Dave Bell <[EMAIL PROTECTED]> wrote on Saturday, February 14, 2004 6:30 PM Subject: Re: solar clock; correction
> On Sat, 14 Feb 2004, Chris Lusby Taylor wrote: > > > I recently bought a 24 hour quartz movement for a heliostat project. > > It took months to arrrive, cost £35 and is of no better quality than the 12 > > hour movements sold for £1.99 by Argos as alarm clocks. So, I'm now > > converting a £1.99 movement by butchering one of its wheels. Actually, I've > > made it go at a quarter speed, so it becomes a 48 hour movement. After > > aligning its axis with the poles, a mirror attached to the hour hand then > > reflects a spot of light in the same direction all day long. Over a year it > > traces a figure-of-eight on a wall. > > > > Regards > > Chris Lusby Taylor > > 51.4N, 1.3W > > Great idea! An anlemmatic calendar... > > Are these the typical, small, plastic quartz clock movements? What did you > do to "butcher" the wheels? Too bad it isn't particulary practical to > modify the electronics; adding two more divider stages would accomplish > the same speed reduction, but everything is so integrated and > encapsulated. I assume there is one chip (probably appears as an epoxy > "blob"), a crystal can, and a tiny pulse motor. It takes little more, now. > > How did you arrange the clock movement and mirror, physically? > > Dave > 37.277N 121.966W Yes, these are the typical small quartz movements driving a tiny pulse motor by a half or quarter turn every second. To butcher the wheels, under a microscope and using a scalpel, I took the gear that drives the second wheel (the one the seconds hand is connected to). It had 8 teeth originally, moving by one tooth every second. Each tooth was actually engaged for about two seconds. I cut off all but two of the plastic teeth, and reduced their lengths so that they now drive for only one second. Unfortunately, this leaves the second wheel disconnected for several seconds every four seconds, so I added a whisker to act as a pawl and stop the wheel moving except when being positively driven. So, now, it moves by one 'second' every four seconds. I have also made a 48-hour dial, so that the clock can be corrected if need be. I mounted the movement on an astro compass that I recently bought on eBay for £9.50! This makes it face towards the Pole Star. On the hour hand, which revolves every 48 hours now, I mounted a thin hexagonal mirror (out of a supermarket laser scanner, as it happens) which, very conveniently, has a small hole in the middle of the hexagon, so the minute and second hands can still be used. Each of the six mirrors has its normal in the equatorial plane. The orientation of the hexagon is arbitrary, and can be selected to suit the location of the wall on which the figure-of-eight display is to be sited. Only one opposite pair of mirrors is used - I ignore the others. I am actually thinking of replacing the hexagonal mirror by a single larger double-sided mirror, but will then have to remove the minute and second hands. Any mirror must be double sided as the opposite side is used every other day. Hope that's clear. I'll bring it to Oxford for those who will be there. Regards Chris 51.4N 1.3W (By the way, I've just bought a GPS, so will quote my coordinates more accurately in future) -
