This month we will look inside a digital watch.
Over the Christmas vacation my long-time (cheap) digital watch decided to stop working, so I took it apart and put it under the microscope. Once you open up a watch there is really not very much inside. A plastic carriage holds all the bits in the right place so that when you press the buttons on the outside to change the time, they contact with the switches on the inside. There is also a place to hold the battery, and keep it connected to the electronics inside. Apart from this carriage there are really only 2 parts to the watch; the liquid crystal display (lcd) and a small printed circuit board (pcb). This pcb is really just a piece of plastic that holds the chip that does all the functions of the watch, and has wires on it to take the signals from buttons on the outside to the chip and to take the signals from the chip to tell which bits of the lcd to be black (In my case, since the watch didn't work, none of them were black!)
When we look at the pcb in the optical microscope we can several different things. First, it is sort of round - so that it fits inside your watch, but it has a hole at the top. That is where the battery fits. In the middle is the silicon chip, encased in epoxy a hard glue-like substance which protects the chip and its wires. Running from this are copper wires, which go either to the sides of the pcb, or to one of the 2 lines of metal pads. (the first one on the lower row is shown by a red arrow). The ones on the sides go to the contacts for the push buttons, the two rows take the signals to the individual lcd segments which sits above the pcb. In the picture on the right I have removed some of the epoxy to show the chip underneath - it is marked by the black box.
The days of the week you see in the top picture are actually painted onto the lcd panel, but not on the top where they could get scratched. When the watch worked a segment of the lcd above the appropriate day would be black to tell the correct day. On the left you can see that the writing does not have very sharp edges - but you can still read it easily. At higher magnification on the right you can see why the letters do not look sharp. The ink is very ragged at the edges, and does not have the same density of color everywhere. In fact our eyes are very good at making allowances for this very small scale variation in am image. Try looking at a picture in a newspaper from a normal distance and then from very close up - see the difference?
In order to get the signals from the pads on the pcb (remember the arrow in the second picture) to the individual segments of the numbers on the lcd there are a couple of very neat devices. The first is a flexible conductor that takes the signals from the pads on the pcb, to other pads on the lcd panel. This can be as the row of dark rectangles like a piano keyboard in the image at the left below.
At higher magnification (in the middle below) we can see that the flexible conductor is actually made up of three different materials. The dark coppery colored regions are the metal "wires". These are separated by an insulator - which looks the lightest color in the picture. This prevents signals in one wire leaking over and travelling up the next one; this is needed so that only the right bits of the numbers are black. Finally this alternating metal+insulator structure is coated in the dark, knobbly-looking material - a rubbery plastic which holds it all together. Each of the small pads in the second picture is in contact with 3 or 4 of the metal wires in this conductor, so that even if one wire does not make good contact, there will be another one which does.
In the image on the right we can see (at very high magnification) one of the pads on the lcd panel. It is actually very difficult to see, so I have photographed it with the help of polarized light which is why it looks colored. (This is similar to the way that clouds look much better through polarized sun glasses). The reason that the pad is hard to see is that it is transparent - like glass. It needs to be transparent because it has to take the signals to the segments of the numbers like a metal wire would, and yet still let light through so that we can read the numbers. This almost magical feat can only be done by a few materials, and this one is probably an electrically conductive ceramic. Most ceramics are known to be good electrical insulators, they are used on power lines and in automobile spark plugs, so this one is very special.
What an interesting variety of materials in one small and very inexpensive package! Now I'll just have to go out and buy another one, who knows what new materials will be in that one.
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Last Update: 2/12/96