On 1/19/14 2:00 AM, P Nielsen wrote:

Half coded.

I'll publish all the details..
It's pretty easy.. a Arduino, a clock, a wall wart to power it.  I
haven't tried it yet (no clock to test with until the stores open), but
I'm assuming that it's just a wire from the digital output port to the
clock.  Might need a resistor in series.

The other burning question is "how accurate" does it have to be. The
scheme I have now basically has a table of "rate" vs day of year (which
I still need to calculate).

Right now, the EOT is changing almost 30 seconds/day, which implies that
the clock could be some seconds off during part of the day (although
"true" at noon).

Given the tens of ppm accuracy of the crystal = some seconds/day it
seems that I want a bit better algorithm.
Rather than drive from a table, maybe actually calculating it. the
Arduino is no ball of fire for floating point computation, but still, it
doesn't have that much to do. It could be that I can just calculate the
rate every second.

But then I have to differentiate the equation of time... and I haven't
had enough coffee yet to differentiate the chain of sinusoids analytically.

The usual quartz clock that runs off of a AA cell is a little
trickier to drive than you might think.  You need to feed
its stepper motor coil with an alternating +1.5V and -1.5V pulse.
The pulse follows the rising or trailing edge of a 1/2 Hz square
wave.  I have driven them using a series capacitor and resistor
to ground...arranged as a differentiator, but I don't recall the
part values anymore.

You arduino could certainly be made to generate such a pulse using
a couple of resistors and a couple of digital outputs in a simple
DAC sort of circuit.

-Chuck Harris

Jim Lux wrote:

On 1/19/14 9:17 AM, Chuck Harris wrote:

Yeah.. that is the challenge.
Use two outputs and make a sort of "H bridge"

On 1/19/14 9:40 AM, David J Taylor wrote:

http://www.wsanford.com/~wsanford/exo/sundials/equation_of_time.html

5 Jan 5.2 minutes
6 Jan 5.7 minutes

30 seconds in a day..

The total variation over the year is +/- 15 minutes, but the derivative
is a lot bigger at some times of the year (now)

Given the relatively low currents needed by the clock
motor, and the relatively high currents that can be
sourced/sinked by the arduino, and the fact that the
motor winding is floating relative to the arduino,
one could probably connect the motor like this:

D0---SomeResistor--MOTOR-----D1

Then, how to describe the way to drive it?

Well, starting out with D0=D1=0,

Set D1=1, then in 0.1 sec set D0=1.

Wait 0.9 sec then:

Set D0=0, then in 0.1 sec set D1=0...

Wait 0.9 sec then:

wash rinse repeat...

-Chuck Harris

Jim Lux wrote:

Hi

I’d bet that you can do some sort of simple fit to +/- 3 days from today and get a reasonable estimate of the rate. Exactly what you would fit might vary over the year.

Bob

On Jan 19, 2014, at 1:05 PM, Jim Lux jimlux@earthlink.net wrote:

On 1/19/14 10:11 AM, Chuck Harris wrote:

That's what I'm going to try.

Actually, on an arduino, you can set the output pin to Hi Z..

So it's more like
#define PulseLength1 100 // milliseconds
#define PulseLength2 100
void ClockTick(){
digitalWrite(D0,LOW); //preload the bits
digitalWrite(D1,HIGH);
pinMode(D0,OUTPUT); // now make them an output
pinMode(D1,OUTPUT);
delay(PulseLength1); // wait til pulse first part
digitalWrite(D0,HIGH); // flip direction
digitalWrite(D1,LOW);
delay(PulseLength2); // wait til second part
pinMode(D0,INPUT); // turn off pulse
pinMode(D1,INPUT);
}

That way you're not drawing power most of the time..