Needed parts (Digikey parts numbers) - RS means Radio Shack is probably a better source
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Part Notes for electronics tyros
If you've never built anything electronic you'll want to make sure that things go in the right way. Chips are all marked to tell you which way they go either by a 'dot' near pin 1, or a 'cutout' at the end which has pin 1 or perhaps a groove on the pin 1 side. On my graphics the chips are shown oriented correctly. Diodes have a stripe at the cathode (negative) end and that is the end with the bar on the graphic symbol. The big capacitors have a stripe down one side with a - in it... that's the negative lead. For the 3-leg chips pin 1 will be on the left when you're reading the numbers. The rest of the parts are non-polarized so they can go in any which way.
Needed tools
- Soldering iron
- Dremel tool with cutters and drill bits
- Screw driver
- Pliers
- Small side cutters
- Salve for burnt fingers
- Laser printer
- Single sided copperclad
- Fine steel wool or Scotchbrite (tm) pad
- 99% Isopropyl alcohol
- Glossy photo paper (I use the Staples brand)
- There is a product called Press-N-Peel from Techniks that is supposed
to work very well. This would be much simpler but it costs $33 for 20 sheets
- An old plastic tub big enough for the board
- A clothes iron
- An old toothbrush
- Ferric Chloride etchant (click here)
- Solvent
Here's the board layout
When connecting the heater outputs and the 12v pass through outputs keep a watch on the polarities. For the 12v Aux Out the centre of the RCA plug is always positive ( the inner hole) and ground (the outer hole) goes to the ground tab. For the heater outputs it doesn't really matter which goes to which, but keep them the same for all of the RCA plugs.
The wiring
Here's how to wire the board
The finished board mounted in the enclosure
Here is the finished box. On my final box the computer plug for shutter control is on the hand controller (add-on). On the board shown in the layout above, the computer control is part of the board so the plug can go on the main box. If you want your computer connection on the hand controller see my shutter control add-on and leave out the optocoupler on the main board along with it's resistor/diode.
Photoshop graphic of the layout suitable to use for making an etch mask (scale to 160dpi)
Construction
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The first thing I did was to etch the PCB for the project. You can find instructions below.
Drilling requires a .8mm, a 1mm and a 1.5mm drill bit for
your Dremel tool; then it's just a matter of soldering on the parts.
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Making your own PCB using toner transfer |
I've used 470uF caps on the 8v output for the camera to take care of any minor current requests such as when the shutter is opened. With this much capacitance the 4001 diode to the input is needed. If the voltage on the input side of the 7808 drops the excess voltage on the output side could damage the chip.
I've also used a 470uF cap on the 12v (Vcc2) on the L239D just to handle any drop. This is behind another 4001 diode just to keep the cap from trying to power the whole circuit.
The rest is pretty straight forward. Oh, the anti-dew heat. That's a circuit designed by electronics guru and astronomer extraordinaire Dave McCarter. You can find links to it on my site. The only thing I've changed with this is to adjust the frequency down by 1/10th.
As the circuit COULD draw better than 2A depending on the state of the anti-dew heat, I've used a 5401 diode for polarity protection; it'll handle 3A. I've used 4001s with no problem but I'll bet that they get a bit warm.
The Canon XT draws about 200mA with the shutter open. This means that the 7808 could be dissippating about 800mW. Check the temperature of the chip and add a small TO-220 heat-sink if you think it needs it. With the LM317s that I've used in the past this wasn't an isuue.
I used Cat5 cable between the boxes but a more pliable 6-wire cable would be better. You need to pass ground and and a wire for each focus motor direction; then two for the shutter. Ground goes to one side of each button and the other side goes to the correct hole on the PCB (as marked on the diagram). The shutter just needs to be shorted... so the two wires go to the toggle switch. The 5v is provided in case you want to get fancy and add some LEDs or things on the hand controller.
Some L293D chips don't like their inputs 'floating' so 1K pull-downs are provided on pins 2 and 7 to keep the ouputs both low when no button is pressed.
Test your connections and you're done!
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IMPORTANT NOTE FOR PEOPLE USING A MONO PLUG FOR DLSR POWER If you are NOT going to wire a power on/off switch in the box ALWAYS connect the DSLR power plug before connecting the unit to your power source. Plugs like the 3.5mm mono plug I used for DSLR power SHORT as the jack is inserted. This is a bad thing. |
The box on my CGE
Velcro(tm) is a wonderful thing. Small tabs on the legs for the hand controller (it just needs to hang... not be stuck on for dear life) and larger tabs on the mount.
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