I have a pedal which accepts a tap tempo switch which works (mostly) similarly to many others:
- the tap-input socket on the pedal provides a voltage (3.3v) which is then shorted to ground by two brief presses of a normally-open momentary switch. The length of time between the first and second 'taps' on the switch tells the pedal what tempo to set.
Such pedals usually take the timing information from the two 'closings' of the momentary switch - ie when you press your foot DOWN on the switch for the first and second times, shorting the control voltage to ground.
However, this particular pedal takes its timing information from the two 'openings' of the switch - ie when you lift your foot OFF the switch, breaking the short from the control voltage to ground.
This is most inconvenient, as unless you make the taps of your foot on the pedal very consistent and short, the timing comes out wrong.
So my idea is to add a small circuit to the tap switch, powered by the 3.3v coming from the pedal, which, as soon as it detects the voltage being shorted by the pressing of the switch, very rapidly shuts off the current flow, allowing only a very short pulse, as if you had lifted your foot from the switch extremely quickly.
This seems to my electronically untutored brain as though it should be pretty simple to implement (a transistor acting as a switch to shut off the current...?) but sadly my electronics knowledge is far too feeble to know how.
So - any advice on how to do this would be gratefully received!
PS My problem is not caused by the pedal in question requiring a normally-closed momentary switch rather than a normally-open one, as eg some Boss pedals do. I'm familiar with this distinction, and have experimented with a normally-closed switch. This doesn't work at all.
Thanks in advance, Paul
Hi, can you post a schematic of the actual tap tempo circuit? I think that would help generate ideas, seeing what there is to work with. :icon_lol:
The solution to this problem is to take the firmware programmer outside and give them a good kicking until they promise to use their brain and not do it again. :icon_evil:
Tap tempo on switch *release*?!? Jeez.... :icon_eek:
I'm far from any kind of expert, but can't you get a 555 timer to do this?
Quote from: ThePracticalPeasant on February 16, 2025, 07:41:19 PMI'm far from any kind of expert, but can't you get a 555 timer to do this?
That's on the line of what I was thinking. Or some way to invert the operation of the tap!
Quote from: ElectricDruid on February 16, 2025, 06:30:33 PMThe solution to this problem is to take the firmware programmer outside and give them a good kicking until they promise to use their brain and not do it again. :icon_evil:
Tap tempo on switch *release*?!? Jeez.... :icon_eek:
This seems a sound suggestion; a more benign and relaxed attitude towards the annoying pedal would surely result.
I name Digitech as the guilty party. Fetch the pitchforks!
I would use a CMOS hex inverter and a transistor. The CMOS would square up whatever the switch did and provide a single pulse to drive an NPN transistor base or 2N7000 mosfet gate for a short pull-to-ground-and-release.l
Tom, I agree. Kicking is appropriate, flogging may need to be brought back. The right way to do an off-board switch is to make it a momentary contact to ground.
Quote from: GibsonGM on February 16, 2025, 05:42:23 PMHi, can you post a schematic of the actual tap tempo circuit? I think that would help generate ideas, seeing what there is to work with. :icon_lol:
Sadly I have no schematics, and tracing or any modding would likely be very tricky as it's all made out out of Digitech's finest teeny tiny components...
So really all there is to work with is what I've already described - a tap input jack with 3.3v on the tip and 0v on the sleeve. And a normally-open momentary SPST footswitch that gives the pedal timing information... when you take your foot OFF it...
So yes, walk of shame for Digitech's programmers, but I'd still like to find a solution!
Very grateful for the pointers so far (as well as the general agreement that it's an infuriatingly foolish arrangement) but I'm afraid my electronics knowledge is so scanty that I will need fairly detailed guidance, if possible. :icon_redface:
Clearly (and unsurprisingly) the solution is not quite as simple as I had initially hoped!
R.G.'s suggestion of a CMOS hex inverter and an NPN or mosfet transistor seems the most fleshed-out suggestion so far.
Anyone willing and able to elaborate on the details?
Would such an arrangement be able to be powered from the 3.3v coming from the pedal's tap input, and so not require an additional power supply to the tap switch?
I've been trying stuff like this, but I haven't got it yet:
https://tinyurl.com/27foc5bz
The idea here was the cap would charge through the 10K, then when you close the switch, it discharges through the 100R, turning the transistor on briefly. The 10K on the collector represents the internal pull-up on the tap tempo input.
