StompLFO CV send and return?

[patrick398]

Since I've been using the stompLFO a lot recently I though it would be really cool to 'clock' different pedals from a single stomplfo. I'm thinking of trying this using 1/8 inch sockets and patch cables, like modular stuff uses. If each pedal that uses a stomplfo had a CV send and return jack I think this would work but I can't get my head around the switching mechanism that the sockets need. Anybody have a schematic for a circuit that does something similar? So plugging a cable into the CV send would take the frequency voltage from one pedal and plugging the other end of that cable into a CV in, which would need to disconnect the frequency voltage of the second pedal. I hope that makes sense, I'm kind of thinking out loud for my own sake

[Ben N]

I don't know the answer, but I eagerly anticipate hearing from someone who does--although I suspect the answer may not be one-size-fits-all. Unlike modular stuff, pedals are generally not designed around standards in terms of control, so, unless designing circuits from the ground up, retrofitting this kind of thing may be a bit of a kludge. I imagine Tom W will chime in in due time.

[patrick398]

I should clarify, I'm only thinking about implementing this on pedals I'm building, all of which use the stomplfo, so thankfully I won't  have to worry about retrofitting it to other pedals, it's more a case of clocking one or more stomplfo chips off a 'master' stomplfo

[ElectricDruid]

If you just want to sync various pedals together, this is a simple way:



Rather than wiring the tap tempo switch to ground in the usual position from the pin, you can wire it to +5V at the Sync input. Thus when you press it, 5V goes to the transistor, turns it on, switches the tap tempo input low as required.

The nice bit is that if you connect another/other pedals to that sync input, pressing the tap tempo button on any of them (assuming they're all wired like this) will tap the tempo on all of them.

There are probably lots of other ways.

[patrick398]

Thanks for the reply Tom, that's an interesting suggestion. So it's essentially clocking all the chips off a single tap tempo input..I guess that means it can only be triggered by the tap footswitch rather than manually setting the frequency with the pot.
How about something like this? I think it makes sense and I don't see why it wouldn't work. Hopefully my crude drawing is good enough



So voltage at pin 7 of one stomplfo goes to a 1/8 socket (sync out), you plug a cable in and that voltage is sent to the sync in of another pedal. When you plug into the sync in the socket disconnects pin 7 of the second stomplfo from its frequency pot and is given the voltage from the first chip. I hope that's not complete jibberish

[patrick398]

The more I think about it the less useful I feel like this is going to be. What I think is more important is just having a sync in, which the chip already supports. So my question now Tom is what can you clock the pedal off? If I wanted to sync in with a drum machine or synth can I use the CV output of one of those? Just say if this is all in the documentation, it's been a while since I read it

[ElectricDruid]

Not a CV output, but you should be ok with any sort of pulse/clock output. The only thing you need to think about is what the output rate of the pulses might be. The StompLFO will respond at 1 waveform per clock period. Squencers/drum machines might output 24PPQN or 16PPQN, so you'd have to see if that's what you want or if you need a divider in between.

[patrick398]

Ok great. And if I wanted to use CV could I just use a sync in socket which disconnected pin 7 from its potentiometer so pin 7 saw the voltage coming in instead? Sorry for all the questions

[ElectricDruid]

Ok great. And if I wanted to use CV could I just use a sync in socket which disconnected pin 7 from its potentiometer so pin 7 saw the voltage coming in instead? Sorry for all the questions

Yes. If you want a socket that you can use as either an expression pedal input (expression pedal instead of pot) or a CV input (0-5V CV input from elsewhere) then there's an example in the FilterFX schematics:



This is shown replacing the "Offset CV" on the StompLFO (so it shifts the filter's centre frequency up and down) but *exactly* the same thing works on any of the StompLFO's CV inputs; Rate, Waveform, Depth, or Offset.

This is is pretty much exactly what you drew in your picture:



The differences are:
1) You need the ground on the sleeve connection.
2) I added R25/2K2 to reduce the chance of someone frying the chip with a too-big CV. The resistor limits the current to something the internal protection diodes can cope with. Hopefully . As a side effect, it smooths the input CV which is good because it's not read *that* fast.
3) I added R24/220R to limit the current that flows if someone inserts a mono jack providing CV into the socket. In that situation, the sleeve of the jack shorts the +5V ring connection to ground, so the resistor limits the current to 22mA. 220R is small in comparison to the 10K pot, so you don't notice any reduction at the top of the range. 220R is pretty small. 470R or even a bit bigger would probably be fine too.

These are just implementation details to make it safer/more robust. Your idea is spot-on.

[patrick398]

Thanks so much for all the great info Tom, really looking forward to trying this out! Syncing some of these pedals with synths and drum machines is going to be nuts! Can't wait