A simple motor controller

Although the circuit is fairly simple, this is not really a beginners project. In order to complete it successfully you need to have a good understanding of electronic circuitry and be able to solder to a high standard, as well as being able to overcome the mechanical problems involved in attaching the drive to your telescope mounting. You will also require the use of an oscilloscope for fault finding, should the unit not work first time!

The controller was designed as an alternative to the manual slow motion knob fitted to most equatorial mounts that come with small to medium sized telescopes. In most cases the slow motion knob is held in place by small grub screws and can easily be removed to be replaced by a motor drive.

The controller is used in a similar way to the manual control knob but has several advantages.

1. On the unit we built for our three inch refractor we found that once set up correctly the unit tracked objects for up to fifteen minutes without any intervention.
2. When an object starts to drift, all that is required is a quick press of a button to bring it back in line. Unlike manual controls this does not introduce vibration into the mounting.
3. The hand controller does not assume difficult positions as the instrument rotates.

Circuit description

The controller generates a variable pulse width signal to control the motor speed. This enables the motor to be driven at very low speeds but still maintain almost full torque.

IC2 is a standard 555 timer circuit, running in astable mode. But instead of using the square wave output from pin 3 we use the triangular waveform that appears across C1.

This triangular waveform is fed to the inverting input, IC3A pin3. On the non-inverting input IC3A pin2 is a D.C. voltage the level of which is dependant on the condition of the four switches S1 - S4 and the settings of R16 and R19. You may notice from the circuit that if R19 is set fully to one end, this would disable either S2 and 3 or S1 and 4. In practice however  R16 will be set only slightly off centre (provided the gear ratio of the drive is not too high) so this should not be a problem. The output on IC3A pin1 is a square wave, the mark/space ratio of which is dependant on the D.C. voltage present on pin3.

R3,R4 and IC3B act as a simple inverter, the output on IC3B pin7 being the complement of the output on IC3A pin1.

IC1 provides a higher drive current for the output transistors than could be achieved using IC3 alone and produces fast transitions on the output. This makes the circuit much more efficient and allows smaller output transistors to be used.

D1 - D4 act as protection diodes for the output transistors.

Circuit diagram

The Circuit diagram is in .GIF format and may be downloaded as a normal image. Use the back button on your browser to return to this page.

Setup and testing

Once the circuit is completed, you should fully inspect your work before applying any power to it.

You will now require a 12V D.C. power source, this MUST be fused or current limited at no more than 1A. When our unit is powered from the car we use a cigarette lighter plug with a built in 500mA fuse, for more mobile use we use a pocket sized power source that takes six AA batteries and provides 12V D.C. at 200mA (max). The design for this will be given at a later date.

1. Set R16 and R19 as close to half way as possible.
2. Connect the unit to your power source (do not connect anything to the output yet) and switch on. Check that none of the components are getting hot, if they are then you have made a mistake!
3. Connect a resistor of between 3k3 and 10k across the output and with a meter measure the D.C. voltage across this resistor. You should find this is very close to zero. Check that all four LED's are lit.
4. Adjust R19 until the output voltage measures approximately 0.9V. R19 will be set accurately once the drive is fitted to the telescope.
5. Remove the resistor from the output and connect the motor and gearbox. The spindle on the gearbox should be rotating slowly in one direction.
6. Press S4, the motor should speed up a bit but still rotate in the same direction.
7. Release S4 and press S1, the motor should run even faster in the same direction.
8. Release S1 and press S3, the motor will do one of three things. It may slow down to virtually nothing, it may stop or it could run very slowly in reverse. All three scenarios are acceptable.
9. Release S3 and press S2, the motor should run fairly quickly in reverse.

The drive can now be fitted to your telescope and the final adjustments made. Depending on how you arrange the gears you may find the telescope runs backwards! If it does then simply reverse the two motor connections. Check that R16 is still set to half way. Using an eyepiece that gives around 60X magnification, aim the telescope at a bright star or planet. Adjust R19 for minimum drift of the objects position in the field of view, R19 can now be sealed with a small spot of nail varnish.

With the absence of any positional feedback from the drive some drift is inevitable, the precise amount will depend largely on the quality and mechanical condition of the mount.

Finally complete the assembly and test the unit with the telescope, checking that all four buttons do what you would expect and that the fine speed adjustment (R16) works.

The completed controller built into its case. In the unit shown above the LED's are an integral part of the buttons. The brass knob near the top of the unit is fitted to R16 and is used for fine speed adjustment.

Although the unit is designed for position control and to run off 12V D.C. it could easily be adapted for focus control and operation from other voltages (below 15V).

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