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How can I build a latching circuit without a Silicon Controlled Rectifier (SCR)? Answered

I was wondering if there was a way to build a latching circuit without using an SCR because, after all, you can't get them at your neighborhood Shack, and I was wondering if you could use a couple of transistors or something similar.  Thanks in advance


KMH's idea is pretty good, but if you want solid state, here's how you do it with transistors.

SCR transistors.jpg

That's really elegant, Steve.  Could I trouble you to outline the theory of operation?  It's been way, way too long since my electronics courses, and I don't ever do circuit design as part of my work (we have real EEs for that). 

It looks to me as though the base-collector loop is doing the equivalent of what I described with the relay.  But I'm having trouble with the connection from the PNP base over to the NPN collector.  It seems like the current is going the wrong way.  Or does the PNP C-output pull "open" the NPN base, which causes a voltage change (as opposed to a current) on the NPN collector, which in turn holds the PNP base open?

I'm fairly sure that what I just wrote is nonsense, so I'd appreciate an expert helping me to get it right.

Inject current into base of NPN transistor:
current in collector rises, extracting current from base of PNP transistor:
Current in PNP collector rises, injecting more current into NPN base, increasing current in collector:
Runaway: latch-up.

Thanks, Steve!  So I wasn't talking complete nonsense after all, but I did have some wrong terminology, and I clearly didn't have the best picture in my head.

Do you have to use solid-state components?  A simple NO relay can be latched fairly easily, by looping the closed output pin back to the input to hold it closed.  You have to cut power to release the latch.