A while ago I made a marathon repair of many of these real-time clock modules, that plug in at
the back of an old HP desktop calculator. And while we were repairing it, we found out that,
to our surprise, inside there was a chip from Texas Instruments, that was used as the clock.
And they reused this. This Texas Instruments chip is actually quite famous: that's the one that
was made for the Texas Instruments LED watch, and that brought the LED watch - the digital
watch - down from an expensive curiosity to a very cheap everyday item. And it had never been
sold separately, except apparently to HP, so they could put it in their modules to keep the time.
And of course that TI watch chip was highly proprietary, so there is no documentation
on it, but we were able to reverse engineer it and figure out how it hooks up in a previous episode.
I wanted to do a little homage to Texas Instruments LED watches and build one with the
last chip. And this is all it takes. The TI watch clock chip, and two bubble LEDs.
The vintage bubble LEDs are from HP by the way,
who led the way with the very first LED displays developed for their test instruments.
At the time, they could not make the low power ones big enough, so they put a tiny plastic lens
in front of them to make them bigger. No kidding. These are hybrids by the way, consisting of
7 tiny dot LEDs with a little light pipe in front of them to make them look like strokes,
and individually wired bonded. When there is a will, there is a way.
Two buttons, a quartz crystal, a capacitor and a battery. And that's all we need to make our
watch rendition. So I'm not going to use a PCB, I'm just going to wire it directly and use the
dead bug technique. So here's my dead bug. And I need some buttons, so let's put the buttons.
One button. So there is Set
and Read. Because the watch is actually off most of the time, because of course LEDs consume way
too much power, so you need to press a button to view it. So for wires I was going to use
enamel wires, just because it looks cool. That's the wire that you use to make transformers.
Okay, that wire is a bit of a problem, that's not gonna work.
I'm using a combination of wire wrapping and enamel wire.
Okay, now that's the tough part, I have no idea if that will succeed or not.
And if I make a mistake, it's going to be pretty hard to repair it. So I'm going to
try to wire wrap everything, and then solder it on top, see if that works.
Okay, it's folding.
Okay, not bad actually. All right, it's complete. If it doesn't work it's going
to be a bear to debug. All we got to do, is put the battery in. There we go.
And I think it's this button. Yeah, it works! So when it comes on first, it's at
one o'clock, zero zero, one one.. So we'll put the time, it's very late.
So we hit the Set. No, we turn it on. No. Turn on? Oh, the set button doesn't work!
Okay, I have to work on the Set button, can't set it. And I could find nothing wrong with it. I
checked the pin for the Set button, it goes up as requested. I tried to jump wire it directly, that
didn't work either. So I think the chip is bad. I should have tested the chip beforehand. And I
don't know that I have the courage to redo another one with another chip. That was hard enough.
Oh wait, it works! I took the battery out and powered it with a power supply,
and now it works! So it's my battery that's not up to par.
I wonder if it's not up to par, or the voltage is too high.
It
says: three Volts on it. I'm trying another fresh three Volt battery on it, and I have
the same problem. Yep, 3V is too much. I tried the power supply at 3 Volts,
and I have the same problem. So I need to put a Zener and drop it down to 2.5.
So I am a little bit stuck between a rock and a hard place, because this chip is so sensitive to
voltage, and also its range of operation is from four microamps, when it's not on and just running
the oscillator, to 20 milliamps, depending on the selection of buttons, when it's on.
So, I tried a silicon diode, and drop was too much, I tried a Schottky diode,
the drop was not enough, I tried a combination of resistors,
that doesn't work because there's too much variation between the on and the off state.
And of course I can't afford any regulator, or 2.5 Volt regulation, because that would consume
too much current in the off state. So the only thing I found that was working, is put a simple
Zener regulation at 2.5 Volts when it's on, and I need to do the adjustment,
and then just add a switch to turn off the Zener when it's not needed. So that works on this one,
but here I would have to add a switch somewhere, which is kind of annoying.
Okay, I found a solution that works. It turns out, it is the voltage on the Set pin going too high
or too low that trips it, not the voltage on the chip. So if I can use the Set pin with a silicon
diode... Actually it kind of did work with the silicon diode, it didn't work all the time.
With a Schottky diode it works also. With a resistor it doesn't work. So either Schottky
or silicon. Okay, I think I finally got it. I
added the little diode here in series with the Set button, and now Set works. Let me focus...
It's 2-something PM.
One,
seven,
and go!
There we go! So now it shows us 2:03.
And the number of seconds. And if I press twice, I have the date.
All right, success!
The clock is going to join the collection of weird clocks. The LED clock - the HP brethren
also with bubble LEDs - the punch clock, the Soyuz space clock, Nixie clock, flip clock,
blinkenlight clock with the IBM panel,
Cesium clock, then our newest addition here, the LED clock. bBt
you have to tickle it to see anything.
Tickle, tickle, tickle.
Thanks for watching!