But it doesn't work because when the switch is closed, the transistor base is connected to Vdd by the 100K. You could use a SPDT to get around that, but it seems like it should be possible with a SPST.
It's basically a very simple monostable we need - you press the trigger, it fires a short pulse, turning a transistor on which shorts the input to ground briefly. The logic won't be inverted exactly, but if the pulse is a fixed length and very short (few msecs) then the problem goes away.
Edit: This might work:
https://tinyurl.com/22ew7omn
Like this, the base voltage is too low to switch the transistor on as soon as the cap discharges, so you get a brief drop in the collector voltage, but only very brief. You then have to wait a little while (50msecs) before the cap recharges and you can tap again.
It probably needs the values juggling, but I think it's getting nearer!
Have you tried the old DPDT-as-a-momentary-switch trick? Using a *latching* footswitch, when you press the switch there is a moment where the wiper has not made contact yet, so it is like opening a switch and closing it very quickly. Admittedly, the click is not such a nice 'feeling' on your foot as a soft momentary switch.
(https://i.postimg.cc/8sx7BhGN/107.jpg) (https://postimg.cc/8sx7BhGN)
Quote from: ElectricDruid on February 17, 2025, 05:46:07 AMI've been trying stuff like this, but I haven't got it yet:
https://tinyurl.com/27foc5bz
The idea here was the cap would charge through the 10K, then when you close the switch, it discharges through the 100R, turning the transistor on briefly. The 10K on the collector represents the internal pull-up on the tap tempo input.
But it doesn't work because when the switch is closed, the transistor base is connected to Vdd by the 100K. You could use a SPDT to get around that, but it seems like it should be possible with a SPST.
It's basically a very simple monostable we need - you press the trigger, it fires a short pulse, turning a transistor on which shorts the input to ground briefly. The logic won't be inverted exactly, but if the pulse is a fixed length and very short (few msecs) then the problem goes away.
Edit: This might work:
https://tinyurl.com/22ew7omn
Like this, the base voltage is too low to switch the transistor on as soon as the cap discharges, so you get a brief drop in the collector voltage, but only very brief. You then have to wait a little while (50msecs) before the cap recharges and you can tap again.
It probably needs the values juggling, but I think it's getting nearer!
Thanks very much for your reply and work on this!
I think I could have a go at actually implementing this to see if it'll fly.
Couple of questions though:
1) If I'm understanding you correctly, inside the Digitech pedal I would need to identify the point you've marked as the 10K resistor representing the internal pull-up resistor on the tap input, and make a connection to the 'inward' side of it to connect to the new 100K resistor. Have I understood this right? This could be tricky, but I could give it a go!
This has also made me rethink the way I should try to put this into practice:
My original aim was to modify a specific external tap-switch pedal for dedicated use with the Digitech pedal.
But because of the need for an extra connection to the far side of the pullup resistor (if I've got it right!), this idea would be far easier to implement inside the Digitech pedal, at the tap-input socket.
This has made me realise that (obviously!) modifying the tap-input of the Digitech pedal itself so that it works properly with any standard tap-switch would be a much better solution anyway, so I'd like to try to do that.
2) Due to my ignorance, I would need some guidance as to what kind of transistor is needed here. Would this be a standard NPN BJT like 2N3904 or similar?
Quote from: merlinb on February 17, 2025, 09:16:15 AMHave you tried the old DPDT-as-a-momentary-switch trick? Using a *latching* footswitch, when you press the switch there is a moment where the wiper has not made contact yet, so it is like opening a switch and closing it very quickly. Admittedly, the click is not such a nice 'feeling' on your foot as a soft momentary switch.
(https://i.postimg.cc/8sx7BhGN/107.jpg) (https://postimg.cc/8sx7BhGN)
I had not, but have just tried it - didn't work for me.
I guess this would be causing the DPDT to act like a briefly-pulsing normally-closed momentary SPST, whereas what I need is a briefly-pulsing normally-open momentary SPST. So close!
Cunning idea though! :)
Quote from: paulbuzz on February 17, 2025, 11:11:07 AMI had not, but have just tried it - didn't work for me.
