Switching neck/bridge bickup via MIDI and custom pedal

[SierraLima]

Hey Guys,

   So I've had an idea for a while for a project to modify my Jackson SL7Q Soloist. The main idea would be to switch pickups via a custom Arduino powered foot pedal that can either be triggered manually by my foot or switched via a MIDI message every time I change preset on my POD HD Pro X via it's footswitch (or any other MIDI out capable effects processor). No more fiddling with the pickup selector or having to change both the pickup selector and using the foot switch to change effects processor preset simultaneously to get the tone I want. In addition, this pedal would have an on/off mute function.
   My guitar has 2 humbuckers with a 5 way pickup selector, volume knob and tone knob. The reality is I only ever use 2 pick up selector positions: full neck humbucker and full bridge humbucker, I never use a mix of the pickups or the pickups as single coils. I also only use the tone knob on full treble and I only use the volume knob on full volume or off to just mute it. As part of this, I would remove all the electronics (pickup selector, volume knob and tone knob) with the obvious exception of the pickups. I'd still need a jack for the cable, but I'd replace the stock mono 1/4" with a 1/4" TRS jack (1 conductor each for ground, neck pickup, bridge pickup).
   I'm not terribly worried about anything that goes into the pedal itself, I've done several non guitar Arduino projects successfully and I'm reasonably handy with electronics, soldering and writing software. This will by my first guitar mod project though, so I figured I should double check some assumptions I'm making before I mutilate my guitar in a way I'll regret due to ignorance, so here goes:

1) A tone knob is just a low pass filter based on a potentiometer and a capacitor? If it's on full treble, it's just a bypass? So I could completely remove it from the circuit and get the same tone as a tone knob stuck on full treble?
2) The volume knob is just a potentiometer and with it set at full volume it has 0 resistance? And hence, again I can just remove the volume knob from the circuit>
3) Are all input jacks basically the same size wise? Would something like https://guitarpedalparts.com/collections/1-4-jacks/products/1-4-switchcraft-stereo-trs-jack?variant=31256480710791 work? I attached a picture of my current stock jack.
4) I'm thinking of using 1 of these signal relays (https://guitarpedalparts.com/products/5v-dpdt-latching-relay?_pos=1&_sid=0cb05e3f2&_ss=r) to switch between the pickups and 1 to simply mute the guitar by opening and closing the circuit on the ground conductor from the 1/4" stereo TRS cable. Anything stand out as wrong with this approach?
5) Any other potential problems or anything else that might alter the tone of my guitar that I should be thinking about?

Thanks,
   Omri

[curtisa]

14 hours ago, SierraLima said:

A tone knob is just a low pass filter based on a potentiometer and a capacitor?

Yes.

14 hours ago, SierraLima said:

If it's on full treble, it's just a bypass?

Sorta. If it's on full treble the resistive element of the pot is at maximum, so the lowpass filter is wound up as high as it can go.

14 hours ago, SierraLima said:

So I could completely remove it from the circuit and get the same tone as a tone knob stuck on full treble?

Removing it completely from circuit would result in a marginally brighter tone. Even with a tone circuit at max treble you still have some lowpass filtering going on, so it's never fully bypassed. Taking the tone circuit out altogether removes that last little bit of tone throttling that you'd otherwise interpret as being 'maximum treble'. If the resulting tone is too bright for your liking you can always re-introduce the stock 'tone pot at max' setting by replacing the pot with a fixed resistor of the same value as the potentiometer.

 

15 hours ago, SierraLima said:

The volume knob is just a potentiometer and with it set at full volume it has 0 resistance?

Sorta. The volume pot is wired as a 'potential divider', which is a fancy way of saying it can variably 'pass through' the output of the pickups at one extreme of its rotation, and ground (ie, silence) at the other.

15 hours ago, SierraLima said:

And hence, again I can just remove the volume knob from the circuit

Yes, although like removing the tone pot altogether the result of removing the volume pot is a marginally brighter and louder tone. The volume pot does present a static load to the pickups, even at full throttle, so disconnecting the pot will remove this slight slugging of the tone. Again, this can be restored if you wish by wiring a fixed resistor between the output and ground of the same value as the volume pot that you removed.

 

15 hours ago, SierraLima said:

Are all input jacks basically the same size wise? Would something like https://guitarpedalparts.com/collections/1-4-jacks/products/1-4-switchcraft-stereo-trs-jack?variant=31256480710791 work?

That jack should drop in fine as a replacement.

 

15 hours ago, SierraLima said:

I'm thinking of using 1 of these signal relays (https://guitarpedalparts.com/products/5v-dpdt-latching-relay?_pos=1&_sid=0cb05e3f2&_ss=r) to switch between the pickups and 1 to simply mute the guitar by opening and closing the circuit on the ground conductor from the 1/4" stereo TRS cable. Anything stand out as wrong with this approach?

The mute circuit would work better if the second relay was wired to short the guitar signal to ground, rather than opening/closing the ground connection. Have a look at guitar kill switch circuits for an example. Opening the ground connection as a method of muting would likely result in excessive hum and buzz everytime you engaged the mute function. On the other hand, shorting the signal to ground is electrically the same as winding the volume pot to zero.

