Re: how to build a clock with pulse for the hours!

On Sat, 06 Jun 2009 11:13:34 -0400, default <default@xxxxxxxxxxxxx>
wrote:

On Fri, 05 Jun 2009 18:27:28 GMT, Jon Kirwan
<jonk@xxxxxxxxxxxxxxxxxxx> wrote:

On Fri, 05 Jun 2009 10:03:33 +0100, dave <dave@xxxxxxxxx> wrote:

Wonder if some kind person can describe how I can build a circuit that
is basically a black-box (clock) that provides a short pulse - that
outputs 1 pulse for 1 o'cl*** 2 pulse for 2 o'clock etc?

I don't even need a display - but would need some means to set the
time for the pulses to be on the hour.

What I need it for is to drive a small solenoid to which is attached a
hammer that strikes a (real) metal gong. It's a novelty clock!

I can build the circuit ok but I don't have the expertise to design
the it.

The comments already made provide some range to think over. I'm going
to jump in and take a specific shot.

I'll assume that you'd really like some precision -- something on the
order you've come to expect from a common watch. That's going to be
"hard" even for a microcontroller supporting a 32kHz crystal, because
watch makers have gone to some lengths now in designing low cost means
with relatively high precision. Achieving that in a new design with a
microcontroller will take effort and equipment. In fact, doing that
with any newly designed circuit will, regardless of whether or not it
includes a microcontroller. (So that's out -- unless you can consider
something on the order of 20 seconds a day drift, or worse. If this
were on the order of some minutes a day drift, it would be cheap and
easy with a micro.)

It costs about US$10 at my local shopping store to buy a watch with
hands. You might consider doing what any decent bomb-maker may do --
modify the watch to provide an electrical contact for you. These
watches have delicate hands but I suspect that the gearing for the
hour hand is sufficient that a slight loading would be acceptable. A
fine wire, for example. To avoid bounce, relying upon a contact time
that is unknown but probably long compared to what you want, etc., you
may want to add a second point of contact for resetting a circuit -- a
flip flop, for example. And then add a one-shot to the output to give
the desired pulse duration.

The watch face cover may be glass or plastic. If plastic, you might
simply drill through it with a tiny drill bit to make a penetration
near the hour position. If glass, remove it entirely or penetrate it
with a drill, too. The fine wire might be 40 gauge, wrapped lightly
over the hour hand and arranged so that the tip extends outwards
towards the hole. Arrange another wire (any convenient diameter) so
that when the hour hand's fine wire goes by the hole that it will have
to brush across it. The tip of the hour hand wire should be just
barely long enough to be forced to brush across it in its travel.

You'd need to tweak things to get the initial timing of the hour pulse
at the right point.

Another possibility would be to use a tiny neodymium magnet on the tip
(or even midway, perhaps) of the hour hand. Use a "reed relay" in a
glass ampule and locate it near enough to the hour hand that the
magnet will be able to close the relay. This one seems good in ways,
tricky in others. Good in that you may be able to find a small magnet
with enough power that you can close the reed relay even when it is
outside the face cover. Which leaves everything sealed. Bad in that
if that idea can't work at all, you may still face difficulties
finding any magnet tiny enough, yet powerful enough to consistently
close the reed relay at any marginal distance apart. Traditionally,
the magnets used are rather large, so this would need to be a matter
of experimentation before proceeding. But there are companies on the
web selling such things. Personally, I'd tear apart a cd-rom drive
for the magnets near where the tiny coil near the optical lens is at.
They are often VERY small and yet VERY powerful. A mere fragment of
one of those may do the trick.

Jon

I like your thinking. I don't know if a magnet is practical the
clock hands are relatively heavy - I notice when testing batteries
with a clock and dummy load, the clock almost always stop with the
second in the west most quadrant trying to climb past the "9."

I have no idea how practical it is. I imagine (and only imgaine) that
it _may_ be possible to get a small enough 'bit' of a neodymium magnet
and place it far enough back towards the axis of the hour hand that
the combination might function. I also worry that the remaining size
of it may be still too big to remain underneath the watch face cover,
so that cover may still be a problem. Besides, it just forces the
distance to the reed relay that much farther away, which is part of
all the factors that need to be optimized together. However, might be
fun to find out.

A photo interrupter, that the hand moves through, would probably be a
better idea. A single one with some method to set the bell count
would probably be easier than trying to find both the hour and minute
hands and the 13 interrupters that might take.

That's good. I have used interrupters. In this case, I might instead
paint a highly reflective spot on the hour hand and use reflection
instead of interruption. But I like this.

Or take a digital clock and use logic or a pic to decode the display,
or just use the guts from a quartz analog clock to provide an accurate
one second pulse to a counter or pic.

Yes, that crossed my mind. But then that hauls in the coding, tools,
etc. So I avoided mentioning it. Just because I have all that stuff
doesn't mean others want to buy into all that trouble.

Jon
.

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