I guess this would be causing the DPDT to act like a briefly-pulsing normally-closed momentary SPST, whereas what I need is a briefly-pulsing normally-open momentary SPST. So close!
Cunning idea though! :)
If this scheme works for what you want, just use the briefly-pulsing normally closed switch to shunt a pullup resistor to ground, and tie the pulled-up point to the gate of an N-channel MOSFET. The MOSFET gate goes up briefly, and this turns on the MOSFET briefly, and the MOSFET drain goes down to the MOSFET source voltage briefly.
Does that fix the issue?
Quote from: R.G. on February 17, 2025, 07:59:52 PMQuote from: paulbuzz on February 17, 2025, 11:11:07 AMI had not, but have just tried it - didn't work for me.
I guess this would be causing the DPDT to act like a briefly-pulsing normally-closed momentary SPST, whereas what I need is a briefly-pulsing normally-open momentary SPST. So close!
Cunning idea though! :)
If this scheme works for what you want, just use the briefly-pulsing normally closed switch to shunt a pullup resistor to ground, and tie the pulled-up point to the gate of an N-channel MOSFET. The MOSFET gate goes up briefly, and this turns on the MOSFET briefly, and the MOSFET drain goes down to the MOSFET source voltage briefly.
Does that fix the issue?
Thanks for the suggestion R.G. - definitely sounds potentially promising!
As Merlin points out though, a DPDT switch is much less satisfactory as a tap-tempo switch than a momentary SPST, because of the 'CLUNK CLUNK' factor.
I'm hopeful that I may be able to get Electric Druid's proposed solution using a momentary SPST to work, though identifying the correct pullup point on the Digitech pedal pcb to attach the 100K resistor may be a challenge/stumbling-block. (This is assuming I have even interpreted his proposal correctly!) I'll get in there and see what I can discover!
So this other approach combining Merlin's and your suggestions of using a DPDT and an N-channel MOSFET definitely sounds worth investigating as well. I think I broadly understand the idea, though I'm rather sketchy on the concept of the pullup resistor - will need to do some Googling to figure out where this would go!
Bed is calling now as tomorrow's work looms, but after that I'll try to draw myself a diagram or two to see whether I can follow this scheme well enough for some practical tests.
Any hints on suitable MOSFETS and pullup resistor values? ;)
As another alternative, I'm also still very interested in your original suggestion of using a CMOS hex inverter and a transistor, but I'm afraid I would need some more detailed guidance to have any chance of getting this together! ;)
Paul
Many years ago, Motorola brought out the MC14490, a hex switch debouncer. It had an internal clock that clocked the input through a 4-bit serial shift register and didn't change the output until all of the bits in the register were the same. The problem with this IC was you couldn't initialize it - the output was in an indeterminate state until the bounce period was over. The clock timing was set by one external capacitor, so you could set any denounce period you wanted.
Other designers have used counters that had "all zero" and "all one" outputs and did the same thing. If you have a number of identical switches with identical debounce specs, this is an idea that has been used before.
Quote from: paulbuzz on February 17, 2025, 10:30:11 AMCouple of questions though:
1) The actual pull-up in the pedal is probably internal to some processor or other, so you can't connect to that specific point. But any 3.3V supply inside the pedal would do, so find some chip you recognize, get the pinout, and find where the 3.3V goes in.
2) Yes, any standard NPN BJT would be fine. 2N3904 or similar is perfect.
What we don't know is what the software does about debouncing. Incidentally, that's one explanation for the "unusual" tap behaviour: My experience is switches bounce a lot less when softy released to "off" than when you firmly stamp them "on". It's very difficult to "unstamp" a switch with the same kind of force you can stamp it with!!
Anyway, debouncing time (if any) might affect the time that the negative pulse needs to be negative. It'll either work or it won't, so it needs to be adjusted until it does.
Quote from: ElectricDruid on February 18, 2025, 03:57:51 PM1) The actual pull-up in the pedal is probably internal to some processor or other, so you can't connect to that specific point. But any 3.3V supply inside the pedal would do, so find some chip you recognize, get the pinout, and find where the 3.3V goes in.
2) Yes, any standard NPN BJT would be fine. 2N3904 or similar is perfect.
