Boss switching circuit problem [solved, sorry to waste everyones time]

[Fancy Lime]

Hi there,

I have a strange little problem. I cannot get the Boss switching system to work on the breadboard. The problem is the flip flop (multivibrator) circuit. It switches sometimes but mostly not. Works maybe 1 out of 5 tries. I used this schematic:

https://www.electrosmash.com/images/tech/ce-2/boss-ce-2-jfet-switching.png

Replaced the transistors with 2N3904's and the 1S1588 diode with a 1N4148, rest is as in the schematic.

It may just be some stupid build error, although I already rebuilt it completely to exclude that with identical results. Any reason why this would not work with 2N3904 NPN's? Just thought Id check before trying all sorts of stupid things.

Thanks,
Andy

[Fancy Lime]

Never mind, got it working with 2N5088's. Seems to be more stable with higher gain transistors.

[Rob Strand]

FWIW, that's the old boss switching ckt.  They made some changes in later versions see for example the BF2.  They ditched the diode in the emitter circuit, changed the input caps to 220pF's, and the 1k is changed to a 100R.  I suspect they did that for a reason.

[Fancy Lime]

Hi Rob,

thanks for the info. Interesting. I was aware of the "diode" and "no diode" versions but had simply assumed that the version with the diode was newer without checking. I did try the version without the diode in the meantime and can tell no difference. I would guess the actual difference in function is minute but saving a part is always good when it does not change the function.

Andy

[Rob Strand]

The smaller input caps may make a difference.  Also I've see that 1k replaced with 100R in a few places I think it's probably a good idea as well.  I could be in the input pulse is not rapid enough to kick the circuit with the 1k.

I have looked at variations of this circuit a few times over the years and you can get it to misbehave.  I'd have to go over it all again to tell you any more.

EDIT:
I had a quick look at it.  I know I found some quirks with that circuit in the past.  I definitely had problems in the past dropping the collector load resistors too much.   Today (on the simulator) it seems to work with anything I throw at it.  I tried supplies at 5V (flat battery) and 9V normal battery,  different switch resistors (100R, 1k, upto 4k), input caps 56p to 1.5n.   I also tried low gains.  It all works - odd!

[Fancy Lime]

...
EDIT:
I had a quick look at it.  I know I found some quirks with that circuit in the past.  I definitely had problems in the past dropping the collector load resistors too much.   Today (on the simulator) it seems to work with anything I throw at it.  I tried supplies at 5V (flat battery) and 9V normal battery,  different switch resistors (100R, 1k, upto 4k), input caps 56p to 1.5n.   I also tried low gains.  It all works - odd!

Actually I would not expect it to make much difference in a simulation, where all parts are idealized to a certain degree. I think the biggest problem that needs fine adjustment of the resistors and caps is the switch actuator itself. There is going to be a bit of bounce, there is going to be a bit of resistance and there is going to be a bit of capacitance in the mechanics, especially after some time of use. A simulation will not take that into account and likely assume the switch action to be infinitely fast, with no parasitic resistance and capacitance. I tried actuating the same circuit by just touching a wire bridge to ground and then by using an actual switch (Omrom). Theoretically the same thing but with the actual switch it works much much more reliably. So I would assume that all these parameters, R, C, gain of the transistors, are not critical with very good mechanical switches but may become important when the mechanics deteriorate over time. The only Boss pedal I own (NS-2) has a switch in it that looks like a mechanical keyboard switch. More precisely it looks like a Cherry MX knockoff. I am still trying to find an enclosure that will fit a Cherry MX switch without awkward workarounds.

Andy

[Rob Strand]

Actually I would not expect it to make much difference in a simulation, where all parts are idealized to a certain degree. I think the biggest problem that needs fine adjustment of the resistors and caps is the switch actuator itself. There is going to be a bit of bounce, there is going to be a bit of resistance and there is going to be a bit of capacitance in the mechanics, especially after some time of use. A simulation will not take that into account and likely assume the switch action to be infinitely fast, with no parasitic resistance and capacitance. I tried actuating the same circuit by just touching a wire bridge to ground and then by using an actual switch (Omrom). Theoretically the same thing but with the actual switch it works much much more reliably.

Maybe you were switching too quickly?   The Boss circuit works by discharging the first cap through the switch.  Because the pull-up resistor is so large the cap voltage doesn't rise quickly so it locks out switch bounce.    A 100R input resistor vs a 1K might help.    I can actually simulate fast switch activations.   There has to be a point where a fast pulse it won't trigger the flip-flop.  The 100R will definitely have an impact since it limits how quickly the cap to ground can discharge.

Interestingly the Ibanez "Lady Bug" enclosure pedals from the early 90's had a quite a small cap across the switch.   When the switch wearz out the pedal becomes unreliable.  By increasing the cap across the switch you can make it work again.  This a case of switch bounce and the input cap not locking it out long enough,   which is different to fast presses.

I'll see what happens with fast pulses.

