I've got an old circuit design I need to replicate, function wise, to fit into an existing larger design. Logic I can usually handle, anything with transistors starts to get me lost. My strength is in DFM and manufacturing, not necessarily electrical design. The original design functions as a simple on-off switch on a TTL logic circuit, with a separate IR LED source. Top circuit is the original design, bottom is my proposed alternate.
D1 is a photodarlington, Motorola MRD370, long obsolete and difficult to find at a decent price. Due to cost and availability I'd like to use a phototransistor instead of a photodarlington, but am concerned about enough gain. Hence the 2nd schematic with an additional 2N3904. Resistance value I haven't even tried to calculate yet. The original circuit is a TTL logic circuit, just can't figure out how to show that in the schematic. I have enough activity to post links now, so here's the original schematic I'm working from. Identical circuit is duplicated a few more times on the board, each for an independent photodarlington.
Would my proposed circuit be completely off the mark here? Photosensors are totally outside my practical experience. Switching time really isn't a big concern here, as the circuit is detecting the presence/absence of a stationary object. I added a 2nd proposed circuit, based on suggestions received. Left the first one to hopefully avoid confusion with changes.