Arduino-based tempo/expression pedal

[ElectricDruid]

Yes. Now that you mention it, that's probably why it uses two ADC inputs: Ring (with the lower value pullup R) to detect whether a load (exp pot) is connected and Tip to measure the value (position). Unless Behringer decided to do it differently, e.g. with a switching jack. Can be easily confirmed with a resistor once you have the pedal.

The jack input is described as "Expression/Control" by Boss, and can accept a footswitch for tap tempo control too. So presumably the processor uses the second input to work out which of those two options has been plugged in, as you surmise.
What we *don't* know is *how closely the Behringer clone follows the Boss design*.

[Baran Ismen]

Yes. Now that you mention it, that's probably why it uses two ADC inputs: Ring (with the lower value pullup R) to detect whether a load (exp pot) is connected and Tip to measure the value (position). Unless Behringer decided to do it differently, e.g. with a switching jack. Can be easily confirmed with a resistor once you have the pedal.

I'll get the pedal tomorrow probably. How can I test that?

Meanwhile, I was checking the astable 555 circuits that generate indefinite sine waves. Using 2 potentiometers instead of fixed resistors, highs and lows can be separately adjusted, right? At least this page says so.

One thing that, is it possible to reduce the input of NE555 to 3.3v, It seems when fed with 9v, it gives the same voltage as Vout, is that correct? Would a 3.3 Zener work on that?

I've made a schematic as below, yet I'm unsure where to connect the Ring. Pot values can be changed upon experiment.

[ElectricDruid]

That's a Square/Pulse wave, not a Sine wave. You can't get a Sine out of a 555, except by adding heavy filtering. You can get a sort-of "shark's fin" triangle shape, which is pretty useful, but that's across the timing capacitor, not at the actual output pin (that's always a pulse wave).

If you wanted to reduce the output level, a voltage divider would probably be the easiest way.

HTH

[Baran Ismen]

That's a Square/Pulse wave, not a Sine wave. You can't get a Sine out of a 555, except by adding heavy filtering. You can get a sort-of "shark's fin" triangle shape, which is pretty useful, but that's across the timing capacitor, not at the actual output pin (that's always a pulse wave).

If you wanted to reduce the output level, a voltage divider would probably be the easiest way.

HTH

For now, pulse wave would work for my scenario, as I'll be needing just hard up and down, 0 and 1 namely.

[ElectricDruid]

For now, pulse wave would work for my scenario, as I'll be needing just hard up and down, 0 and 1 namely.

Cool. Carry on, Sir!

[Baran Ismen]

For now, pulse wave would work for my scenario, as I'll be needing just hard up and down, 0 and 1 namely.

Cool. Carry on, Sir!

Cheers man.

What about the ring connection of the schematic I've shared? Where shall I connect it, do you think?

[Baran Ismen]

Okay so i got the pedal today and gave it a try. Well cloned, i would say

Apart from that.

1- 50k pot seems to work fine, getting 0 to 2.7v from bottom to top,

2- 100k pot seems to work better(better peak points), getting 0 to 3v from bottom to top,

3- 220k doesnt work, but getting around 3.4v from top to bottom, i think it exceeds the limit somehow.

4- I used 3 crocodile cables with a TRS Jack, removing any of them disconnects the expression function without physically removing the Jack, so thats good.

And the most important thing is, I think I've been beating a dead horse since now, and no one warned me about it.

Expression input is provided 3.3v from the pedal, (I knew that), but I've come to realize just now that I just can't feed it with some extra voltage, as it's just there. The circuit I'd connect should not provide any extra voltage, right? Or am I too sick to think correctly? (got sinusitis)

[Baran Ismen]

Just found out that this "TWA Side Step" product does exactly (actually more than) what I want! Yet it's quite expensive for me to afford 

[ElectricDruid]

Just found out that this "TWA Side Step" product does exactly (actually more than) what I want! Yet it's quite expensive for me to afford 

You'll be glad to hear that the SideStep is very much like a StompLFO in a box.

