So, who won the mistery circuit? and what was it?
Luftek, you made the best analysis of it, so the circuit goes to you. You only had a bit left wich would have given away the purpose for it: “Q1”
That transistor sits in parallel with 555’s main capacitor…so…shorts it out whenever it’s told to. And if you short the 555’s capacitor, what happens? Yep, it starts charging again, efectively retriggering it.
So, what does the circuit actually do?
The circuit is an EDGE detector plus retriggerable 555, so, it can detect the change in state (not the state itself) of an input, and, given enough frequency (0,1 Hz or less) it mantains the output ON. If the water sensor (or other) fails to give life signals for some seconds, it will shut down the whole laser machine. (provided you wire it that way, of course)
Edge detection circuit:
Internet says that just a NOT + AND, will work, however the 555 required an inverted pulse for it’s activation, hence, the NAND. R3 as pulldown so input doesn’t float, and C3, because otherwise, the propagation delay from U2 as so short, the circuit didn’t work. (yep, 20uF is a LOT, I know)
But, why bother to make the edge detection rather than just retrigger the 555 from the sensor? Safety (heh, XD) reasons. Imagine the worst case scenario:
Water pump stops and the simple flow sensor also stops in the ON position, you have no way to know that the pump has stopped if you retriger by level (+5V, for example). The edge detector waits for the rising level shift, so, pump stopped = laser shutdown.
I hope the winner has some use for the circuit, and that someone can use the edge detection info.