Wednesday, December 07, 2011

Modifying the motor and controller for Mark-X

I have found source of very cheap 3.68MHz crystal and so I asked myself, why not give it a trial?

By using a pair of gears, the complexity of the system increases and the cost also increases.  So before I go that route, I should try to do it myself.

Order has been placed for the crystal, and tonight I shall also order the shaft coupler.  Or shall I wait to see if the speed was successfully change roughly or not first?  Of course, the shaft coupler is not that expensive after all, and thus, I guess I will proceed to order the coupler as well.

Mounting the motor should not be impossible for me as a DIY project.  I should be able to keep the cost of these hacking well below $100 HKD which is way cheaper and simpler than using a pair of gears.


Updated on 2011/12/7

- Crystal ordered at $0.99 USD

- Shaft coupler ordered at $4.99 USD

Updated on 2011/12/8

- Both items were marked as shipped, I'm feeling excited!

- In order to attached the motor to the mount, I guess all I will need is to buy a long bolt and some matching nuts, and to drill a hole on the motor mounting plate.  It should be well manageable to me as long as the crystal hack works as expected.

- After reading the document here:

I found that HC49U and HC49US differs by the pullability only, and thus my hack is going to be okay in that case.

- A local astronomer confirms that:

Dear Oldfield,

HC49U is standard size (~13 mm high).  HC49US is low-profile size (~ 5 mm high).

Since your clock circuit is using non-high frequency crystal (5.257 original  vs 3.68 MHz presumeably), I think HC49U and HC49US makes little difference in trimming capacitor.
Just try it.


Another step closer!

Updated on 2011/12/9

Simple calculation yield that the unmodified motor should do one rotation in 8 minutes (1440/180), and I'm trying to do a rough measurement by using a stop watch.  The simple measurement confirms that it's indeed a 180 teeth motor.

The above measurement was conducted by providing the rated 9V DC to the controller.  By reducing the voltage to 6V, the speed is the same.  The hand controller has a rated voltage range of 4.5V to 12V, and at 9V, it consumes 160mA.  I was doing that because 6V is simpler to get by using 5 AA rechargeables rather than 8 of them.

By using the simple formula of Power = Voltage * Current, if I use 5 AA rechargeables at 6V, the current consumption will then be 240mA.  Suppose I'm using 2000mA eneloop, a set of 5 will provide enough juice for 8 hours which translate to a whole night of tracking!

After replacing the crystal, it should do one rotation in 11.4 minutes (1440/126) if it's done correctly.  Since it will be running at a slower speed, the energy consumption should be slightly lower as well.  Sounds like a good setup for wide field astrophotography.

On mounting, I will need a single long bolt of 9-10cm, and that mounting hole has a diameter of around 6-7mm (like 1/4 inch?), so a M6 or 1/4" bolt of 9-10cm would do the job.  I will need 3 matching nuts at least.  The bolt should hide under the motor cover, and fixed on the other side of the cover with a nut, and then fixed to the mount with two nuts, one of either side of the mounting hole on the mount.

Found that the motor cover was pretty thick, and maybe thick enough to tap thread there...? and there should be at least 2mm to sneak hide the bolt head inside, good!

Updated on 2011/12/10:

This morning, I searched round my scratch box to see if there's anything useful.  I found a 4 AA battery box with an on-off switch.  The hand controller of my motor does not have any on/off switch and it should be nice to have one on the power supply.  4 AA rechargeables yield 4.8V which is near the lower limit of the input voltage.  The motor did turn but the LED on the controller keeps blinking and it should be an indicator of low battery.  On timing the speed of the motor, I found that it was slowed down to more than 7 minutes for a half rotation and that's too slow even without changing the crystal.  Worst yet, the motor might turn even slower when the power level drops.  Therefore, I don't think it's viable.

I think that I should find a battery box with 6 AA and an on/off switch, it will draw 200mA at 7.2V and a set of fully charged eneloop should be very safe for a whole night of operation.

Updated on 2011/12/12:

I've conducted experiment to tune the trimming capacitor of the crystal and I found it quite an ineffective way to change the speed of the motor.  On making two complete turns, the speed only differs by 8s.  Therefore, it's a way to really fine tune the clock speed.

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