Pendulum-Controlled Clock

Here’s how to build a
pendulum-controlled clock which can be made really accurate. Retro? –
yes, but an interesting project all the same. You’ll need a spare quartz
clock which must be adapted by first isolating the two pads on the chip
which lead to the coil. You then have to connect wires to these pads
and feed them out through a hole in the case (see SILICON CHIP,
December 1996, p38, for full instructions, or October 2001, p37, for
brief notes.) You’ll also need a spare battery driven pendulum from
another, or the same, clock. As originally used, these pendulums are for
appearance only and play no role in timekeeping.

The salvaged unit should be mounted on a substantial vertical
backboard. You’ll find that the pendulum swings pretty fast and it must
be slowed down by adding weights near the lower end. However, it’s not
the mass of a pendulum that controls its rate – instead, it’s the
distance from the support to the centre of mass that counts. The aim is
to make the pendulum operate so that it takes exactly 1s for a full “to
and fro” swing – ie, 0.5s “beats”. Fine adjustment on mine was made by
adding an adjustable (up and down) weight to the pendulum rod. This
consisted of a small G-clamp fabricated from a brass strip and held by a
small screw.

Circuit diagram:

Pendulum Controlled Clock Circuit

Pendulum-Controlled Clock Circuit Diagram

At the bottom end of the pendulum attach an inverted T-shape
aluminium vane, about 10mm wide and as thin as possible. This should be
painted black. This vane is used to trigger a photo-interrupter which is
attached to the backboard. The lengths of the arms of the “T” are made
so that when the pendulum swings one way, the interrupter triggers – ie,
the light is no longer blocked. Conversely, when the pendulum swings
the other way, the vane must continue to interrupt the light. This means
that, with the pendulum swinging in 0.5s beats, we get a short pulse
from the photo-interrupter at 1s intervals.

This pulse is inverted by IC1a and inverted again by IC1b which then
clocks IC2, a 4013 flipflop. IC2 alternately produces 1s-long pulses at
its pin 12 & 13 outputs. These outputs are then fed to IC1c &
IC1d respectively, where they are gated by the short pulses on pin 4 of
IC1b. This produces two short pulses to drive the clock in alternate
directions at 1s intervals. And that’s all you need to drive the clock.
Alternatively, this circuit could be a master clock and could be used to
drive several slaves, all remaining in time. And model train
enthusiasts could drill one or more holes in the vane to make their
“railway” clocks run at what ever speed they need.

The circuit can be built on a small piece of strip board. Note that
the photo-interrupter should be mounted with the photocell facing the
backboard. This minimizes the risk of interference by ambient light. The
photo-interrupter is available from Jaycar – Cat.ZD 1901. A footnote
for horologists – if you have a clock with a Hipp butterfly escapement,
you could rid yourself of the trailing arm and contact arrangement and
replace it with a vane and photo-interrupter set so that as the arc of
the swing becomes too small, a pulse is missed. This could then be
detected by a 555 missing pulse detector circuit which would then
energize the impulsing magnet.

Author: A.J Lowe – Copyright: Silicon Chip Electronics

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