Quartz clock movement

[duncan webster 1Participant @duncanwebster1]

So here’s a puzzle. I want to d

rive a quartz clock movement from an Arduino instead of its internal electronics. Taking it apart and putting flyleads onto the coil needs a steady hand, but successfully completed. The coil has a resistance of 250 ohms, so I added 150 ohms in series to get within the drive capability and connected across 2 pins of the arduino. Sending alternating polarity pulses should drive the movement, and at 10ms it did, but BACKWARDS. Changing the circuit to what I’ve used before, 470 ohms resistor, coil, 470uF capacitor in series connected to one pin and ground it drives the correct way. Anyone care to speculate what is going on?

[Robert Atkinson 2Participant @robertatkinson2]

Did you measure the timing and amplitude of the existing pulses? It sounds like you may be over-driving the coils. With a 1.5V battery a 250 Ohm load will draw 6mA. So for a 5V logic output you would need a total of about 830 Ohms or a 500-600 ohm series resistor.

Robert.

[duncan webster 1Participant @duncanwebster1]

I’ve tried it again with 570 ohms and no cap and it goes the right way. If I have 500ms pulses the needle does a peculiar shuffle, goes most of the way to next position but then does a little further move when de-energised. According to wikipedia that’s what it’s supposed to do. However I’m sticking with the capacitor circuit as it removes any possibility of inductive spikes getting back to the Arduino

but the question is still why did it go backwards when it got too much drive.

[duncan webster 1Participant @duncanwebster1]

Just re-reading the wikepedia, the pulse length for 2 pin mode is around 31 mS, so must be pretty close to my capacitor version

[Michael GilliganParticipant @michaelgilligan61133]

For no immediately obvious reason, Duncan’s hot-link to Wikipedia failed

This is is an interesting page, so let’s try again:

https://en.wikipedia.org/wiki/Lavet-type_stepping_motor

MichaelG.

[John HaineParticipant @johnhaine32865]

Tony Jeffree started a thread on Lavet motors here:

https://www.model-engineer.co.uk/forums/topic/a-prototype-lavet-stepper-motor/

I found these two links I posted in the thread.  The first patent describes a way to drive Lavet motors to do weird things such as make them go backwards and rotate faster to enable setting watches more easily:

https://worldwide.espacenet.com/patent/search/family/004292664/publication/EP0103542B1?q=pn%3DEP0103542B1

The second is a page by Tom Van Baak describing some measurements on those continuously rotating quartz clocks:

http://leapsecond.com/pages/sweep/

I suspect you have found one mode to make them go in reverse.

For another reason I recently wrote a simple routine for a Nano that reads a pot voltage between 0 and 5V (0 and 1023 binary), maps it to a range of desirable pulse widths (in my case 0 to 250ms), and generates a 1Hz pulse train with the selected value (which is printed out on the serial monitor), driving a MOSFET to switch the current to an electromagnet.  Could easily be modified to drive a stepper via two output pins so one could explore how it behaves with different pulse widths.

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