What we don't know is what the software does about debouncing. Incidentally, that's one explanation for the "unusual" tap behaviour: My experience is switches bounce a lot less when softy released to "off" than when you firmly stamp them "on". It's very difficult to "unstamp" a switch with the same kind of force you can stamp it with!!
Anyway, debouncing time (if any) might affect the time that the negative pulse needs to be negative. It'll either work or it won't, so it needs to be adjusted until it does.
Fantastic, thanks! I had started to wonder whether that might be the answer to my question 1, so it's great to have that confirmed.
I had also just about managed to come to the right conclusion about my question 2 after spending some time playing with your suggested circuit in the simulator. It took me a while to realise that the whole thing was dynamic and tweakable on the fly... (doh! slaps forehead!) It looks absolutely perfect in the simulation at least, with various elements adjustable if necessary, as you say.
I will dig in with the soldering iron and breadboard and see what happens. I'm feeling uncharacteristically optimistic about the outcome ;D Thanks again for your work on this!
I have now breadboarded the circuit suggested by Druid, and hooked it into the tap tempo circuit of the Digitech pedal.
Frustratingly, after my previous wild optimism, I can't get it to work.
I'm fairly sure my actual implementation is ok, unless I reveal anything foolish below:
- I found a spot for an extra hookup into the 3.3v supply. As Druid suggested, I used the Vcc pin of an identifiable IC: specifically pin 20 of an 74HC574D octal flip-flop. I assume this is suitable - it certainly has 3.3v on it.
- All the rest of it is exactly as per Druid's simulation. I hope I'm right in thinking that the new circuit's output to the pedal's existing tap input should be coming from the collector of the 2N3904 transistor, and ground...?
I think the circuit is operating at least vaguely as it should, as far as I can tell:
- With the tap switch open I get 3.3v between the pedal ground and the 2N3904's collector.
- If I connect my ancient analog meter between these points and press the momentary tap switch, I can see just a tiny flicker of movement in the needle - it twitches downwards and then back up, even if I keep holding the tap switch down. This seems right to me - presumably the needle has too much inertia to keep up with the switching of the voltage by the transistor.
But the tempo of the pedal stubbornly refuses to respond.
I have tried playing with the component values, both in the simulation and in reality - I wondered whether the 'close and open' pulse of the switch was too rapid for the pedal to recognise, so I tried tweaking the values (mainly with a larger capacitor to lengthen the pulse due to the longer recharge time if I drained it by holding the tap switch down)
These tweaks looked in the simulation as though they should behave as I intended, but had no effect on the pedal's lack of response to my wild tapping.
I've learned one or two things about the pedal along the way, but am not sure that any of it is helpful information:
- the pedal's tap tempo input has a threshold at 1.65v (as I suppose you would expect from the 3.3v control voltage).
- This corresponds to putting a resistance of 10k across the pedal's existing tap input.
- If there is a minimum pulse time required for the pedal to identify and respond to the tap pulses, it's quicker than any method of tapping I have tried.
So I'm a bit stumped - any further suggestions, or alternatively recognition that I've screwed up in some very dumb way? ;D
Paul
There is always the old-school option of using monostable multivibrators from the 7400 TTL logic series. The 74121 is a commonly-used monostable, the 74122 is a retriggerable monostable with clear and the 74123 is a dual retriggerable monostable with clear. These devices are easy to use but you may need level shifters to translate TTL to CMOS or whatever logic levels you are using.
Quote from: paulbuzz on February 16, 2025, 07:59:57 PMSo really all there is to work with is what I've already described - a tap input jack with 3.3v on the tip and 0v on the sleeve. And a normally-open momentary SPST footswitch that gives the pedal timing information... when you take your foot OFF it...
Ah. This. Only a tip and sleeve, and 3.3V on the tip, sleeve grounded.
I forehead palmed on re-reading this. There is not 3.3V AND a signal line, only a signal line that reads 3.3V to a meter. That means that there is no power supply available to run any kind of circuit, other than the signal wire. My guess is that there are pullups and/or series resistors and caps inside the unit.
I could only come up with one thing to power a circuit to make the pulses. This would involve using the tip signal feeding a capacitor through a high-ish value resistor so the cap could store enough charge to run the pulsing circuit through the short time of switch operations. That is, a variety of phantom power for the circuit. My first leap is to use CMOS inverters to do the real-life switch sensing, and creating a short positive-going pulse. The pulse would feed the gate of a MOSFET (maybe 2N7000) to pull down and release the tip signal.
