Hi! I'm new to this forum, so I hope this is in the right section of the forum.
Oh, and I'm Spanish, so I'm sorry if my English is a little bit fuzzy.
So I've been doing electronics for a few years, so I know all the basic stuff, but eventually I find myself thinking about something that looks simple but I don't completely get it. This particular example I've never fully understood how it worked, even when I understood it partially (Ignore the diode, it's a feedback element whose importance is not relevant to my doubt):
I saw this circuit somewhere used to trigger a 555 timer, and as we all know, it needs a negative input as a signal. The 555 is meant to trigger when the LDR(a potentiometer or a digital signal could be used) raises over certain level. I fully understand how the first part works:
- Before the LDR raises its value, the voltage divider (adjustable by the potentiometer) blocks the transistor, so it doesn't let any current flow from the capacitor to ground, so the capacitor is fully charged, so V(TRG) is 12V.
- When the potentiometer raises its value, the transistor allows current to flow from the capacitor, so it drops its value to (almost) 0, and it needs some time to charge the TRG side to 5, effectively getting a negative pulse there.
But I still have some doubts, like: Does the capacitor discharge through the NEGATIVE terminal? Whenever I see electrolitic capacitors, the negative terminal is usually directed to ground and it's the positive terminal the one that varies it's state, so the positive terminal charges the capacitor, and then it gets discharged, but in this case I can't see any logical current line going from the positive terminal of the capacitor to GND.
So that's why (paradoxically enough) I understand how it works, but still don't understand how it works. I've tried to google this but I never came into any good explanation of this method of creating a negative pulse. (How would this be called? inverse latch? Because it makes the opposite of a latch, it makes a pulse out of a "constant" change in value).
Thanks a lot for your help