how to build a clock with pulse for the hours!

  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'clcok 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.

Thank hopefully.

 On Fri, 05 Jun 2009 10:03:33 +0100, dave wrote:


The "stock" answer for any digital logic problem involving more than a
dozen gates is "use a PIC" (or other microcontroller). Whether that's the
right answer depends upon how many of these you're planning on making.

If it's just one or two, the cost of a programmer (~$35) and the time
taken to learn microcontroller programming will outweigh the cost and
effort of a discrete solution. Unless you're planning on making similar
projects in the future, in which case you may as well start learning
microcontrollers now.

If it's between dozens and many thousands, using a microcontroller
(whether PIC, AVR, etc) is the logical option. For very large quantities,
a custom chip will eventually win out, but I guess that you're not talking
about that kind of scale.

The main question is how you're going to adjust the time without a
display. That's a far more involved problem than generating 1-12 pulses
every hour.

 

It does not *have* to be involved at all. It depends on the
requirements. You could, for example, have one button that must be
pushed at exactly 1 o'clock. Extremely technically simple, but not
very practical.

On the other end of the scale you could use GPS or DCF77 to fully
eliminate any need for adjustments, even after a power outage.

Finding that perfect balance between simple (cheap) enough and
practical enough is the challenge. I can't come up with a better idea
than a simple 7 segment display and a couple of buttons.
--
RoRo

 On Fri, 05 Jun 2009 19:09:43 +0200, Robert Roland wrote:


Which is still significantly more complex than generating the pulses.

 

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