[ElectricDruid]

4- I used 3 crocodile cables with a TRS Jack, removing any of them disconnects the expression function without physically removing the Jack, so thats good.

Ok, so which pin does the wiper go to? The tip?

And the most important thing is, I think I've been beating a dead horse since now, and no one warned me about it.

I think we did try...

Expression input is provided 3.3v from the pedal, (I knew that), but I've come to realize just now that I just can't feed it with some extra voltage, as it's just there. The circuit I'd connect should not provide any extra voltage, right? Or am I too sick to think correctly? (got sinusitis)

If you're using a pot, then the pot needs to be provided with a voltage - which is what the 3.3V is for.

If you're using an LFO, then *you're* producing the voltage, and you need to ignore the 3.3V.

Get a StompLFO, or even your 555 oscillator. Scale the output down so the maxmimum output is 3.3V. If you have any questions about how to do that, ask us, because it's simple enough. Then feed the reduced output from your LFO to the wiper connection of the expression pedal with ground connected to ground.
Safest of all is to use a TRS lead with the ring not connected (if ring is the 3.3V connection as I suspect) but Boss/Behringer are sensible enough to have protected thier circuit if you use a simple mono jack too.

[Baran Ismen]

4- I used 3 crocodile cables with a TRS Jack, removing any of them disconnects the expression function without physically removing the Jack, so thats good.

Ok, so which pin does the wiper go to? The tip?

And the most important thing is, I think I've been beating a dead horse since now, and no one warned me about it.

I think we did try...

Expression input is provided 3.3v from the pedal, (I knew that), but I've come to realize just now that I just can't feed it with some extra voltage, as it's just there. The circuit I'd connect should not provide any extra voltage, right? Or am I too sick to think correctly? (got sinusitis)

If you're using a pot, then the pot needs to be provided with a voltage - which is what the 3.3V is for.

If you're using an LFO, then *you're* producing the voltage, and you need to ignore the 3.3V.

Get a StompLFO, or even your 555 oscillator. Scale the output down so the maxmimum output is 3.3V. If you have any questions about how to do that, ask us, because it's simple enough. Then feed the reduced output from your LFO to the wiper connection of the expression pedal with ground connected to ground.
Safest of all is to use a TRS lead with the ring not connected (if ring is the 3.3V connection as I suspect) but Boss/Behringer are sensible enough to have protected thier circuit if you use a simple mono jack too.

As per my trial, the ring was the wiper, and the tip and sleeve were outer lugs.

I've made a circuit as attached, I don't know if it would work like that. Can you check? And someone else in a Turkish forum suggested another upgrade to it so that it'd give other types of waves.






What still confuses me is the voltage supply. With any sort of LFO circuit, I'll be providing 3.3v to expression input, isn't it right? Wouldn't it do any harm to the circuit? Be it a pot or LFO, there's already a voltage inside and I should be playing with it externally, right?

Just measured the naked TRS jack and saw that, when the pedal is powered, the voltage between the Sleeve & Tip is 3.31v, and Sleeve & Ring is 3.34v.

[Baran Ismen]

4- I used 3 crocodile cables with a TRS Jack, removing any of them disconnects the expression function without physically removing the Jack, so thats good.

Ok, so which pin does the wiper go to? The tip?

And the most important thing is, I think I've been beating a dead horse since now, and no one warned me about it.

I think we did try...

Expression input is provided 3.3v from the pedal, (I knew that), but I've come to realize just now that I just can't feed it with some extra voltage, as it's just there. The circuit I'd connect should not provide any extra voltage, right? Or am I too sick to think correctly? (got sinusitis)

If you're using a pot, then the pot needs to be provided with a voltage - which is what the 3.3V is for.

If you're using an LFO, then *you're* producing the voltage, and you need to ignore the 3.3V.

