Breakout box with heat
For Rajiva MCU Update

You may have read on some of my other pages that I have built a circuit board for anti-dew heater control, and another to breakout the functions of the Rajiva MCU Update for the HEQ5 and EQ6. Well, the perf-board solution, while working fine, is just not elegant enough and having two boards fighting for space in a single box was not what I would call the best way to go.

That being the case, I decided to integrate the two into a single board and try my hand at creating a proper PCB to put the electronics on; here it is.

The board
    The PCB contains all of the functionality provided by the Rajiva MCU Update board. These are:
      Focus motor support (with indicator LED)
      ST-4 autoguider port
      RS232 port

      Along with the Rajiva support, there is a complete anti-dew heater control section on the board to allow the deliverance of controllable power to resistive heater strips with a duty cycle adjustable between 5% and 95% at a stable frequency of 265Hz.

    The size of the board is 2.15 x 3.45 inches and all of the traces are on a single side. The trace widths and spacing should lend themselves to inexpensive toner- transfer etching.

    There is a single source for 12v power and outputs are provided for all of the functions mentioned above. Connection to the Rajiva MCU Update board is provided through a 15-wire ribbon cable attached to the board through a 16-pin ribbon board connector (one unused pin).

Board layout
    Revision 2.2 of the board is electrically equivalent to the original but adds a bit of space around some of the components and arranges the external connections more logically. The original works fine but some things are a bit cramped.

    Here some JPEG images of the component side of the board.

     


    For a Photoshop image of the board with all the stuff on seperate layers that you can use to print the mask needed to etch your board click here.

    To scale the PhotoShop image correctly (I use The GIMP but it should be the same in PhotoShop) you want to hide all of the layers except the copper side. Make sure it is backwards left to right and scale it from 72dpi to 161dpi. Print it and then check with calipers that the spacing on the chip leg holes is .1 inches.

The components
  • 1 x L293D quad H-bridge motor controller
  • 1 x 7806 6v positive voltage regulator
  • 1 x 555 timer
  • 1 x IRF530A power MOSFET or equivalent
  • 1 x 3N246 small rectifier or equivalent
  • 1 x 10000R 1/4W resistor
  • 1 x 560R 1/4W resistor
  • 2 x 2200R 1/4W resistors (could use 1/8W)
  • 2 x 1N4001 general rectifying diode (4007 etc. work fine)
  • 1 x 1N914 fast switching diode
  • 2 x .1uF ceramic capacitor
  • 1 x 470uF electrolytic capacitor
  • 1 x 330uF electrolytic capacitor
  • 1 x .01uF ceramic capacitor
  • 1 x 50K potentiometer
  • 1 x Red LED
  • 1 x 16-wire ribbon cable board connector
  • 1 x 15-wire female crimp-on D-shell
  • 2 x DB9 D-shell or RJ11(RS232 and ST-4)
  • Suitable 15-wire ribbon cable
  • Suitable wire to run to connectors

    The 7806 can be replaced with a 7809 if you have a 9v motor, or if you have a 12v motor you want to use just put a bridge in place of the 7806 and leave the .01uF capacitor off. The minimum voltage you can provide to the L293D to control the motor is 6v as the voltage MUST be greater than the 5v provided to the chip.
Enclosure
    I've figured this all out for a Hammond box that is about 6.5 x 3.5 inches in area and about 2 inches deep. This gives clearance for the 7806 and the IRF530A and provides lots of room for the connectors etc. I use RCA plugs to provide outputs for 12v (for accessories), heat and for the focus motor but you can use whatever you wish. I bring 12v into the box with colour coded banana plugs (again, my preference) and also provide a hard-wired connection to provide 12v to the mount. This gives me a single power lead running to the telescope. Also, I've put an RJ12 plug in the box where my hand-controller connects. Again, this isn't really needed, but I now have all my connections in one place and the new location for the hand controller plug is more efficient than the one on the HEQ5.

Heater output
    There is a single output on the board for heaters. Just connect whatever plugs you wish to use IN PARALLEL between the board and ground (-12v). You can put an indication in of the amount of heat provided by running an LED (with a 2200R current limiting resistor) in parallel with all the heater plugs.
See my pages on my power enclosure/heat controller and my heater strips.

Need a link to a page that tells you everything you need to know about making your own PCBs? It is HERE. Below is the short version of the instructions.

Making the board
    If you've never made a printed circuit board before now is the time. Tom's website gives a lot of good instructions but here are some from me:
    1. Use the Staples Glossy Photo Paper (as mentioned in Tom's website).
    2. Print the copper side layer of the Photoshop image. Set your printer to the highest density and insure that the text is backwards. You'll have to scale the image for correct pin spacing; on The Gimp this equates to 161dpi. This may be the same for PhotoShop; try it.
    3. Clean the copper side of the board with 0000 steel-wool, or with a scouring cleanser and a clean scouring pad. Rinse with warm water and dry thorouly.
    4. Set your iron to it's highest temperature (NO STEAM) and then press the iron onto the paper for 30 seconds. For the next minute or so move the iron slowly around the paper pressing down.
    5. Leave the iron on the paper while you fill a container with HOT water.
    6. Drop the board into the water and leave it for 2-3 minutes... then start SLOWLY to rub off the paper with your thumb. Take small amounts at a time and let it soak for 5-10 minutes between layers.
    7. When you get down to the last paper, use a soft tooth brush with very little pressure and small circular motions to remove the remaining paper from the voids on the board. Use good lighting and you'll see when all the voids and holes are free of paper.
    8. Drop the board into your etchant. If you don't have an agitator or bubbler then just swish the board around every once in a while. At room temperature it should only take about 1/2 hour to 45 minutes to etch. Check it every 5 minutes; when you see all the copper gone from the voids and around the edge rinse and dry the board.
    9. Remove the toner with any solvent that will remove laquer. You're board is ready to drill.
    10. Holes for components such as capacitors, resistors and connectors can be drilled with a #64 drill. Use a #55 for the transistors and a #38 drill for things like power and heater leads. If you have steady hands you can drill the holes by hand with a Dremel tool and a good lighted magnifying glass.
    11. Use a low-power fine-tipped iron and .8mm rosin-core solder for the components. For heavier soldering such as wires etc. I use 1mm rosin-core.

      Here is my etched board ready for drilling and populating. As you can see, the toner-transfer method can make very nice boards.


    Final build
      Well, I populated the board today. The connectors for the ST-4, RS232 and 50K pot I put onto in-line pin connectors (the spacing on the board is correct. The only soldered connections from the board to the project box are the heater out and focus motor leads. I just didn't have any small enough connectors. Anyway, the board works as designed so build away. Here is a view of the box that holds all of the stuff; it attaches to the mount using industrial Velcro from Home Depot.






    New additions to the breakout box

      Well, as things move along so does my gear. I've recently added a plug and LED for my powered focuser to the box along with a plug to provide 8v to my DSLR (no more dead batteries) and a plug to control the shutter from my laptop. The box is looking pretty full inside with voltages raging around everywhere. It's about time to build a new one.

    NEW FEATURES
      Anand Rajiva has added features to his electronics, therefore it behooved me to add support for those features to my board. For a board that has full support for dual-focus motors, DSLR (Canon Rebel) shutter control, cooling fan control and dual-speed focusing go HERE.

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