BTW I once worked on a project where a microswitch (like the omrons) was activated so fast by a machine it was in the order of the switch bounce time.   The solution was to use the cap discharge method, quite similar to the Boss idea.

EDIT 1: OK I tried fast pulse and it seems to work well beyond what a switch would do.  So maybe your problem was bounce?   

EDIT 2:  That circuit is totally crap at removing bad bounce!   The cap across the switch needs to be about 10 times what it is now to be immune against bad switch bounce.   That lines up with the Ibanez problem.  Those Boss foot switches must be pretty good quality!

[Fancy Lime]

Actually I would not expect it to make much difference in a simulation, where all parts are idealized to a certain degree. I think the biggest problem that needs fine adjustment of the resistors and caps is the switch actuator itself. There is going to be a bit of bounce, there is going to be a bit of resistance and there is going to be a bit of capacitance in the mechanics, especially after some time of use. A simulation will not take that into account and likely assume the switch action to be infinitely fast, with no parasitic resistance and capacitance. I tried actuating the same circuit by just touching a wire bridge to ground and then by using an actual switch (Omrom). Theoretically the same thing but with the actual switch it works much much more reliably.

Maybe you were switching too quickly?   The Boss circuit works by discharging the first cap through the switch.  Because the pull-up resistor is so large the cap voltage doesn't rise quickly so it locks out switch bounce.    A 100R input resistor vs a 1K might help.    I can actually simulate fast switch activations.   There has to be a point where a fast pulse it won't trigger the flip-flop.  The 100R will definitely have an impact since it limits how quickly the cap to ground can discharge.

Interestingly the Ibanez "Lady Bug" enclosure pedals from the early 90's had a quite a small cap across the switch.   When the switch wear out the pedal becomes unreliable.  By increasing the cap across the switch you can make it work again.  This a case of switch bounce and the input cap not locking it out long enough.   Which is different to fast presses.

I'll see what happens with fast pulses.

BTW I once worked on a project where a microswitch (like the omrons) was activated so fast by a machine it was in the order of the switch bounce time.   The solution was to use the cap discharge method, quite similar to the Boss idea.

EDIT 1: OK I tried fast pulse and it seems to work well beyond what a switch would do.  So maybe your problem was bounce?   

EDIT 2:  That circuit is totally crap at removing bad bounce!   The cap across the switch need to be about 10 times what it is now to be immune against bad switch bounce.   That lines up with the Ibanez problem.  Those Boss foot switches must be pretty good quality!

Ah, yes, the "cap across the switch" trick. Should have tried that. Has helped me in the past. To the quality of the Boss switches: I think they are indeed mechanical keyboard switches. These are relatively cheap because they are made in enormous quantities compared to more specialized switches, and they are extremely reliable even after millions of activations. I should start thinking about how to use them in a standard enclosure.

ANdy

[Rob Strand]

To the quality of the Boss switches: I think they are indeed mechanical keyboard switches. These are relatively cheap because they are made in enormous quantities compared to more specialized switches, and they are extremely reliable even after millions of activations. I should start thinking about how to use them in a standard enclosure.

Hey cool, so they are.  Back in the days when keyboards had really good quality switches.

http://telcontar.net/KBK/SwitchCollection/switch.php?id=238

(click the Series link for more options)

[Fancy Lime]

To the quality of the Boss switches: I think they are indeed mechanical keyboard switches. These are relatively cheap because they are made in enormous quantities compared to more specialized switches, and they are extremely reliable even after millions of activations. I should start thinking about how to use them in a standard enclosure.

Hey cool, so they are.  Back in the days when keyboards had really good quality switches.

http://telcontar.net/KBK/SwitchCollection/switch.php?id=238

(click the Series link for more options)

Thanks for digging that up, Rob! "SKM J-M0404, tall stem, upright" seems to be the rather catchy name of the switch in my Boss pedal.
https://deskthority.net/wiki/SMK_J-M0404_series

BTW: "Back in the days when keyboards had really good quality switches."? While it is true that the average keyboard today is of abysmal quality, switch wise at least. But super high quality mechanical keyboard diy is very much a thing these days and it is quite possibly even more crazy than what we do here. People tune their switches with very specific springs, argue about what lubricants to use for the switches (which work fine without lubrication but can be made a smidge smoother with), fight about the right material for keycaps and write their own keyboard drivers. It's a lot of fun, is what I'm saying.

Andy

[Rob Strand]

People tune their switches with very specific springs, argue about what lubricants to use for the switches (which work fine without lubrication but can be made a smidge smoother with), fight about the right material for keycaps and write their own keyboard drivers. It's a lot of fun, is what I'm saying.

It can be fun.  I've done some mechanical tweaking in my time.   It's made me appreciate what it must have taken to get those old mechanical printers and mechanical calculators to work reliably.   IMHO the old switches (and jacks) are much more reliable than the new ones.   I've actually had switches loose contact because the lubricant spread over the contacts and went hard over time!   Good stuff is around but it's expensive and hard to find.