Further Modifications of the CG-5 mount drive


By Pawe³ £añcucki, Pawel.Lancucki@pl.ibm.com


Here I described modifications of the original duaI-axis drive for use with the CG-5 mount from Celestron.


Electronic drive


Original dual axis drive from Celestron consists of four main parts: two stepper motors with gear boxes; hand controller, which includes all the electronics and connectors and power supply in a form of a battery box accepting 4 R20 (D type) 1.5V cells.


Components of the original drive are shown below:

The drive is quartz controlled and gives good accuracy for visual use. Main drawbacks of the drive are:

Not a lot can be done with the gears, so I decided to focus on rebuilding / rearranging the electronic parts. I also decided not to play with the original printed circuit board (PCB), but rather to connect external components to it. Also, I decided that components build should be (as far as possible) compatible with a motorised / computerised telescope drive as described by Mel Bertels.


The new electronic parts consists of three main components (apart of the motors):


The components were build exactly in the order described above to minimise any time the drive is non operational – as described in the following text. The circuits described below are very simple. The tools needed are those of a radio-amateur and can be readily purchased in some stores (e.g. Radio Shack). I used a couple of screwdrivers, small hand tools to cut or bond wires and component leads, 15W soldering iron, small electric drill, fine files to make holes and universal electronic multi-meter (I use mostly voltage, current and resistance settings as well as a circuit continuity tester).


Just a word of warning – any changes you do to any electronic components may and will violate the manufacturer guarantee and therefore will be done at your own risk. If you are not confident you can modify electronic components, ask a friend or limit yourself to the power supply box only!


Power supply components are shown below:



Power can be sourced from 220V mains (European standard), from 12V car battery or from any battery pack capable of driving the scope through the night.


Celestron dual axis drive requires 6V at approximately 500mA. However, a stabiliser IC cannot be connected to it directly – the drive “hangs up” with higher power consumption, ant this may cause stepper driving transistors to burn. Apparently the original circuit uses internal resistance of the batteries to limit motor coil current, so I taken a provision to include additional 2 Ohm resistance in the power line.


At this time, I also decided to use 6.3mm mono Jack plugs and sockets as a standard for all power supplies. One 6.3mm mono Jack plug on a 3 m speaker cord (2 x 1.5mm2) provides supply to the unit. Five 6.3mm mono Jack sockets are mounted on the front cover and can be used to provide supply to other devices, like dew heaters, mirror cooling fan, small CCTV camera etc. You may opt to increase the rating of the power cord depending on your particular power needs, however, high power consumption accessories or accessories generating electromagnetic noise should use separate power supply. Front of the power supply box looks as follows:


The entire power supply box is housed in a small (ca 8 x 14 x 3 cm) plastic “project” box. Four mono “jack” sockets are not fuse protected and can be used to supply power to auxiliary devices.


The power supply box circuit diagram is shown below:


Power supplied to the drive electronics is protected by 800 mA fuse F1. Diode D1 - 1A/50V - protects the circuit from reverse polarity in input lines. Power is the conditioned by an IC stabiliser IC1 - LM7806 - down to 6V. It should be mounted on a heat sink to dissipate 3 – 5W of power. In my case, I have used ca 100 cm2 heat sink with tree-like fins, which heats warm in use but actually never overheats even after many hours of use. Two small capacitors C1 and C2 – 470nF/50V are necessary to prevent unwanted oscillations of the IC. Small red LED diode D2 fed by resistor R1 - 1.5kW 1/4W - is mounted on the front cover and confirms that the power is on. Two 1W resistors rated 1W each (shown as R2) simulate the internal resistance of the battery pack, and output capacitors C3 – 1000mF/25V and C4 – 470nF/50V are necessary to moderate any current spikes caused by stepper switching circuit. Internal electronics is shown here:



Finally, power is fed to the stepper driving circuit by 1.5m of 0.5mm2 wire with standard “power supply” plugs on both ends. In my case, I used a spiral wire form an old headphone set. However, I may opt in future to exchange these plugs because the standard plug tends to slip out of the socket in the stepper controller!


