[time-nuts] Sidereal timekeeping

[iovane at inwind.it]

Neville and Jim and all,

there are good pictures of tuning fork crystals at 

http://en.wikipedia.org/wiki/Crystal_oscillator

I think it is the 8x3 mm in size, which is available from China (10 units for 
2.5$ including worldwide shipping). Assuming as a reference the diameter of the 
baseplate, say 2,5 mm without the can, the tines appear to be long in the order 
of 5 mm. It looks suitable for trimming (see, just below the tuning fork 
crystal, the picture of a trimmed round crystal). Trimming that way is common 
practice in crystal manufacturing, and this is what happens when you order a 
custom crystal (they have blanks which cover ranges). So I'm confident that the 
Q will not be affected so much.
I think it is worth the try.
Anyway, Neville, as a backup solution, I would be pleased to consider your 
design, and I think that others might have an interest too.
I would recall that I actually know what sidereal time it is now, by a 
computer program, but my problem is that I have to rotare a mini torsion 
balance by means of a low power drawing setup.
Cheers,
Antonio I8IOV

>Hi Antonio,
>the 32kHz Xtals are 2mm long tuning forks (that is what I believe  
>although I have not opened one).
>You would have very little chance of modifying it and still have  
>enough Q left for it to oscillate.
>As an alternative you could build an external circuit (a few uA at 3V  
>supply) and generate a signal to inject into
>the existing Xtal osc with the Xtal removed.
>The type of circuit that I would build would be a cmos binary divider  
>connected to a quad gate.
>The 4 gate inputs connect to selected binary stages of the divider.  
>When the gate decodes the
>selected number, an extra pulse is added to the count chain. The  
>output is thus shifted to a higher
>frequency.
>If you are interested I can try to design the circuit for you, I have  
>intend to build a Siderial clock
>dial for my TBolt.
>cheers, Neville Michie
>
>On 17/05/2011, at 7:58 AM, iovane at inwind.it wrote:
>
>> Neville,
>> at present I have not enough skill with micros to solve the problem.
>> I think I will try modifying a crystal. This would not be that  
>> difficult using
>> a lapping sheet or the like. And opening the can would be quite  
>> easy using hot
>> air. This is the fastest way for me, and the device will continue  
>> to be powered
>> by a simple AA cell, which is a non negligible advantage in my  
>> application.
>> All the best,
>> Antonio I8IOV
>>
>>> Antonio,
>>> it is quite easy to make an external circuit that uses a 32kHz  xtal
>>> and divides
>>> it down to siderial seconds. It is also easy to drive most analog
>>> quartz clock movements from
>>> an external circuit.
>>> Just what signal do you need? What frequency? and what does it drive?
>>> (an alternate polarity
>>> quartz clock motor?)
>>> It can also be done with a micro if you have the skills.
>>> cheers, Neville Michie