Build Your Own LED Digital Wristwatch

This is a 12 hour, 6 digit, LED wristwatch using discrete 4000 series CMOS dip IC's.

This sounds like an impossible task, that is designing and building a 6 digit watch with discrete thru-hole IC packages to fit inside a case, small enough to be worn as a wristwatch.  Also, to use power-hungry LED's makes it even more difficult.  That is why I attempted this project, just as a challenge.  The result is a watch that measures 1 1/2 x 2 5/8 x 1/2 inches.  Not real small, but not too reasonably large either, as compared to some of the fashion watches folks are wearing nowadays.

 

Before you read further, I want to point out that although I am happy with how the project turned out, it is really not practical for use as an everyday watch.  The reason I say this is because even though the circuit draws only 12 microamps or less with the display off, it will draw up to 50 milliamps when the display is enabled.  No problem, if you only look at it once a day, but if you are like me, I was pressing the display enable button several times a day which resulted in the batteries being exhausted to a point where the display was noticeably dim within three weeks!  In theory, the batteries should last close to a year if you didn't enable the display, but that makes it a little hard to tell the time!  However, if you want to build one just to show off or want a simple circuit for a 6 digit clock, read on...

The goals set forth for this project were:

1.  Use common CMOS dip IC's and other easily obtainable parts.

2.  Have a display that was large, bright, easily readable and non-multiplexed.

     Also, one that is some color other than the traditional red.

3.  Have a full 6 digit display showing the seconds as well as minutes and hours.

4.  Have it display the time in the 12 hour format.

5.  Make the circuit as simple as possible to reduce the number of parts.

Click here for the complete schematic in PDF that is printable.

For the main counting circuit, I decided to use the 4026 because they contain a decade counter and a seven segment decoder with tops on the "6" and bottoms on the "9".  They will drive high efficiency LED's directly without current limiting resistors.  They also have an additional output that is very useful for making clocks.  More about this later.

For the displays, I first wanted to use some very bright blue LED's that I already had on hand.  However, they proved to be too large (1/2 inch) to fit into the case size I had in mind.  As an alternate, I chose a smaller, 3/10 inch green display made by Hewlett Packard, the HDSP-F503.  These are readily available as surplus from several sources.  Actually, you can use any efficient LED display that fits your case, as long as it is common cathode.  One thing to consider, is that you will need an appropriate filter to cover the display, which is sometimes hard to find.  As of today, I have not found a material to make a filter for this display.  Green just takes too much away from it because it appears as yellow-green.  Until I find one, I don't have a filter on it, just a clear plastic lens.  Still, it looks pretty darn good.

For the time base, I used a 4536 which is a programmable divider with inverter stages up front that can be used to make a crystal-controlled oscillator.  With a 32,768 Hz. crystal and setup to divide by 2 fifteen times, it puts out 1 Hz. that feeds the seconds counter.  The "Fast Set" and "Slow Set" functions are obtained by changing the division when the buttons are pressed.  Pressing "Hold" temporarily stops the dividers.  You can connect a frequency counter to pin 4 to set the 32,768 Hz. with the trimmer capacitor.

The seconds and minutes circuits are virtually identical.  They both use 1/2 of a 4013 to create a reset pulse when the tens of units reach six.  This is accomplished when the "Carry Out" pin on the tens of units counter (4026) goes high with the "5" count follow by "Clock In" going high on the "6" count.  This toggles the 4013's "Q not" output which then resets the 4026.  It then counts from 0 to 5.  The units counters are straight divide by ten or decade counters.

To provide the 12 hour format, a 4013 counts the tens of hours plus works with the 4001 to reset then insert an extra count into the units of hours when 13 hours is reached.  This makes it count from 1 to 12 hours.  Part of this is accomplished by utilizing a special output on the 4026, the "c" segment, that is available independent of the ED state.  This "c" output is low only when the count is at "2" and goes high on count "3".  This is how the hours count of "13" is detected.

The display is enabled by applying V+ to the ED (Enable Display) pins on the 4026's and to the tens of hours display directly.

I used the decimal points on four of the displays to simulate colons between hours and minutes and between minutes and seconds.  You have to wire some of the displays upside down and backwards to get this to work.  Discrete LED's would have looked better, but would have made the overall length of the watch much longer.

I chose 6 volts for the circuit to provide the additional brightness for the display and help compensate for the extra battery drain when you don't multiplex.  I used four LR44 alkaline cells because they are robust and cheap.  I bought 50 of them on eBay for less than $5.00 including shipping.

A friend of mine has a 1970's vintage LED watch that he was showing off to me in a fairly dark room.  I whipped out my homebrew watch, pressed the button and about blinded him in comparison to his tiny, dim display...


View of the inside with the back cover removed

As you can see from the photo, it was a tight fit.  Also, consider that over half of the wiring is on the other side of the case. 

The case was made from a 3/8 inch thick piece of epoxy linen composite material, sandwiched between 1/16 inch Garolite front and back pieces.  The center was cut and drilled out to provide for the battery and other circuit compartments.  The brackets for the wrist band were fashioned from two pieces of 1/16 inch brass stock, then nickel plated.  They were made to fit a common 18mm band.

Near the top center, are located the "Fast Set" and "Slow Set" switches which are accessible through two tiny holes in the top of the case.  I didn't install the "Hold" switch, as shown in the schematic, because you can set the time pretty close without it and I needed the real estate.  The display enable switch is on the right side, as viewed from the front.

You can set the oscillator to exactly 32,768 Hz. by connecting a frequency counter to pin 3 of the 4536 and adjusting the trimmer capacitor.  After a month, the watch was only off by two seconds as compared to my Radio Shack atomic desk clock.  Not too bad...

Improvements?  A six digit display looks neat and gives the impression that it is a super accurate watch.  However, a 4 digit version would be more practical, making it smaller.  A red or perhaps blue led display would be easier to find a filter for.  Also, the height of the displays played a big part in the thickness of the watch.  A thinner display would be nice, if you could find one.  Older LED displays are thinner, but tend to be much less efficient than the newer types which are much thicker.

Another way to possibly reduce the size is to use the more expensive 3 volt lithium coin cells instead of the 1.5 volt cells.

Several of you are probably saying: "Why not use a microprocessor?"  You could, but not the challenge this was.

I hope the information presented here will help those who want to build a simple clock circuit or even a wristwatch.

Mike


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