Any voltage between 9 and 15V can be applied to this unit, however it should be compatible with power requirements of any auxiliary devices connected to jack sockets. Direct connection to car battery (12V) or 12V wall transformer (unregulated) is usually adequate. To complement the power supply box I have made a car lighter to jack socket adapter cable and one 10 m (30 ft) extension cable, which extends the use both in my yard and in field. Also, a jack socket was mounted on the end of cable from a commercial 12V wall transformer to provide supply while using the scope in my backyard. Warning – I always place the wall transformer in house and use 10 m extension cord. Bringing a wall transformer outside may create a risk of electric shock!


You can prepare the power supply box without any changes to other components, while you continue to use the battery pack. Once the power supply box is complete, you can start to use it immediately even if you do not plan to make any other modifications. Here, the power is supplied from a wall transformer through the power supply box to the original hand controller:



Hand controller is slightly more complex. The front panel is shown below:



The main features are:


The hand controller circuit diagram is shown here:



 All buttons operate by connecting appropriate output line to ground, while in a “non-active” state, output lines are “floating”.


Fife small mono-stable press switches are used for direction and pause buttons. Pause button is connected directly to the output socket. Direction buttons are grouped in pairs (N+S and W+E) and connected to two switches, which reverse operation within each pair. Output lines of the switches are connected directly to the output socket.


The three-position switch output lines are connected directly to the output socket.


Another three-position switch is used to activate LED map lights. Two diodes are used for both red D5 / D6 and white light D7 / D8. Configurations of diode pairs and current limiting resistors depend on required diode voltage and were adjusted to get 20mA current through each diode – this gives a decent brightness without possibility to burn the diode. In my case, red LED diodes are connected in series, while white LEDs are connected in parallel, with separate current limiting resistors. In my case, all resistors turned to be 150W. Power supplied to diodes by one of the connecting lines is connected to the stepper driving circuit. The voltage may vary when steppers are in the sleeving mode, but the changes in LED brightness are negligible.


Finally, four small LED diodes D1 to D4 with current limiting resistors R1 to R4 (all 1.5kW) are connected directly between power supply and ground. They glow dimly in the night and provide very dim backlight to the controller buttons. Actually, you may opt to use resistors of even higher value, depending on the required level of button backlight. Values shown here were tested in a decent dark, but not ink-black environment.


All resistors are rated 1/4W.


A DB-9 “computer type” male socket is used to output the controller lines. The individual pins’ functions are as follows: 1 – ground, 2 – RA +, 3 – RA-, 4 – DEC+, 5 – DEC-, 6 – three position switch pos1, 7 – three position switch pos2, 8 – pause, 9 – power supply 6V.


A custom made cable with 9 lines and male / female plugs on both ends connects the controller to the stepper drive. I used soldered joints, which are very reliable. I decided to make the cable in a way which enables easy replacement or extension – see below:



Most of components were mounted directly on a universal printed circuit board 100 x 60 mm, which is held inside a black plastic project box 120 x 65 x 20 mm. This size is a good compromise between ease of assembly and convenience of use.


Two direction reverse switches and extra switch for computerised drive were mounted on the cover of the box.


The order of operations was as follows:

1                    Select a plastic project box which fits your hand and which will accommodate all components.

2                    Select a universal printed circuit board and cut it to fit inside the project box. Prepare mounting holes / spacers to secure the board inside the box.

3                    Select positions for direction / pause buttons and mark PCB holes which coincide with centres of the switches. Next, drive a thin (1.5mm) drill through the board while mounted to the box – this will ensure that buttons will mat the holes after they are soldered. Make button holes slightly oversized – for my 9.5mm buttons I made 10mm holes.

4                    Cut or drill remaining openings in the box – for direction reverse switches, MEL Bertel’s switch, DB-9 socket, 4 LED diodes and light switch.

5                    In my case, I had to cut or drill holes in the printed board to provide clearance for switches which are attached directly to the box cover.

6                    All drilling / cutting should be completed before you start to assembly the electronics. While assembling the electronic components, always check for fit to the prepared holes / openings.

