As drawn, R1 goes to ground, then there’s the 10n capacitor, then R2 in series with a diode to ground, and R3 to gate and on to R4 and a second diode.
R2 and a diode seems a bit weird — surely you want to get rid of the negative edges quickly? — so I thought the cap was drawn wrong and it should go to the second strip instead. That’ll give you a diode to ground which makes sense, but then you have R2 and R3 in series, which is weird,
And then I looked at the fartbox, which is supposedly the same circuit, but to get that circuit you have to move the capacitor and replace R3 with a wire… (i.e. diode to ground, 47k to gate). So two steps from weird to not weird? Weird.
Actually, after some thinking, it seems that it only works because of the imperfect opamp input impedance (R5 on my schematic): that’s the only way to sink the current from the cap!
If it doesn’t work in real life, try this circuit instead:
I think you missed the 100k to ground on the + input (forgot to mark that path in my annotation), otherwise your quickly drawn version looks good
With that in place, the discharge path would be R3+R4+100k = 300k for a time constant of 3 ms. I was more puzzled by the 47k, which felt a bit pointless, and it’s in a different location in the fartbox schematics:
If you change the cap location you get the 47k in the same place, but you then end up with three resistors in a series – get rid of one of them, add a 1M pullup, and you have the fartbox schematics.
But as your last example shows, as long as the overall structure is roughly right, you can shuffle things around and get a similar trigger-to-gate mechanism, so maybe I’m just being overly pedantic here
I guess that R37 is useful when you have negative voltage at GATEIN (to avoid short-circuits), but I don’t get the utility of R46?
By the way, the t=RC equation only applies when you’re comparing the cap voltage to 63% of the input voltage (t=3*RC for 95% and t=5*RC for 99%). And here, it’s compared to 18% of 12V, but as we are discharging the cap we can say it’s 100%-18%=82%, which is between RC and 2*RC. Here it doesn’t matter at all, but I often fall into that trap, so I thought it’s worth sharing it! (more info here: https://en.wikipedia.org/wiki/Time_constant#Time_constants_in_electrical_circuits )
Except that it’s used for trigger to gate here, not gate to trigger
(But all these circuits are crazy imprecise, since they depend on the trigger voltage, which isn’t exactly standardized. See here for a bit more on that.)
Yeah, it keeps the voltage away from the upper rail in case someone goes all in and uses +15V signals (which is what this circuit outputs, btw). The LM358 can handle 0 V just fine, but needs a 1.5-2.0 V margin to the positive rail.