Get a StompLFO, or even your 555 oscillator. Scale the output down so the maxmimum output is 3.3V. If you have any questions about how to do that, ask us, because it's simple enough. Then feed the reduced output from your LFO to the wiper connection of the expression pedal with ground connected to ground.
Safest of all is to use a TRS lead with the ring not connected (if ring is the 3.3V connection as I suspect) but Boss/Behringer are sensible enough to have protected thier circuit if you use a simple mono jack too.

Any ideas, Tom ?

[FiveseveN]

Wouldn't it do any harm to the circuit?

See the end of reply #17.

The circuits above look fine. You might need a ~100 K resistor from Ring to ground for that detection function we were talking about.

[Baran Ismen]

Wouldn't it do any harm to the circuit?

See the end of reply #17.

The circuits above look fine. You might need a ~100 K resistor from Ring to ground for that detection function we were talking about.

All right, then imma giving it a try tonight.

I've added a voltage divider for decreasing the 9v to 5v in the beginning, so that the output will be 3.3v (2/3 of the vcc datasheet says) but kept the Zener just in case.

[Baran Ismen]

Ok, I got success!

With a minor change, when I connected the ring to the tip (not to the sleeve) with a 100k resistor, it worked quite well.

I'll tweak around more for timings as one of the rate knobs tweak T0 and T1 timings I guess, and the other one is the general rate. Or even make a different LFO circuit that would yield different waveshapes. Output is between 0 and 3.1v (I should tweak it just a bit more from the voltage divider section I suppose)

[Baran Ismen]

Just found out that this "TWA Side Step" product does exactly (actually more than) what I want! Yet it's quite expensive for me to afford 

You'll be glad to hear that the SideStep is very much like a StompLFO in a box.

I was checking this, I assume it is the schematics of the side step clone, is that right?

I have some questions for this if you mind answering @ElectricDruid;

1- I see that for 4 control, there are CV outputs. Depth, Offset, and Freq are just fine but got stuck with the waveform one. How do I set the waveform? Different voltages mean different waveforms? Also found the answer, it is voltage-dependent, but still is it possible to implement an 8-way rotary switch here? Maybe adding various resistors on each lug of the rotary would do the trick like 12.5k and its multiples until 100kohm? But those will be weird resistors, never seen any with 12.5k, 37.5k, 62.5k, or 87.5k of values.

2- Filters are optional I see, but what's the purpose of them? For my scenario, passive one would be fine?
I see its for ticking, all good, I can keep it I suppose.

3- What are the min and max freq values? Found this on datasheet, its quite wide.

I think I can order one (or write one if I can find a suitable PIC) if I can solve these questions.

[ElectricDruid]

I was checking this, I assume it is the schematics of the side step clone, is that right?

Yeah, pretty much. The Sidestep has one waveform different, but otherwise they're basically the same thing.

1- I see that for 4 control, there are CV outputs. Depth, Offset, and Freq are just fine but got stuck with the waveform one. How do I set the waveform? Different voltages mean different waveforms? Also found the answer, it is voltage-dependent, but still is it possible to implement an 8-way rotary switch here? Maybe adding various resistors on each lug of the rotary would do the trick like 12.5k and its multiples until 100kohm? But those will be weird resistors, never seen any with 12.5k, 37.5k, 62.5k, or 87.5k of values.

No, you don't need weird resistors because the waveforms are equally spaced. There's an explanation in the TAPLFO datasheet, page 9. The STOMPLFO works the same way:

https://www.electricdruid.net/datasheets/TAPLFO3Datasheet.pdf

2- Filters are optional I see, but what's the purpose of them? For my scenario, passive one would be fine?

I see its for ticking, all good, I can keep it I suppose.

This is mixing up two different things. The chip's firmware includes a waveform smoothing filter which softens the hard edges of some waveforms and helps reduce ticking. That's internal and done in firmware.
The other thing is the external filtering. The output is provided as a high frequency PDM pulse train, and that needs lowpass filtering to turn it back into an analogue LFO waveform. That can be done either with a passive filter or an active filter, but it needs to be done somehow.

