Help me avoid using a 3pdt toggle here

[patrick398]

Greetings Comrades,

I've run into a bit of a problem with cap switching and want to avoid using a 3PDT toggle as a solution (which i currently am).



On the schematic and PCB layout i'm using a DPDT toggle to select between either a 100nf or 1uf between pins 9 and 10 on pt2399. Really it needs to be about 900nf to avoid runaway oscillations but i'm still tweaking that.

The problem is that when the 1uf (or 900nf) is selected the signal is considerably hotter so to solve this i built the proto pedal with a 3PDT toggle and used the outer lugs to split the output to the bypass switch. When in the 100n position it goes through to bypass as normal. When in the 1uf position, the output goes through a 220k resistor before the bypass switch. The levels are fairly well matched doing this but i really really want to avoid using a 3PDT toggle.

Can anybody think of anyway of having equal outputs using a DPDT? Also i think i need a 1M between the switch contacts, it kind of freaks momentarily in between contact positions.

Hope that makes sense

Thanks!

[todd.dukes]

Does it freak out if you do something like  this? How much does the "hanging" capacitor affect the one that is in the circuit?

[GGBB]

You only need one pole for the cap switching, and the other pole can be used for the resistor:



Upper position: resistor is jumpered and cap is 100n, lower position: resistor in series cap is 1u.

An alternative if you don't like hanging caps is to wire two capacitors in series and jumper one via the switch:



Upper position: resistor is jumpered and caps in series for ~107n, lower position: resistor in series 120n cap jumpered for 1u.

[todd.dukes]

I like the series solution without leaving the cap hanging.

[ElectricDruid]

You could do a similar thing with the caps in parallel. Have one cap "always on", and then use the switch to bring the other one in to give you a larger value. Since you ideally need 900n, a 100nF and an 820nF would get you pretty close. But that does give you a hanging cap, although I don't see any problem with that. What worries me more is there being nothing at all connected while the switch switches, and both of these solutions fix that.

[patrick398]

I like the idea of doing caps in parallel but my god, for the life of me i just can't figure it out. Switching really isn't my strong point and i just spent an hour trying to hash it out to no avail 
I put together Gord's suggestion which i think i have implemented correctly...

[duck_arse]

move your 100nF to the left of the switch common, and it is then connecting to "CP1_IN" and "CP1_OUT" at all times, hence always on. then your *whatever* cap is also correct, being switched by the switch and adding to the 100nF.

switching problems are easy.

[edit :] bang for one.

[patrick398]

Ok i think i've got it...

[GGBB]

Not quite - move the 910n back to the bottom switch position so that it's in parallel with the 100n for total 1010n while the resistor is un-jumpered. If you want that ~900n value replace the 910n with 820n as Tom said earlier - that will give you 920n when in parallel with the 100n. (And if you have an assortment of 820n caps, you could measure and probably find one that's only 800n to get to 900n total.)

[patrick398]

Jeez it seems so simple when somebody else says it haha. I think i've got it now (fingers crossed)



Thanks you so much for your helps guys!

switching problems are easy.

I got 99 problems and a switch is most of them...i'll see myself out.

[GGBB]

There is a way to have the best of both worlds - parallel cap switching and no hanging cap ends. If you flip the lower switch around - 820n cap to the common lug and 100n cap to the lower position "on" - you can use the upper position "off" to tie the hanging end of the 820n cap back to its other end so that it is jumpered but only connected at one end:

[PRR]

I don't understand "hanging cap ends". I assume, since you use it sometimes, you have *mechanical* support for the stray end. If you don't need it right now, leave the end electrically "hanging".

Look at the wiring in your house. If you don't plug something into a wall-socket, it "hangs". If you plug in a lamp but switched off, the power "hangs" at the switch contact.

[anotherjim]

I suppose the worry is popping noise, which in a delay with regen would go on a while and possibly kickstart runaway.

[patrick398]

Yeah there is a slight pop when i change toggle posistions, that's using the old 3pdt wiring though, i haven't tried with this new dpdt wiring but i imagine it'll be the same. Is there not a way to use a resistor to stop this?

[ElectricDruid]

Don't imagine, try it. This is engineering.

I would suspect that these new schemes would be better because the chip is not left with nothing connected during switching. There's always a cap present, so that's much better. But I haven't tried it either, so until one of us does, we're both just guessing

[patrick398]

Don't imagine, try it. This is engineering.

I would suspect that these new schemes would be better because the chip is not left with nothing connected during switching. There's always a cap present, so that's much better. But I haven't tried it either, so until one of us does, we're both just guessing

Yeah good point, i'll try it tomorrow

[duck_arse]

there is one more thing - your drawing shows a centre off switch.

[patrick398]

there is one more thing - your drawing shows a centre off switch.

Thanks Duck, i didn't even think of that, just assumed it was an 'either, or'  scenario but obviously arrows in the middle indicates centre off, makes sense. I've updated the schem now, thanks again.

Good news is that the switching works pretty well, there's still a slight glitch when you flick the toggle but i guess that's just the pt2399 reacting to a sudden change in capacitance between pins 9 and 10?

[patrick398]

Any obvious reason why this switching method would cause bypass switch pop? Not sure if it's conventional switch pop or something to do with having two paths to the output via the switch