VC200L MICRO FOCUSSER PROJECT

This project used the same PIC microprocessor and software as my Takahashi focusser project,the real difference being in the mechanical design.In the other design I used a commercial gearbox that was found as a secondhand part that was cheap.This gearbox if I had to buy it at full price would have cost a fortune and made the project impractical.However in this case I realised I could make my own worm and wheel gearbox by hand with no more tools than a file,hacksaw,some drills and a drilling machine for less than £20 UK.The gears could be bought off the shelf for a few pounds each.The stepping motor in this case was second hand but a new one can be bought for around £25 UK.Considering commercial microfocussers exceed £300 in many cases it is a worthwhile project to make.The makeup is the same using a 200 steps per rev motor driving a 20 to 1 ration worm and wheel gearbox that drives the telescope focussing knob direct.When focussing by hand the coupled knob is released to run freely and then locked up to the gearbox when powered focussing is required.For materials I used two 2.25 inch long pieces of aluminium right angle that was itself 2 inches with a wall thickness of 3/16 of an inch.If you look a the picture below you can see how they were cut to fit the endplate of the motor.The endplate of the motor was approximately two inches square with four mounting holes.If you have a motor that has a smaller endplate than this you would have to make a bigger endplate for it out of aluminium plate preferably 1/4 inch thick to adapt it to fit.After that you only have to drill acurately two 6mm holes in the mounting brackets as shown for the bearings for the gear shaft.For the gear shaft I used a length of 6mm silver steel that was long enough to go through the gearbox with 3/8 of an inch sticking out for the focusser knob connection.The motor can be any 200 steps per rev motor that has a working torque of 15 ounce inches or greater that has 5 connecting leads that are arranged as one lead goes to plus 12 volts and the others are driven by the switching transistors in the driver circuit.The motor should take something like 150 milliamps per winding current during stepping.The gears in this case were from RS components UK as part numbers..262-2359 as the worm gear which needs a matching pinion 262-2400

In the image above it shows the motor holding the two aluminium brackets that mount the gears.On the right is a U shaped bracket that is a spacer to the back of the telescope with the brass locking screw fitted.When on the telescope this screw goes through the scope mounting bracket into the tapped hole in this bracket.On the bottom is the simple box for the electronics and in the side is the 5 pin Din socket for the power and the two switches that make it go backwards or forwards.The blue adapter at the back slips onto the telescope focussing knob and the brass screw locks it in place for turning the focusser knob,it is released for manual focussing.If it is difficult to make one of these then a simple brass sleave can connect the two shafts if the focusser knob is removed.There are two brass sleeve spacers either side of the brass worm wheel to keep it in place with the worm pinion fitted directly to the motor shaft.

A stepping motor can be run from DC or 3 phase ac current.In the mode used here using DC current is switched into the four magnetic coils following a binary pattern controlled by the parallel port of the pic microprocessor.This PIC output can only supply a small current of a few milliamps which is not enough to drive the motor itself.The pnp and the NPN power transistors amplifiy this current capability up to a level capable of powering the motor.There are four binary states for this motor that run through one by one then repeat themselves over and over again.Each one creates a magnetic change in the motor structure that makes its shaft step around a circle like the fingers of a clock in 200 steps per rev of the shaft.The microprocessor also controls the rate at which these codes are stepped and repeated so controlling the R.P.M of the motor.If the codes are sent in reverse order the motor reverses.In the case of this design one step moves the focussing knob 1/4000 of a rev or approx one tenth of a degree via the 20:1 gearbox.Sending 8 codes puts the motor in half stepping mode which means even more resolution and the motor goes slower.As most of the circuit runs at slow speed the electronics can be built in any simple form such as veroboard if it is not possible to make a printed circuit.All of the parts for this were sourced from companies off the internet, even the simple PIC programmer that runs from the computer serial port.Code complilers and programming software is freely available in the form of MPASM and pony programmer etc..