Hello,
this question is about the external base lead on the 3-pin photodarlington packages, and what to do with them.
I bought some CL138 photodarlingtons on ebay. Googling 'CL138 datasheet' returned very little of use, the datasheet is mostly white space.
NTE3036 looks like an equivalent, and on its datasheet it states 'external base for added control.' Initially all was well, but when I took my contraption outside to calibrate the light sensitivity I got some unreliable behaviour from the photodarlington. I suspect it's because I haven't dealt with the external base lead properly - I just left it floating - some stray current was entering the base perhaps, which of course is bad news given the large gains of these devices. But I'm not qualified to say.
Searching on this forum returned a thread about optocouplers with external bases (specifically about using them in their linear regime to send analogue signals.) Someone suggested the base can be connected to the Emittor via a 100K resistor. Someone else suggested that it might be best to have the base at a lower voltage than the Emittor(!?)
All I want to do is use the photodarlington as a switch (in line with my camera's wired shutter release, so it only works during the day) . I don't want an external base for 'added control.' The 100K resistor suggestion seems to work, but I'd like to know for sure, since this thing has a habit of working regardless of what I do with the base, but then suddenly behaving badly for no apparent reason.
Any advice, explanation, discussion would be of interest. Seems like such a trivial problem.
Thanks in advance,
AR.