3- What are the min and max freq values? Found this on datasheet, its quite wide.

Yes, 0.05Hz (1 wave every 20 seconds) to 25Hz. If it's too wide for a specific job, you can always add a resistor above/below the pot to limit the voltage range.

I think I can order one (or write one if I can find a suitable PIC) if I can solve these questions.

Sure. The code is provided on the website and it's free to use for personal use, so go ahead and program one up if you can get some blank 16F18313 chips.

[Baran Ismen]

I was checking this, I assume it is the schematics of the side step clone, is that right?

Yeah, pretty much. The Sidestep has one waveform different, but otherwise they're basically the same thing.

1- I see that for 4 control, there are CV outputs. Depth, Offset, and Freq are just fine but got stuck with the waveform one. How do I set the waveform? Different voltages mean different waveforms? Also found the answer, it is voltage-dependent, but still is it possible to implement an 8-way rotary switch here? Maybe adding various resistors on each lug of the rotary would do the trick like 12.5k and its multiples until 100kohm? But those will be weird resistors, never seen any with 12.5k, 37.5k, 62.5k, or 87.5k of values.

No, you don't need weird resistors because the waveforms are equally spaced. There's an explanation in the TAPLFO datasheet, page 9. The STOMPLFO works the same way:

https://www.electricdruid.net/datasheets/TAPLFO3Datasheet.pdf

2- Filters are optional I see, but what's the purpose of them? For my scenario, passive one would be fine?

I see its for ticking, all good, I can keep it I suppose.

This is mixing up two different things. The chip's firmware includes a waveform smoothing filter which softens the hard edges of some waveforms and helps reduce ticking. That's internal and done in firmware.
The other thing is the external filtering. The output is provided as a high frequency PDM pulse train, and that needs lowpass filtering to turn it back into an analogue LFO waveform. That can be done either with a passive filter or an active filter, but it needs to be done somehow.

3- What are the min and max freq values? Found this on datasheet, its quite wide.

Yes, 0.05Hz (1 wave every 20 seconds) to 25Hz. If it's too wide for a specific job, you can always add a resistor above/below the pot to limit the voltage range.

I think I can order one (or write one if I can find a suitable PIC) if I can solve these questions.

Sure. The code is provided on the website and it's free to use for personal use, so go ahead and program one up if you can get some blank 16F18313 chips.

All perfect, got the point for rotary switch as well. I'll check around for a PIC option, else I'll order one from you. Thanks so much!

The output being 3.3v shouldnt affect anything, though, right?

[ElectricDruid]

All perfect, got the point for rotary switch as well. I'll check around for a PIC option, else I'll order one from you. Thanks so much!

You're welcome.

The output being 3.3v shouldnt affect anything, though, right?

No. You've got various choices there. You can run the StompLFO off 5V, in which case, you'll have to drop a bit of voltage on the output like you did for the 555 LFO. The passive filtering might do that anyway, since it'll get loaded down.
The other option would be to run the StompLFO from 3.3V, in which case the output would be the level you need straight away. The standard 16F18313 runs from 2.3V up to 5.5V, so you can choose.

[Baran Ismen]

All perfect, got the point for rotary switch as well. I'll check around for a PIC option, else I'll order one from you. Thanks so much!

You're welcome.

The output being 3.3v shouldnt affect anything, though, right?

No. You've got various choices there. You can run the StompLFO off 5V, in which case, you'll have to drop a bit of voltage on the output like you did for the 555 LFO. The passive filtering might do that anyway, since it'll get loaded down.
The other option would be to run the StompLFO from 3.3V, in which case the output would be the level you need straight away. The standard 16F18313 runs from 2.3V up to 5.5V, so you can choose.

I think I won't bother dealing with finding the right PIC for this and order it now: Says back-ordered, how much time would it take to ship?

Quick question on the schematics that, whats the use of J letters? Especially on the Tap Tempo where there's a button already?

I think I'll go with L78L33 which is a 3.3v converter.