). Not only will the timer need to count up from zero, but I'd also need it to have the option of starting at a preset time and count down to zero.
Power supply - 12 AH -12V rechargable battery(if possible)

HISTORY: I built a basic timer 0-> up countdown timer with 8" digits, with the help of an electronics friend (I know enough to be dangerous, I soldered in 300 LED's without frying a single one!) and scabbing different components together. Works great but now I want to take the next step of more features and not using parts out of other devices.

If some sort of PIC is needed, I'm willing to learn and do that, but as of now, I'm starting from ground zero.
I have the LED's, I have the circuit board material in my CNC and am drilling the holes for the LEDs and am in the process of building the case for the unit but I do need help with the schematics, and programing, if necessary.
Between my machine shop, family of 5,this clock, Rotary, running club, building a bicycle from tube and a host of other adventures, so many projects so little time, but I love to learn and try new things :-/

Any guidance would be appreciated , thanks!
(existing schematics, kits, reference books, or from scratch...it's all good :)

Oxbo Rene

Joined Feb 20, 2009
Ha ! ! !
Good luck ! !
Here's a couple threads I have gone through in my scoreboard project, perhaps you can garner some helpful info, etc.....
I am closing up my "console" part this weekend and finally that will be complete.
I have a 25 pin connector on the back which will feed the necessary data to my large display board (4" 7-seg LEDS).
Only a couple folks here offer much help, but I sure couldn't of made it to day two without em.
Welcome to the forum.
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Thread Starter


Joined Oct 14, 2009

thanks Oxbo Rene, I'll get it figured out, sooner then later, I hope.
Just took the display off the CNC and stuffed the LEDs.
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Joined Jul 17, 2007
Cool :)

One of the big decisions is to try to build it from discrete ICs, or take the plunge to learn uC's (microcontrollers).

The big advantages with uC's is that your parts count will be very low; and if you keep your hardware plan pretty basic and "bulletproof", you can do practically everything else via software. When you're new to programming, it might seem a lot easier to change the hardware; but after you get used to the programming aspect, it certainly takes less time to update the software.

I don't know how accurate you need this to be; but if it's for some kind of sporting event , it will need to be pretty doggone accurate. A uC's internal clock won't be good enough. Even a plain crystal oscillator will drift with temperature, throwing your timing off. At the very least, you'll need a temperature compensated crystal oscillator (TCXO); better yet an oven-controlled crystal oscillator (OCXO).

Found someone who'd decided to make his own OCXO's on the web somewhere; cheap and effective. Don't have the link offhand.

Anyway, you might consider starting off with something like Microchip's PICkit 2 Debug Express, around $50, which comes with the PICkit 2 ICSD programmer, development software, and a PIC16F887 processor on a board. Plenty of I/O pins for a project like this.

Thread Starter


Joined Oct 14, 2009
Simple is good and I won't shed a tear if I wind up soldering a few less connections. It's been a few years since I have done any programming but learing BASIC in the late 70's, FORTRAN in ME class, and C++, just for the heck of it, I think I should be able to pick up PICs fast enough.

Yeah, the accuracy of the clock, does not need to be nu-que-ler;), but
pretty good. I am using them to time events for my running club.
This pic is from last weekends XC race, here in Portland, using my clock
ver 1.0 (yeah, I have a portion of a segment I have to fix :-/)


Joined Jul 17, 2007
Portland Oregon, or Portland Maine?
It's a good idea to put city & state in your profile, so folks have at least a vague idea where on the planet you are. This helps others a great deal when trying to decide on what kinds of components to suggest. What you can get in the States can be quite different from the UK, Asia, Down Under, etc.

I have an idea that you may have lost some LEDs by overcurrent. They usually have a very long lifespan, if they are operated at less than maximum rated current.

You can buy a batch of LEDs, and 75%-85% of them will operate near the typical Vf (forward voltage) @ specified current; but the remainder will be up to 5% lower or higher at the same current. Those with the higher Vf's will look dim; those with the lower Vf's will burn out quickly - unless you match them up or otherwise compensate for their differing Vf's.

Don't know what LEDs you're planning on using.
If they need less than 40mA current, you might consider using something like a 7446 or 7447 7-segment driver IC. Multiplexing will save on ports, but will also make the display less bright.

You may want a small LCD display for feedback from commands entered via a keypad. That'll take up an 8-bit port. It's possible to do it with just a couple of lines.

A keypad will be useful, but it'll take some bits on a port.