Wednesday, June 22, 2016

finished mirror control cable

Finished the custom serial mirror cable build. Just needs a proper project box...

I wanted to make my own cable for controlling the mirror of the Canon 40D DSLR while using Backyard EOS. This will allow still photography with the mirror locked up to eliminate vibration. Overcame my anxiety.

schematic of mirror control cable using opto-isolator

Ultimately, this design is based on Paul Beskeen's circuit where he uses an opto-coupler. Initially, I had made a cable based on Michael Covington's circuit with MOSFETs, which worked, but I was attracted to the isolation benefit and the status indicator feature. I changed to discreet yellow and red LEDs from Beskeen's plan. I added the switch on the input side to block junk falling out of the serial port on starting a computer. Used the fantastic Express SCH and PCB once again.

The parts:
  • old Psion serial cable with female DB-9 connector
  • 470 and 330 ohm resistors, from the spare parts bins
  • diffuse red LED (for exposure indication)
  • diffuse yellow LED (for standby indication)
  • small SPST switch, from the spare parts bins
  • Lite-On LTV-817 opto-isolator transistor from digi-key
  • female 2.5mm submini audio stereo jack (with screw terminals) from Sayal 
The other cabling to make it all work:
  • Neewer Canon N3-to-2.5mm submini plug from Amazon
  • serial 9-connector extension cable
Learned a lot on this build (ironic, given the small circuit). Serial communication signals and that voltages flip from positive to negative and can vary by 12 to 15 volts. What MOSFETs are and how they work—field effect transistors are not unlike regular (bipolar) transistors. What opto-couplers are and how they work—they too act like a switch; the input side, with the internal LED, must be current limited. And that the focus (half-press) and expose/shutter release (full-press) channels carry voltages from the camera. And that I do not enjoy soldering submini jacks.

Again, it works (on the bench). I look forward to trying it in the field!

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