Sunday, March 29, 2015

Next clock?

I have several ideas for clocks I'd like to build next...  Here they are in no particular order.
  • A torsion pendulum clock.  Maybe with a pivoted detent escapement (see below)
  • A gravity escapement.  Not sure which kind; there are several good options.
  • A clock with a conical pendulum.  (This doesn't seem like it ought to even work... though of course it apparently does.)

Aesthetics and adjustments

Since I started constructing the clock, I imagined cutting out the front plate to reveal the gears.  Here's a sketch of where to cut, drawn by hand on a tracing of the front plate.

I cut out the paper plan and traced the edges on to the plate itself.

This was easily cut out on the scroll saw and finished with fine sandpaper.

The clock with the cutout front plate shows the gears as desired...

But the weight pulleys were still held on with wire.   Aside from looking sloppy, the weight has a tendency to be very close to the pendulum, sometimes colliding with the pendulum.  I added two small, but sturdy wings to hold the pulleys farther out.  This was easy, and keeps the weights clear of the pendulum.

Monday, March 16, 2015


Now that I've been running the clock for some time, it looks like it was running consistently fast.  Disassembling the clock and examining, I found that
  • The pendulum rod weighed 200g
  • The pendulum rod (from arbor center to end) was 43"
  • The pendulum bob weighed 100g
Putting this together, the farthest the center of mass could be from the arbor is about 29".  That's far shorter than the nearly 39" it should be.   Tweaking these values a bit, it looked like I needed to reduce the weight of the rod to around 50g and increase the bob weight to over 300g in order to be able to make a band of adjustments around the correct length.

Given that direction, I used a router to grind down the pendulum rod to quite bit thinner.  This took off about 100g, but also made somewhat of a mess.  Fortunately, I could make this the back so it's not so obvious.  I then tapered the rest of the rod using a plane.  In the end, the pendulum rod weighed 60g and is quite delicate.

I also cut two more circles of plywood to add additional weight to the back of the pendulum bob -- now it is 325g.

Given those two changes, I found that the clock had difficulty running.  Earlier calculations suggested that this would happen, because more lifting force needs to be applied at the anchor.  Therefore, I very carefully filed back the anchor teeth on the escapement to reduce the lift.  I may have overdone the exit tooth, but after applying more Slip-It to the gear teeth and pivots, the clock runs. 

After these changes, with the pendulum bob roughly in the middle of its range, the clock actually runs slow by about a minute every hour.  That's a success: I can now adjust both faster and slower as needed.  So now it remains to slowly bring it into regulation over the next few days.

Sunday, March 15, 2015

Edwin's relay latch circuit

My son Edwin loves Elenco Snap Circuits.  He has many, many components, and usually builds circuits that are not in the books that come with the kits.  Recently, he's been playing with relays, I think because he likes how they control things.  For instance, about a week ago, he built an oscillator by putting the relay coil in series with the normally closed contacts.

Yesterday, he discovered relay latches.  He did so in an odd way, by building the circuit into his Snap Rover.  He used the rover's motor as a generator to drive the relay coil.  Once the coil activated, the normally open contacts supplied an alternate path for the battery to run the motor.  The effect is that you can push the rover to start it, and then it continues along its way.

Here's a different version of that circuit that he built today, where instead of the motor he used a lamp.

His layout is a little confusing, so here's a scan of the circuit diagram I drew.

As the diagram indicates, there are two power sources: one (on the left) that drives the light when the relay is deactivated (S1 is open), and the other (on the right) that both controls the relay and supplies an alternate source of power for the light.

There are a few nonstandard things about this circuit even though the latch circuit is completely standard (he's never been told about it before).  The two power supplies could be combined by just using the one on the right.  More surprising is that the lamp is in parallel with the relay coil; usually a designer would place them in series.  I think this is because of Edwin's design history.  If you replace the lamp with a motor (removing S2 and the 4.5 V source), then the circuit sustains the motor once it's been pushed.  Although I didn't draw the diagram for his previous circuit, I suspect that is what he had done.