7                    Carefully solder all components – one at a time. I started with LED diodes as they must match the holes in the box. Next – direction buttons, diode driving transistors / resistors and small LEDs and their resistors. Finally, connect the components which are mounted outside the printed board (direction reverse switches, MEL Bertel’s switch and DB-9 socket) with short lengths of thin stranded wire – I used small pieces cut off a 10 strand tape.

8                    Check proper resistance for the current limiting diodes before soldering the components – I did it by just connecting diode and resistor to 6V power supply and ground leads carrying these components in hands, while watching the readout of my mili ampero-meter. Also, check all buttons / switches before assembly – they are quite difficult to take apart from the PCB once they are mounted.

9                    As I used an universal PCB, there was a need to connect individual printed paths of the board. I made these connections from cut-over resistor / diode pins (short ones) or from very thin stranded and insulated wire (longer ones).

10               After soldering each component, check for possible bridges between adjacent printed paths. In case of doubt, check with resistance meter. All eventual bridges must be cleared right on spot, as they will be even more difficult to remove after more components are attached.

11               I always solder one component or one group of components at a time and take few moments to test the circuit. In case of any problem, it can be contributed to the most recent mounted components. This is a very practical advice – I did quite a bit of custom electronics development and it always pays to do as much testing as possible at various assembly stages.


The entire controller can be mounted in three – four evenings – one to do the machining and the rest to solder and test the circuit. You can continue to use the original drive while you build the hand controller, so there is no rush – just do it well.


Those who already know the design of the Mel Bertel’s computer controlled drive have already noticed that the hand controller is not entirely compatible with the drive – in particular the buttons and switches do not operate in required way. I decided to make the hand controller more suitable for the current use, while compatibility with Mel Bertel’s computer controlled drive will be achieved by adding some ICs to properly decode buttons and switches at a later stage.


Stepper driving circuit is the most tricky part, because you actually need to play with the factory made PCB            . Also, you cannot use the drive while the original stepper driving circuit is disassembled.


I decided to retain both the original stepper driving circuit as well as its plastic housing. The add-on components are mounted on a separate universal PCB, which is housed in another plastic project box – a twin to the hand controller. The original driver box is “piggy-backed” on the add-on components box (glued with double sided tape and fixed by four M3 screws), both circuits are connected by a number of wire strands which pass through a couple of holes between both boxes.


The stepper driving circuit diagram is shown below:



Direction buttons (N, S, E, W) of the hand controller drive four identical amplification circuit. I used these amplifiers to make my circuit compatible with auto guiding units. In a low input state, the driving unit must be able to sink less than 0.5mA – typical MOS ICs can sink up to 2.5mA, while TTLs can sink up to 10mA. Therefore, this unit is quite safe to use with commercial auto guiders.


I will describe operation of the amplification circuit based on the RA+ line. Input line is connected to a base of a low-power PNP transistor T3 through a network of two resistors R31 – 2.2kW and R32 – 4.3kW and a protective Zenner diode D32 – 7.5V 1A. This circuit will ensure that the transistor is saturated (open) while the input voltage is close to ground, but it will also protect transistor from damage in case a high or low voltage is accidentally connected to the input line. Transistor T3 operates miniature relay P3, which in turn duplicate the function of the existing direction buttons. Therefore, the internal circuit of the stepper driving circuit is separated from any voltages that may appear on the input lines. Transistor is protected from any voltage spikes which can be caused by turning on and off inductive load of the relay coil by a reverse polarised diode D31. Normally open pairs of relays’ contacts are connected by pairs of thin wire (cut from 10 wire colour tape) to the back of the button PCB from the original controller. Make sure that RA and DEC buttons are not mixed. Actually, it does not matter if you connect RA+ relay to the West or East button of the stepper circuit – if the operation of the buttons is opposite to what you desire, simply put the swapping switch on the hand controller in a second position!