 

15 hours ago, SierraLima said:

Any other potential problems or anything else that might alter the tone of my guitar that I should be thinking about?

Relays have a propensity to introduce a pop in the audio signal when operated due to the coil being energised/de-energised rapidly. The pop is induced from the coil to the nearby audio-carrying contacts, and is generally difficult to avoid without adding specialised support circuitry to either suppress the audible pop, or 'condition' the coil switching signals. The fact that you're proposing switching the pickup signals directly may make the resulting pop somewhat difficult to control (the output of a typical guitar is a relatively fragile signal that is prone to induced noise).

One method of combating the pop while switching between two signals is to momentarily mute the output while the switching operation is performed. As you're building a mute function into the pickup switching system, you could incorporate this feature into it quite easily. All you'd need to do is engage the mute, switch between the two pickups and disengage the mute again. The mute only needs to be active for 20-40msec, which should be trivial to program up in an Arduino; short enough to be unnoticable to our ears, but long enough to mask any pops from the switching relay. The mute function in its own right would then just be the activation of the mute relay by itself.

The only drawback is that as you're proposing to use a relay for the mute function as well, the very device you're using to kill the pop may introduce noise of its own. In that situation look at replacing the mute relay with something like a TLP222 photomosfet switch.

[SierraLima]

Thanks curtisa! Definitely good things to know that I wasn't aware of.

[SierraLima]

curtisa, I have one more question regarding your TLP222 suggestion. Can I just forget about using relays altogether and just use 3 of these photomosfets? (1 to short to ground as the kill switch circuit as you suggested, 1 to close the circuit for the bridge pickup, 1 to close the circuit for the neck pickup). Would using these to carry the signal from the pickups be noisy or introduce any other problems? One thing I really like about these as opposed to the relays is that according to the spec they only need 10 mA of forward current which is less than the 20 mA I can pull off of a Teensy 2.0 digital output pin. The relays need more current than the digital output pins can deliver and thus require me to introduce transistors and diodes as a relay driver circuit and simplicity and less components is good!

Thanks Again!

[curtisa]

If in doubt consult the datasheets.

Relay data suggests the coil resistance for the single winding 5V latching model is 250ohms (assuming the picture on the webpage you linked to earlier is an accurate representation of what you get). Current draw for the relay coil when energised is therefore 5V/250ohms = 20mA.

Teensy claim the 2.0 (or rather, the ATmega32u4 chip on board) can supply a recommended max of 20mA output per pin, so that's right on the theoretical limit for the relay coil, although that isn't quite the full story. The datasheet for the ATmega32u4 suggests that the absolute maximum current output per pin is 40mA (page 383), beyond which "... may cause permanent damage to the device...". So the suggested 20mA limit quoted by Teensy is more likely to be a safe limit that you shouldn't exceed continuously for long periods of time if you want the 32u4 to perform as guaranteed by the manufacturer. The datasheet only mentions the 20mA limit in regards to their "test circuit", which further reinforces its use as a continous max rather than a short-term max.

But. You're looking at using a latching coil relay, so the drive signal only needs to be applied to the coil in a brief pulse, only long enough to get the contacts to "stick" in one direction or the other. So the 20mA coil drive current only needs to be supplied for a fraction of a second (pulse width timing graph on page 4 of the relay datasheet suggests 10msec pulse would be more than sufficient to get the relay to seal in). The continous safe max of 20mA per pin would be perfectly safe to drive this relay direct from the Teensy, as the chip is only asked to supply this current for a brief pulse, well below the absolute maximum rating of 40mA per pin.

It gets better if you're still worried about stressing the Teensy out too much. The relay data claims that the 5V coil must operate at a minimum voltage of 3.75V, so you could put an extra resistor in series with the relay coil to reduce the drive voltage slightly, which has the knock-on effect of lowering the coil current. Say you wanted to give the relay a bit of headroom to operate and reduce the drive voltage to only 4V, a reduction of 20%. To achieve this reduction you'd need to increase the total coil resistance by the ratio of the voltage difference, ie  5V / 4V x 250ohms = 312.5ohms. So the extra series resistor required to drop the coil voltage is 312.5ohms - 250ohms = 62.5ohms. Pick the nearest E24 resistor value, so use 62ohms in series with your relay coil (or 68ohms if you can only find E12 resistors) . The subsequent relay drive current now drops to 5/(250 + 62) = 16mA.

 

2 hours ago, SierraLima said:

Can I just forget about using relays altogether and just use 3 of these photomosfets?

I've only ever seen these things implemented as a shorting (shunt) switch, so I'm assuming they're not suited to be in series with the audio signal (possibly to do with inherent capacitance in the switching element? Dunno...).

You could get all fancypants and get away with just two TLP222's, one shorting out each pickup. You'd just turn one on to mute the pickup you didn't want to hear, turn both off to hear both pickups, or turn both on to mute both pickups. But you'd need to construct a mixer circuit to prevent the shorting action of one TLP222 from erroneously muting both pickups simultaneously. Circuit complexity starts skyrocketing...