Chesser Shutter Modification
The Chesser Shutter Modification is a hardware enhancement for the CB245 camera. It was originally designed by Doug Chesser and is included here with his permission. The Win245 software utilizes the shutter through pin 16 of the Printer port. This is a previously unused output pin and if the Shutter is not present, no harm is done. If the Shutter is present, you can automatically take dark frames without having to cover the end of your telescope.
The interface uses a spare buffer on the U1 74LS14 chip on the Interface card. Pin 3 of this buffer should be tied to Pin 16 of the DB25 connector through a 100 Ohm resistor. Pin 3 should also be tied to +5v through a 4.7K Ohm resistor and a 100pF capacitor should be connected from Pin 3 to ground. The output of the buffer (Pin 4) is tied to the Chesser Shutter Control circuit.
SIMPLE SHUTTER CONTROL FOR THE CB245 CAMERA (USED TO TAKE AUTOMATIC DARK FRAMES)
NOTICE: READ THE FOLLOWING VERY CAREFULLY BEFORE YOU START THIS MODIFICATION TO YOUR CAMERA, IT MAY NOT WORK IN YOUR SYSTEM:
THIS DESIGN UTILIZES THE WIN245 SOFTWARE TO DRIVE THE SHUTTER. IF YOU ARE NOT USING THIS SOFTWARE YOU WILL NEED TO DESIGN A MANUAL SWITCH TO SWITCH IT ON AND OFF, OR WRITE THE SW YOURSELF.
If you have any questions email me at chesser@mindspring.com
Thanks! I'd like to hear about your CB modifications also!
Doug Chesser
Overview:
I was looking for a simple and cheap dark frame shutter which I could use to take dark frames with. After some thought, this is what I came up with. It works quite well with Tybee Evans Win245 Software for Windows:
The device involves a Radio Shack 15VDC analog meter, which is disassembled and mounted as a shutter mechanism. The driver circuit is quite simple and is easily assembled onto the back of the CB interface card. The total parts count for the mod consists of 2 resistors, a transistor, and the Voltage meter. The total cost for the parts was less than $15.
Purchase the parts.
Disassemble the radio shack voltage meter. The outside housing easily unsnaps using the tabs on the side of the case. A screw and a glued peg located on the front of the panel scale actually hold the device in place. Remove the screw, and break the glue seal. Using a pair of needle nose pliers push the voltage connectors through the plastic housing. They are held in place with glue. The meter should now consist only of a metal housing and a needle.
Flip the meter upside down, and position it so that the needle will pass in front of the optical window on the front plate of the camera head. Using the screw holes on the base of the meter, attach the meter to the inside of the camera's front plate. Use washers or plastic standoffs to give the needle clearance to move.
Place a small set screw in the path of the needle, so that the needle can only move until is passes just to the edge of the optical window.
Using a very light weight opaque material, (Al foil works well), cut off a piece so that it covers the window. This will be the vane which covers the CCD during dark frame integration. Glue the material to the needle using super glue.
If everything has been done properly, when the needle on the meter is extended the vane you just created should cover the optical window. When it is in its natural position it should rest on one side against the stop you created, at the edge of the window.
Now comes the tricky part. You need to balance the meter mechanism so that you offset the weight of the vane. You will notice that on the back of the meter is a small metal tab which very slightly extends away from the housing by about 2 millimeters. Using a soldiering iron and piece of solder, tack a few inches of solder to the tab. The trick is to now cuts away at the solder until the needle is balanced. When it is balanced, the needle should always fall, and stay to the far end (uncovering the optical window), regardless of the position of the camera head.
Wiring up the meter. Hopefully you have 2 spare terminal connections left on the camera housing to pass the lines from the meter to the interface card. If you don't, you need to feed 2 wires in. The wires should come from where you have the interface card located to where the meter is located.
Attach the first wire (+5V) to the positive side of the meter.
Attach the second wire (+5 Return, Not Ground) to the negative side of the meter.
Break the ground trace going to pin 3 on U1.
Attach a 1.2K ohm resistor to Pin 4 on U1.
Connect the free end of the 1.2K ohm resistor to the base of a 2N2222 transistor.
Connect the emitter of the 2N2222 to ground (Pin 7 on U1).
Connect the line coming from the negative side of the meter terminal to a 2.2K ohm resistor.
Attach the free end of the 2.2K ohm resistor to the collector of the 2N2222 transistor.
Connect the wire going to the positive side of the meter to a +5V source (U1 Pin 14 will work).
If you are using Tybee Evans Win245 CCD control software connect a wire from the DB-25 Connector (Parallel Port) Pin 16 to U1 Pin 3. If you are not using his software, you need to connect a +5V (TTL Logic) switch to U1 Pin 3. These instructions assume you are using his software.
Put everything back together and try it out, note: You may need to adjust the 2.2K Ohm resistor to something higher or lower depending on how fast and strong you want the needle to snap into place. Just remember the lower the resistance the more force the needle will have. To much force may cause the vane to become bent or detached.I recommend using the smallest force required to move the vane into position.