The pause button operation is slightly more complex. Ideally, I should rather disconnect coil driving unit from the micro-controller on the original PCB. However, I did not want to play with the original PCB, so I used two small relays P11 and P12, which disconnect two RA motor coils from the driving circuit. Suddenly switching the coil inductance from the current supply generates voltage spikes that could over time destroy small contacts of the relays. Therefore, on both coils I used a spike protection circuit consisting of 150W 1/4W resistor R11(R12) and 470nF capacitor C11 (C12), connected in series. Furthermore, two 15V 1A Zenner D11/D12 (D21/D22) diodes cut-off any over-voltage spikes.


Instead of simply re-connecting the motor wires to the relays, I used two RJ-4 sockets – this enables me to use any standard telephone cable as connection between the driving box and stepper motors. Coiled telephone cables are by far more convenient in use and also can be easily replaced in case of any damage!


The whole stepper driving circuit consists of two main components:


Close-up of the original drive controller unit interior is shown here:


Here you can see it after the key-board was removed:



And here there is a close-up of the relay board:


Here is shown how it looks inside the plastic box:



The order of operations was as follows:

1                    You do not need to play with the original drive before the relay board is completed and tested – start with it!

2                    Select a plastic project box which will accommodate all components. In my case, it was identical to the hand controller box.

3                    Select a universal printed circuit board and cut it to fit inside the project box. Prepare mounting holes / spacers to secure the board inside the box.

4                    Cut and drill openings for RJ-4 sockets (to motors) and for DB-9 socket (connection to hand controller).

5                    Lay the largest components on the PCB. I opted to put relays in two groups – four relays which operate direction buttons are mounted closer to the hand controller socket, while two remaining relays which operate the pause function are mounted closer to the RJ-4 connectors to motors. I left a lot of space around the relays to mount other components (drivers / amplifiers, relay protection etc.).

6                    Before soldering relays check them for polarity -  my pause relays did not work until the driving voltage was applied to the coil in a specific. Also, check which pins correspond to the relay contacts, which will be opened and which closed before and after applying power supply. I used 6V relays, which still operate safely while powered by a lower voltage (the supply may drop by 0.5 to 1 volt due to the current consumption by stepper motors and additional 0.4 volt for driving transistors).

7                    If you have an opportunity, check the transistors for their current amplification coefficient. It should be at least 100 - 120, and preferably around 200 to ensure proper operation of relays.

8                    Connect two power supply lines – red for positive and white or black (or any other, just remember the colour) as negative.

9                    Lay and solder all other components. I started with direction relays. Again, I opted to assembly one relay circuit at a time. After each section is completed, solder a short coloured wire as an input line and test this section by connecting the input line to the ground – you should hear the relay to click while switching. If you don’t, check that particular section.

10               Attach pairs of thin wires to the connecting contacts of the direction relays. It helps to use colour-coded pairs from a 8 or 10 wire tape. Please note which colour corresponds each relay and save this information until you connect this pairs to the button board.

11               Solder all input lines to the appropriate pins of the DB-9 socket. Also, remember to connect power supply and ground lines to DB-9 socket – otherwise the hand controller would not work!

12               Connect the RA RJ-4 socket to the pause relays. Please note the colour coding of the RJ wires. Red and green wires should be connected to one relay while black and yellow to the second – this pairs drive two stepper coils and need to be connected to the over-voltage / spike protection circuit.

13               Connect a 4 wire tape to the pause relays – each of 4 wires between the original controller and the RA motor will be disconnected. Please use the same wire colours as the ones which were just soldered from the RJ socket – this colours will be important while you will connect these wires to the stepper driving board - this tape will be later soldered to the output pins of the RA line. All these tapes and wires should be quite flexible!

14               There are two pins (6 and 7) left on DB-9 socket – they correspond to the three position mono-stable switch. You may leave them not connected – I opted to connect them (and power supply and ground) to a small auxiliary socket, they can be used to control motorised focuser in the future.

15               Apply 6V power to the proper power lines (warning – this circuit has no protection from reverse polarity of power voltage!) and perform a final check of the relay board – all components should operate properly, hand controller buttons should cause relays to switch. Check if the two wires in each pair get closed as you push hand controller button, check if the RA lines get opened as you push pause button. Also, LED diodes on the hand controller (keyboard back-light and map light) should get lit.

