LED Wristwatch

Clocks, Projects 12 Comments

Sure, this doesn’t use a vacuum tube, but it’s still a neat way to reuse some old-fashioned 7-segment LED displays. OK, so I can’t wear it yet. It still needs a case and watch band. I am kicking around a few ideas, but feel free to post a comment if you have any suggestions.
LED Wristwatch
The LED displays are quite tiny and would have been used for calculators or similar devices back in the day. They were made by Fairchild as you can see by the original packaging:
LED Wristwatch - Fairchild LED Chips
The unique thing is that each display has a single die inside with each of the segments etched into it. In the picture below, you get a pretty clear look at the bond wires and the top metal layer. If you click the image, I have annotated the Flickr page.
LED Wristwatch - Closeup of LEDs

Back to the watch. It keeps the time with a fairly pedestrian PIC16F628A. It has an internal timer that operates with a separate oscillator (which is the watch crystal in the lower right corner) which can run even during sleep mode. This is critical to keeping the power consumption low. When a timer tick occurs, it generates a wakeup event, and the processor can increment the internal timekeeping registers. The processor can also wake up when one of the buttons is pressed. When that happens, it turns on and starts multiplexing the LEDs so that it can display the time. After a short delay it goes back into sleep mode. I haven’t yet calculated or tested the battery life.

The batteries are ZnAir number 10 (a common hearing aid battery). This is a zinc-air cell that uses oxygen as part of the electrochemical reaction, which is why there is a tiny hole in the top of each cell. Any battery holder has to allow air to make contact with the hole. In many states, these are the only batteries you’re allowed to throw away in the regular garbage. California is one of the exceptions, and the state considers zinc to be hazardous waste, so these batteries have to be collected separately in a category called “universal waste”. To me this seems foolish because I suspect that a lot more zinc is released into landfills as bits of scrap galvanized metal. Things like galvanized flashing, nails, and deck screws. Regardless of legislation, zinc is a lot less harmful than lithium, so remember to dispose of your burned-out LED throwies properly (and not in the regular garbage).

Here’s a puzzle for you: the PIC has 13 I/O pins. The LED displays use 8 (7 segments plus 1 decimal point) anodes and 4 cathodes. That leaves a total of 12 I/O pins, and I am not using the 13th I/O pin (RA4) because it is open drain and not useful in this circuit. So how are the two pushbuttons wired to the PIC? In fact, how can either button cause the PIC to wake up from sleep? Post a comment with your theory. I’ll give you one hint: there is a dual diode connected in some fashion to both switches.

And here’s one last photo to give you an idea of how small this thing is:
LED Wristwatch - Coin for Scale

Electroluminescent Display Panel

Projects No Comments

You’ve probably seen LCD displays that use electroluminescent panels for the backlight. They’re not so common since the advent of the white LED, but they were used all the time for those 5×7 character displays. Wikipedia has a nice article about electroluminescent technology.

Well, it turns out they can be used to make displays:
EL Display Panel

Here’s a closeup of the pixels:
EL Display Panel - Closeup

The “ghostly” numbers are letters are the result of the screen displaying the same thing for years and years. Looks like it was from a machine used for rapid thermal processing of semiconductor wafers.

This display panel was made by Finlux. There’s a very thorough history of this company and EL displays here: http://www.indiana.edu/~hightech/fpd/papers/ELDs.html.

It’ll be fun coming up with a project for this one. The display is 640×200, “high resolution” CGA. The video inputs are TTL-level video, hsync, vsync, and pixel clock. It will probably work at other frequencies too since the panel just uses high-voltage shift registers, not some special video ASIC. The tricky part will be getting the pixel clock to the device since video card connectors don’t provide that. Maybe I’ll just drive it with a microcontroller…

New CRTs

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The other day at the electronics flea market I obtained a couple of new CRTs. The one below has a P2 phosphor which is brighter and more energetic than the P1 and has much longer persistence. You can light it up with one of those UV LED flashlights. Notice the inspection sticker.

New CRT - 3JP2

And the one below is a fine example of the P12 phosphor–it lights up amber. The color is similar to that of the old amber MDA monitors but the persistence is longer.

New CRT - 3JP12