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I am a hobbyist working on a project that requires a linear ramp generator (0VDC to 5VDC @ 120Hz) that is triggered by an external pulse. The requirement for the external pulse is so that the ramp waveform lines up with the zero crossing of the 60Hz AC power that I am attempting to "dim" with triacs (so it can potentially be used with inductive loads). The output of the ramp generator (Vo) will go to a comparator that will also look at the voltage of a voltage divider controlled by a potentiometer to fire the triacs for the appropriate amount of time during each half cycle of the 60Hz AC. The schematic below shows what I have come up with so far. I am considering referencing Q1 and Q2 to -5VDC instead of ground and using a diode clamp from GND to Vo in order to keep the output waveform in the more linear region of the RC discharge curve. I have narrowed the width of the pulse trigger at the base of Q1 to 420usec (5% of 1/120Hz), which would be acceptable, but would like to see it narrowed to 0.1% of the period if possible (8.33usec). Possibly some sort of high pass filter? Trying to narrow it more by further increasing the value of R3 causes circuit instability. Q1 and Q2 are currently both 2N4401TF (small signal transistor) devices.

Am I on the right track here, or is there a much simpler and cheaper way of going about what I am trying to do?

Circuit Schematic

user2121874
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  • Just FYI. I've written up something that may be worth a moment's time [here](https://electronics.stackexchange.com/a/433890/38098). I'll look over what you wrote in more detail, tomorrow. Just wanted to put the above out there in case it helps between-times. – jonk Nov 06 '19 at 07:15
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    Just another thought. Have you looked at the SFH6206? – jonk Nov 06 '19 at 07:33
  • Just glancing at the datasheet, it looks like the SFH6206 could replace both the DF04M and the 4N25M in my circuit. Was that the intention of bringing it to my attention? – user2121874 Nov 06 '19 at 07:40
  • Thanks for pointing me at your answer to your other question by the way. It does not look suitable however. I need to trigger the rising edge of the saw-tooth from a pulse input rather than a square wave. – user2121874 Nov 06 '19 at 07:43
  • Yes, I wanted you to look over that part for just those reasons. It might help and it was just a passing thought. – jonk Nov 06 '19 at 08:02
  • As far as the other circuit is involved, fewer parts would allow it to use pulses. Can you see why? I actually added parts to handle the square wave. – jonk Nov 06 '19 at 08:04
  • It looks like the SFH6206 costs about the same as the combination of the DF04M and 4N25M combined. I would save about $3 if I bought 100 of them. It would save a little bit of board space though as well. However the SFH6206 doesn't have access to the transistor base, so you cannot input signals into it to say ground out the base as a default safe start-up condition. Having access to the base is worth the board space for me. – user2121874 Nov 06 '19 at 08:09
  • What about using two 4n25, collectors tied to a shared pull-up? – jonk Nov 06 '19 at 08:11
  • Regarding the other circuit: I am a hobbyist, not a trained EE. From what I can discern it looks like I could throw away C2, R2, D2, and the NOT gate (That much is obvious). During the pulse, Vout would get grounded immediately with Cint rapidly discharging, then start increasing on an RC charging curve via R4 and Cint. I have no clue what Q2, D3, and D4 are supposed to accomplish though. Also, it looks like you are relying on having a voltage rail 3x larger than your logic levels, is that correct? – user2121874 Nov 06 '19 at 08:33
  • Let us [continue this discussion in chat](https://chat.stackexchange.com/rooms/100736/discussion-between-user2121874-and-jonk). – user2121874 Nov 06 '19 at 08:36
  • I'm just a hobbyist, too. No training. I don't have time right now to go into it. But I may have a moment in a half day or so. I have a better idea where you are going than before, but I haven't carefully considered the possible differences between what you posted here and what I was thinking. – jonk Nov 06 '19 at 08:37

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