16               Until at this moment, you did not need to play with the original controller. But now it is the time!

17               Place the original hand controller on a clean surface. Prepare plastic bag or box where you will put various components and screws – you ill need them back to complete the assembly! Also, prepare an anti-static bag or box to safely store the driver circuit.

18               Please obey the anti-static precautions while handling the drive components – any electrical components can be easily destroyed by static which builds up on our body and clothing. The best option is to use an anti-static wrist band connected to ground by 1M resistor – I have purchased one some time ago in Radio Shack. Get your soldering iron grounded and avoid touching electrical components on PCBs.

19               Carefully remove four direction buttons’ caps, remove two screws mounting the power supply socket and four screws holding the cover. Carefully remove the cover. You will see two PCBs – four buttons and one two-coloured LED are mounted on the top board, while the micro-controller and driving units are mounted on the “main” board on the bottom. Both boards are connected by a multi-wire tape.

20               Remove two machine screws holding the keyboard PCB. Locate and remove three remaining screws holding the main board.

21               Locate ten pins where all external cables are soldered. There are two stepper motor cables four wires each and a pair of power supply wires. Consecutive pins are isolated. If you look at the board while these pins are closest to you (natural position according to the printed labels on the cover), RA cable will occupy four leftmost pins, DEC cable four central pins and power supply two rightmost pins. Please note on paper colours of wires which are connected to all pins – this will be crucial later.

22               At this moment you will have to disconnect both RA and DEC cables. Do it carefully and remove cables from the controller box.

23               Carefully remove both boards from the housing – pay attention not to destroy the tape connecting both PCBs. Place the PCBs in an anti-static bag and put aside for a while.

24               Attach the bottom part of the original controller box to the cover of the relay board housing. Pay attention to ensure that you can attach the cover to the relay box – places for screws must be left clear! Also, the RJ-4 sockets should be located close to the output pins of the controller, while the DB-9 socket for hand controller should be located closer to the keyboard side. I used some pieces of a double sided tape to hold the together. Next, I drilled four 3mm holes through both boxes. Four M3 machine screws and nuts are used to keep both boxes together – I secured the nuts from turning out with small drops of paint.

25               Finally, drill two large 10 – 12 mm holes between the boxes – all interconnecting cables will pass through the holes. Clean the boxes (and your workshop) from any pieces of plastic or dust left after drilling.

26               All drilling / cutting should be completed before you continue to assembly the electronics.

27               Pass the wires and cables from the relay box through the two holes up to the driver box. Do not forget to pass the wires from the DEC RJ-4 socket as well!


Here you can see this stage of the project:



28               Take the driver PCBs out of the plastic bag and put them back in the box.

29               Carefully solder RA and DE socket wires to the appropriate pins on the driver PCB. Pay attention not to mix RA and DEC wires and to solder colour coded wires to exactly the same pins as before!

30               Solder the relays box power supply directly to the power socket.

31               Solder pairs of wires to the keyboard PCB. At there is no place to attach these wires on the components side, solder them directly to printed paths below the direction buttons. Ensure that RA and DEC pairs are not mixed (RA + or – can be connected to any of the W or E buttons – you may only need to swap the button reverse switch on the hand controller).

32               Now it is a time to check the entire system. First, double and triple check visually that all wires are connected properly. Check for any short-circuits between the power lines and between the output lines of RA and DEC motors.

33               Connect the hand controller to the DB-9 socket and connect RA and DEC motors to appropriate RJ-4 sockets. In my case, location of sockets correspond to the labels on the controller cover.


Here you can see the stepper driver side by side with the new hand controller:



34               Put the power switch in central (OFF) position and apply power supply. Keyboard illumination LEDs should get dimly lit. Hand controller buttons should operate appropriate relays.

35               Switch the power switch to N. RA motor should start (you can hear it). Check if the original buttons still operate the RA and DEC motors. Next, check if the hand controller buttons operate the motors as well.

36               Check if the pause button stops the RA motor.

37               If everything went well, you are ready to finally assembly the drive electronics. Otherwise, switch the drive OFF and continue testing to isolate failed component or connection.

