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Going back to the 22MEG photo diode circuit if i may....

...Thanks it all makes sense now.

As you discuss kindly, the circuit "works" when there is a *change* of light level on the photo diode.

The actual useage of this circuit is in a home for the blind, where the blind

traverse the corridors by sliding their hand along the hand rails on the walls.....if 2 blind people are there they can knock in to

each other. As such, these photo diode circuits are used in light block cct, where a human passing will block the led

light onto the photo detector, and then a pulse will be sent and an audible alarm goes off, warning the blind

walkers that another person is coming. I have actually now  been terminated from this job as they suggested that I had posted to a forum!. This amazes me because

I have seen this circuit now literally all over the internet , so why they think its their personal secret is

beyond me. And besides, if another home for the blind benefits , then good-o, since these people

need all the help they can get. It most  probably wows all that a common emitter BJT circuit is now considered a "top secret" design!