A while back I posted a couple of replies in the thread that SoD created on his experimental clock on something I was playing around with. I thought I'd create another thread with more details to avoid confusing Dave's thread. The photo shows the nearly complete clock which is now sitting on my desk keeping reasonable time. It doesn't actually lean over to the right, that's just the photo!

The pendulum is to the rear of the clock, suspended from a bracket mounted on two brass channel uprights. Also attached to these is the dial, a commercial dial plate from Meadows & Passmore mounted in a shallow tray CNC millied from 1/4 inch aluminium. Movement is a converted quartz, with the stepper coil disconnected from the IC and brought out on a couple of thin wires. Arduino Nano is mounted on a small bit of perf board on spacers from the plate below the dial. Pendulum rod (bit of 6 mm CF tube) suspended from double spring, brass bob about 75mm long clamped by set screw from behind. 12 x 3 mm Neo magnet in the end of the tube.

This is the back view of the former lash-up. The white block at the bottom has the impulse coil directly below the centre position of the pendulum, and a Hall effect sensor to the right that detects the magnet on its swing extreme to control amplitude. The block is now mounted slightly differently with adjustable height from the aluminium base.

I was very much using bits from the scrap box in making this and focusing on getting it finished so not much "design" but I think it looks at least OK. Still need to polish up all the metalwork, hide the power cable (currently running off a USB lead to the Arduino) and make some sort of "dome" to keep the dust out. Also of course get it regulated!

I'll write another post about the software.

Software is based on the very simple scheme I posted back on Dave's thread. The coil generates this sort of waveform as the magnet passes over it (not mine but from the web article with the simple software).

This applied to an Arduino analogue pin through a 220 ohm resistor and the Arduino samples this at about 2 kHz. During the first negative-going bit of course all the samples are zero. Just after the zero crossing when the voltage is changing quickest, when it reaches a sample value of "10" the analogue input is changed to a digital output and driven high for a period (10 ms currently). Current during this period is ~15 mA, with the 220R plus the 120R of the coil itself. The coil inductance is negligible so there are no time constant issues. At the end of the pulse the pin goes low and remains low (inhibiting sampling) for ~470 ms.

There are 4 other software functions.

• Every 2 pulses from the coil (i.e. every second) a 20 ms pulse is sent to the clock movement coil with a 270R resistor in series. This advances the movement by one tick.
• Slight complication is that the current direction has to be reversed on alternate ticks so this takes 2 Arduino digital outputs.
• The seconds pulsing function is inhibited for 1 s every "N" seconds to adjust the rate, the pendulum period being set slightly short. Currently N is 132 but needs some tweaking.
• Finally the digital output from the Hall effect device, which goes low at the extreme pendulum swing if the amplitude exceeds a limit, raises an interrupt that inhibits impulsing (but not the other functions) for the next two centre crossings. When the clock settles down then roughly there are 7 swings or so without impulse followed by slightly fewer with.

The active bit of the shims are 10mm long IIRC, 6mm wide, 0.1 mm BeCu sheet. I found a supplier some time back, after discussion with a sales guy he said it wasn't worth selling me any so he'd send a sample. I was expecting a couple of square inches, but he sent the end of a coil of 2" wide stuff with several metres on it! Shims are spaced 15mm apart, soldered between brass strips in a little soldering jig. Here's a rather poor photo of the top end of the pendulum.  Main benefit is the pendulum doesn't twist, but it can still swing back and forth on 2 pivots.

My scheme is already within +/1 0.5 seconds, but I agree it leaves a bit of error on the table unless you also trim the pendulum with a bit of weight. I am mulling over a more complex scheme that can be more accurate.

Edited By John Haine on 18/11/2020 14:43:40

Thanks Michael. BTW, the little toggle switch below the dial, if "up" stops the movement which is convenient for setting.

Thanks Duncan, I can see how that would work. But an extra capacitor! As you say, there's no shortage of pins. No h Bridge, just connected to two digital pins via a series resistor.  Normally both pins are LOW, they pulse alternately HIGH for 20 ms to advance the dial.  Mean current is about 0.2 mA. 

Edited By John Haine on 18/11/2020 22:23:53

Not me, on the web in a design for a clock. Using a PIC I think – didn't even have the series resistor! But working on 3V supply.