38               Press the stepper driving PCB back into it’s place. You may slightly pull the wires from the relay board side – there will be lot of place left for wires in the relay board box.

39               Attach three small screws securing the main PCB. Attach the keyboard PCB using two machine screws – be careful, you may need to press cables that are soldered on the soldering side flat before you can tighten these screws.

40               Put the cover in place and attach it using four screws. Attach power supply socket. Put button caps in place.

41               Attach the relay board inside the box. Make sure that all cables fit inside the relay box and attach the cover with four screws.

42               Perform a final check – just in case any wire or joint got broken during this final assembly stage.


The stepper driver piggy-backed on the relay box:



Congratulations – you have completed an important step – your modified dual axis drive was modified and is ready to use! You may need to find a place to attach the drive to the tripod of your mount. I opted to attach the drive electronics with a self-adhesive Velcro tape, while the hand controller is held in a bracket made form 25mm x 1mm aluminium tape – it allows the controller to be easily taken in hand without pushing the telescope mount.


Field tests


My drive was tested after the modification during several nights of observing. The modification proves to be very helpful – with my 3.5m cord I can easily take hand controller to the eyepiece and use it to center a planetary nebula in a field of a high-magnification eyepiece, while moments later I use the map lamp to look in my Star Atlas 2000 for next object to observe.



And the close up of the connections to the stepper driver:



I didn’t try auto-guiding yet, but I think it will be possible with this modified drive, as I have already checked the drive reaction. First, I put the mount slightly out of balance in RA to make the motor work “upwards” – this eats any gearbox play. Then, I simply use the RA+ button at 2x speed to sleeve the scope westwards, and use pause button (and the diurnal motion of the sky) to sleeve the scope eastwards to correct the periodic error. From my eyepiece impression at over 200x, the mount reacts almost immediately, definitely a couple of times faster than if you reverse the direction of the RA drive! I think a similar trick with slight imbalance can also work for DEC axis. Also, it may be easier to track in declination if you correct only in one direction – very slight offset of the polar axis off the true celestial pole can do this, but will also limit useful exposure times due to the field rotation.


Final notes


Just a few final notes on building electronic devices:

  1. It always pays to buy 1 – 2 more components – if one get damaged during assembly, you will save a trip to the shop.
  2. Use only new components. If a component has any of physical damage, discard it. Even if it works initially, the probability of failure is greater.
  3. In case of any doubt, always check components. Even simple multi meter will help. Resistors can be easily checked for their values. Discard any resistor if the value is outside +/-20% of the nominal value. Diodes can be checked using diode checking setting on the multi meter. If you do not have one, you can use 9V battery, LED and 1.5kOhm resistor in series. For transistors the best way is to check their current amplification coefficient. Switches and relays can be checked using circuit continuity checker.
  4. While working with the main board of the stepper controller circuit, I recommend to use an ant-static device to prevent damage to the micro-controller.
  5. Use small soldering iron. Precision 15 W model is the best choice.
  6. Plan assembly in advance. Some components may be easier to mount on the PCB if they are soldered first.
  7. Always perform lots of tests after new component or few components are added. Any failure can be contributed to the components most recently added, so check them first. Isolating a problem in a complex circuit may be quite difficult.
  8. Faults may be contributed to failed components, bridges shorting traces on the PCB, small hair-like wires shortening soldering points, broken wires inside the insulation (rare), broken joints of wires.
  9. Mechanical components (plugs, sockets, switches, buttons, relays) tend to fail first.
  10. If the soldered components are dirty, if you solder to shortly or if the temperature of the iron is to low, you can get a “cold joint”. Cold joint may look as a proper one, but will contribute noise and eventually will break. For good joints, the soldering time should be approx. 2 – 3 seconds.
  11. Do not overheat components – the soldering time should not be longer than few seconds.
  12. Large components (e.g. capacitors) tend to tear joints away from the PCB if exposed to shock - due to their large mass. To relive some stress from joints, you can secure these components by alternative means – e.g. using silicone adhesive. If a component has mounting holes – simply use small machine screws and nuts to secure it in place.