Tuesday, May 03, 2016

got nervous

Was ready to test the photocoupler in the custom mirror control cable circuit. A departure from tested simple MOSFET design.

Before installing the 4-pin DIP in the breadboard, I realised I need the pinouts. Searched Digi-Key and downloaded the spec sheet. I also searched with Google for example circuits. OK. Need to worry about polarity. On both sides. Feeling a little rusty. Suddenly realised all my electronic notes and cheat sheets were in old Psion files. Never converted. Ugh. Fired up the emulator to review. 'Round this time it finally dawned on me. This would need current limiting protection. There's an LED inside. Of course! But what numbers should I use...

Again, for the spec sheet, I found the forward voltage and the recommended amps. Fired up a LED calculator. And hit the amazing wikipedia for the serial port spec. Oh my. Plus or minus up to 15 volts. Nominally varying between +6 and -6 voltages. Stared at the electronic kit for a moment. Then checked Covington's circuit again. My "bottom rail" was negative (or the source for the MOSFETs). Decided I needed to see it.

Connected everything. Fired up Backyard. Loaded the last good imaging profile. Disconnected wires on the input side of the breadboard. In lieu of an oscilloscope, I put the DMM on the serial wires, hoping I'd see the signal. Oh. +6.46 volts. Then I hit the Test Cable button. Briefly, the meter flopped: -6.20 volts. There ya go.

And all of a sudden, I didn't want to go further. Didn't want to blow up my first photocoupler (even though I ordered a spare). Didn't want to press at this late hour. I really wanted to back up a bit. Carefully consider a proper circuit. I tore down the lab.

Happily, after a bit of searching, I found what I was hoping for. On Paul Beskeen's Astrophotography site, in his Projects section, he documented DSLR Serial Control, and showed a schematic with a opto-isolator. Bingo.

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