How to allow only a short pulse of current to flow when a switch is closed?

[amptramp]

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.

[R.G.]

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...

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.

[paulbuzz]

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

[R.G.]

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.

[bluebunny]

If 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!)

[paulbuzz]

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.

This seems like an accurate summary of the task!

Do you have space in there for a DIP IC and some Rs and Cs?

Yes, the inside of the pedal is relatively spacious

If 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!

[paulbuzz]

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...?
     

[R.G.]

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.

[paulbuzz]

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.

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!

[paulbuzz]

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!   

Thanks again for everyone's help,
Paul

[Eb7+9]

Congratulations on finding your solution Paul,

btw, which looper are you working with ?!

[paulbuzz]

Congratulations 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...

[paulbuzz]

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 

[ThePracticalPeasant]

Currently 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!

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.

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. 😆