(Some more info, on Solar Cell interface):
The 1.2 mSec delay I quoted was an average, as both switching directions happen, via 'inverter' chain, in my implementation of a classical Edge Detect; that's with several 'gates' feeding in serial order, into a two input logical 'AND'.
In some related trials, I did quite easily manage to simply connect one of those 'garden light' gates as a straight replacement to a regular push button, in one TOY keyboard. That works to play a piano note, or to cause a canned song to play.
Interesting part of that is the solar light, as a 'gate' does work in EITHER connection config...That is, if we assume that toy does key scanning by pulling down a 'row', while monitoring a set of 8 columns in typical style. One direction of connecting not super good, but reversed worked quite well. I'm a bit concerned, about possibly supplying excess voltage, in that substitution, as solar cell unit, (don't forget, output is 222 khz), can put out actual voltage, vs a passive dome push-button, but seems to work, by way of small flashlight.
Kind of weird, that it works in both directions of hook-up.
I need some more time, for more results, as I need to construct (the optical edge detect, using several gates)
that being on the test bench. Plus, I need to get a periodic action, for best oscilloscope capture, of those optical rise and fall times. That, to characterize for actual switching time, as evb149 has been describing, the large delay seen in photo-voltaic output, in one direction of switching. Thanks.