USB-serial converter
My new computer does not have a hardware parallel, nor serial, port. And because one of those ports must be used to control the shutter and amp-off on the camera, I should not be able to operate it in long exposure mode.
Unfortunately the Sweex USB-serial converter proved not usable to me right out of the box, due to non stable and too low signals at the output pins of the serial port. In order to stabilize these signals and to make the connector compatible to the much wider used parallel port design, I decided to modify the USB-serial converter to be used with a standard long exposure modified camera with a parallel control connector.
I do not guarantee that ALL USB-serial converters need this stabilizing circuit, but it can be very handy and is not difficult to build.
In the schematic below is the circuit I used to modify the USB-serial converter.
The 74HC00 is
wired as an inverter, because a NAND gate with all inputs to
the same source will act as an inverter. I did not use the dedicated
74HCT04 inverter IC, because I did not have it in my parts box. Notice that it
is important to wire the unused inputs of the 74HC00 to the ground (or positive
supply voltage) because unused inputs on logic circuits tend to start
oscillating,with unpredictable results for the whole IC as a consequence, when
left in an undefined state. The main function of the 74HC00, however, is to
supply a true logic signal to the rest of the camera.
Also, the small signal NPN BC547 transistors can be exchanged to similar types,
as can the 1N4148 diodes.
DTR is pin 4 of the serial port, which is used by the capturing program like K3CCDTools to send the shutter commands. RTS is pin 7 of the serial port and is used to transmit the amp-off/amp-on signal.
LPT pin 2 (pin 3 of the 74HC00) now has the same function as pin 2 of a normal hardware parallel port. LPT pin 4 (pin 6 of the 74HC00) functions as pin 4 of a normal parallel port.
In order to send the correct signals to the camera, the capture software must be told to use the correct COM port (from the computer control panel -> system -> device manager) and the correct long exposure settings. In my case I always use K3CCDTools and the serial command signals must be set as shown below.
Finally The
picture below shows how I initially made the connector. I used a 9-25 pin
connector housing, with on one side a female 9 pin connector (to attach to the
serial port) and on the other side a 25 pin female connector, to connect to the
parallel port plug of the webcam. The entire stabilizer circuit is inside the
9-25 pin connector.
The black wire with jackplug is used to supply +5V to the stablizer circuit.
Later I rehoused the complete USB-serial converter and stabilizer circuit into a single project box.