This "shorts" the tip signal to ground, also "shorting" the resistor feed to the CMOS timing circuit, but with the right values of storage cap and feed resistor, the pulsing would be over quickly enough to leave charge in the cap to keep the CMOS going.
The down side is that you have to mess with the CMOS to ensure that it doesn't stay latched down at power on, and that pedal pulses happen infrequently enough to leave the power storage cap charged. Well, and it might take a few seconds at power on before the phantom power cap gets charged enough.
Sorry to all if I've failed to be sufficiently clear - it's hard to strike the right balance between comprehensiveness and a massive wall of text!
I've slightly modified my goal since my original post, as I've realised that the best solution would be to modify the tap input of the Digitech pedal inside the pedal itself, to allow it to be used with any standard tap-switch pedal.
This would additionally allow me to make use of the 3.3v supply in the Digitech pedal, without it being shut off every time the tap switch is closed. I've managed to find a suitable place to grab the 3.3v supply in the Digitech pedal, using the advice of Electric Druid.
So here's a picture of where I'm currently up to.
I'm trying to use ElectricDruid's suggested circuit additions. As far as I can tell, I've managed to implement Druid's proposals successfully, but the tempo just doesn't respond to the taps from the tap switch.
Perhaps because the tap pulse is too short?
I've tried playing around with the component values, but haven't managed to make any improvement.
So any suggestions would be most welcome :)
(https://i.postimg.cc/NKKT9000/tap-tempo-mods-for-digitech-looper-v3-clarified.jpg) (https://postimg.cc/NKKT9000)
Yeah, it's possible that they're too short. It's also possible that the amount of drive to the transistor is not suited to keeping the transistor on for the right amount of time or some such, or the rising/falling edge of the collector of the transistor isn't fast enough, something like that.
I would guess that we're chasing the unknown software inside the pedal controller and/or the unknown condition of the in/out pins on the controller.
Do you have space in there for a DIP IC and some Rs and Cs? If so, it's possible to use CMOS inverters to do things more adjustably. I always fall back to this for reasons of sloth - CMOS inverters can be had with Schmitt trigger inputs, so the input thresholds are well known, and always provide some noise immunity.
I simulated circuit for your purpose yesterday. It had the phantom power setup, but that is clearly optional if you have 3.3V from inside the pedal. I used a two-resistor-one-cap debouncing network on the input switch, a Schmitt inverter to clean this up to one and only one pulse, then a half-monostable to give a predictable width pulse, controlled by one R-C time constant. After that, I used two inverters to give the choice of either polarity of output pulse. A resistor to the gate of a 2N7000 takes the place of the bipolar transistor in your schematic.
I don't know for sure this would work, as we're still guessing at what the software would do, but you do get a whole lot of flexibility. The input from the switch is delayed by about 20mS by the R-C debouncer, but then the transition to switch-grounded is clean and sharp and does not bounce. That edge starts a single pulse for a fixed time. That pulse drives the MOSFET to produce normally-high or normally-low action for the switch input of the pedal. I made that pulse be 20mS just for testing, but it can be made shorter or longer.
The penalty for all this adjustability is that it's more complex, a whole IC and some Rs and Cs; it's bigger and maybe harder to stick somewhere inside the pedal. If you're interested, I can post a schematic.
Quote from: R.G. on February 22, 2025, 10:51:31 AMIf you're interested, I can post a schematic.
Yes please, R.G. Sounds like a handy little building block (and learning nugget). Quite apart from Paul's need here, I'd be interested to see what your description above looks like. (I hope I'm not a minority of one!)
Quote from: R.G. on February 22, 2025, 10:51:31 AMI would guess that we're chasing the unknown software inside the pedal controller and/or the unknown condition of the in/out pins on the controller.
This seems like an accurate summary of the task!
Quote from: R.G. on February 22, 2025, 10:51:31 AMDo you have space in there for a DIP IC and some Rs and Cs?
Yes, the inside of the pedal is relatively spacious
Quote from: R.G. on February 22, 2025, 10:51:31 AMIf you're interested, I can post a schematic.
That's another definite 'yes please' from me.
I'm very pleased that this seems like it might also be of use to others such as bluebunny, so that it's not just me picking everybody's brains for my own benefit!
I've managed to determine one further detail, though I don't know whether it's at all helpful:
In the originally existing setup, before any of these modification attempts, the current drawn across the tap switch when it's closed is 0.33mA.
Don't know whether this would help to explain why the solution proposed by Druid has no effect on the pedal (perhaps because the transistor can't supply enough current...?)
When testing Druid's circuit with the Digitech pedal, I can clearly see that there is a pulse of current when the tap switch is pressed, but the meter is too slow (compared to the length of the pulse) to see how much current is drawn during the pulse.
Maybe this information might be helpful to any other suggestions, such as the idea R.G. has talked about...?
It is helpful.
I have a number of alligators that chew on my time that slows me down. I'll get up a decent diagram soonest I can.
Quote from: R.G. on February 24, 2025, 07:42:34 PMIt is helpful.
I have a number of alligators that chew on my time that slows me down. I'll get up a decent diagram soonest I can.
Of course! Sorry R.G., wasn't meaning to hassle you at all, just feeling slightly pleased with myself that I'd managed to discover a small Thing! ;D
Currently basking briefly in the golden glow of TOTAL SUCCESS with this project :)
Guided by clues from various responses in this thread, and encouraged by the discovery of the falstad.com simulator, I decided that trying to do something with a 555 timer and a MOSFET looked like the most approachable option.
It's now built, installed and working perfectly :)
This is quite a significant and somewhat unexpected win relative to my meagre knowledge, so I'm feeling slightly smug. I picked up lots of pointers in the right direction here in this thread, so many thanks to all who contributed.
One thing I discovered along the way was that the pedal required the timing pulse to be at least 25ms long, which would be why I couldn't get ElectricDruid's suggestion to work. The 555 circuit turned out to be very readily adjustable in this respect.
This was also my first attempt at using perfboard, having used veroboard previously. I'm sure both the circuit itself, and my implementation of it are considerably less than optimal, but it works, so all's well! ;D
Thanks again for everyone's help,
Paul
(https://i.postimg.cc/zyn13N7V/tap-tempo-mods-for-digitech-looper-v5-final.jpg) (https://postimg.cc/zyn13N7V)
(https://i.postimg.cc/t1pGxcvD/board-final-top2.jpg) (https://postimg.cc/t1pGxcvD)
Congratulations on finding your solution Paul,
btw, which looper are you working with ?!
Quote from: Eb7+9 on March 05, 2025, 05:47:44 PMCongratulations on finding your solution Paul,
btw, which looper are you working with ?!
It's the original two-pedal Digitech JamMan; very ancient now, but still seems to me to strike a good balance between functionality and ease of use.
I have wondered whether the 4-button Stereo JamMan which succeeded it still suffered from the same silly tap tempo problem I've been addressing in this thread. I should try to find out...
I have just been experimenting with a friend's 4-button Stereo JamMan, and interestingly, its tap-tempo function works much more sensibly (ie taking the timing from the down-press of the switch, rather than the switch release).
[Although you do still have to either use Digitech's proprietary footswitch, or else make up a slightly ridiculous cable ( T&R → T / S → S ) to use it with a standard tap switch.]
So perhaps, between the release of the original JamMan (like mine), and the Stereo JamMan, somebody did indeed, as suggested near the top of this thread, give the Digitech engineers a kicking until they promised not to be so stupid next time ;D
Quote from: paulbuzz on March 05, 2025, 04:46:41 PMCurrently basking briefly in the golden glow of TOTAL SUCCESS with this project :)
It's now built, installed and working perfectly :)
This is quite a significant and somewhat unexpected win relative to my meagre knowledge, so I'm feeling slightly smug. I picked up lots of pointers in the right direction here in this thread, so many thanks to all who contributed.
Well done! Smugness well deserved!
QuoteGuided by clues from various responses in this thread, and encouraged by the discovery of the falstad.com simulator, I decided that trying to do something with a 555 timer and a MOSFET looked like the most approachable option.
I'm admittedly feeling somewhat smug as well, given that my meagre knowledge of digital circuits was able to help... All that on/off stuff is starting to click, but these analogue circuits are still pretty mystical to me. 😆