2 button 3 LED footswitch?

[the recluse]

I've been combing the internet trying to find schematics for this, and I am having no luck.  Any suggestions will be appreciated.

A little background:

I have a Mesa Boogie Nomad 100.  This is a three channel amp with switchable reverb, EQ, and Solo boost.  It uses a 7-pin DIN cable to connect to the massive 6-button footswitch.  My goal is build my own two button footswitch to replace this behemoth and free up some space on my pedalboard since I really only use the channel switches.

The good news:
The nomad has a ridiculous switching matrix inside the amp, as well as a ridiculous amount of switching logic in the pedal itself, this would be prohibitive to my project EXCEPT that I have gotten my hands on a box that Mesa custom built that splits the DIN cable into 5 1/4" jacks.  This box is switched using standard latching switches, so really the heavy lifting is already taken care of.

The bad news:
When put in footswitch mode, Channel 2 is the default channel. 
When the switch box is used:
Shorting tip to ground on jack #1 toggles to channel 1.
Shorting tip to ground on jack #2 activates channel 3 and overrides whatever channel is active on jack #1.

On the footswitch I plan on building, it will have a TRS jack.  The sleeve is the common ground for both jack #1 and jack#2 on the switch box.  The ring is the tip for jack #1 and the tip on the footswitch jack is the tip of jack#2

Using DPDT switches, I'd like to have 2 LEDs (one for channel 1 and one for channel 2) for DPDT1 and 1 LED for channel 3 on DPDT2. 

If I use a battery or add a DC jack, this is a no brainer, and as a last resort I am willing to do that, but there is voltage being supplied from the switch box that could be used to power the LEDs so I'd like to try to avoid external power.

Here's the part I am having trouble with.  Using the supplied voltage, DPDT2 is easy enough to wire, when the switch is closed shorting tip to ground for jack #2, I can wire the other side of the switch to close the circuit of the LED. Excellent!

On the side with 2 LEDs, the LED for channel one is easy enough, because it would be wired just like the other DPDT, but when I want the LED for channel 2 to light up, meaning the connection between the tip and sleeve of jack#1 is now open, how do I close the circuit for the LED?  taking the voltage from the ring and closing it to ground through the LED will also close the circuit for the switch which will activate channel 1.

I hope I've explained this clearly.  Any suggestions would be helpful.

[the recluse]

OK, I'm still digging,  but I have been coming across some circuits that seem to be using diode pairs to reverse the polarity on the LED. I don't really understand how this works but can someone correct me if I'm wrong on this?

I'm just going to focus on the side of the footswitch with 1 DPDT and 2LEDs.  Actually, for this I think the DPDT can be considered an SPDT too.

Center terminal (terminal 2) of SPDT is the incoming voltage. 

Terminal 1 attaches to limiting resistor, then anode of LED 1.   Cathode of LED 1 attaches to ground.   When engaged, switch box has tip and sleeve shorted,  channel 1 is active.

Terminal 3 attaches to a pair of diodes in parallel (with cathodes at opposite ends) then the cathode of LED2.  Anode of LED 2 attaches to limiting resistor.  Resistor attaches to ground.  This would close the circuit for the LED, but would the switch would see the tip and the sleeve as open.

Would that work?  Do I need another pair of parallel diodes between the resistor on LED 2 and ground?  Do I have it backwards and the ground should be the common terminal with the LEDs connected to the incoming voltage on either side of the switch?

[PRR]

> I'm still digging

I think you are FAR ahead of anybody here.

Background, such as a link to the Nomad plans, might help others to catch-up.

Image with chip-guts overlaid: http://i.imgur.com/ExUwn.gif

{EDIT: Further comments supeceded by next post.}
Studying the M-B plan is frustrating.
The plan I found seems to be incomplete (where does EQ go to DIN?).
The original pedal is a motley collection of fairly standard logic. Three ordinary debouncers for REV SOLO EQ. Three other switches for a one-of-three CH selector.
To jam 3 channel choices on one wire they use a 3-bit DAC to apply a variable analog voltage to the wire, then a good old LM3914 to decode that to two logic lines, with a third state implied. Diode-logic feeds a one-shot to Mute while the relays settle.
But the brain-blowing part is that "5V logic supply" is actually +2V -3V (derived from heater supply). Since the LM3914 and relays are also fed +3V -3V, the channel switching stands alone (overlooking the mute which is ground referenced). The REV SOLO EQ functions get offset by 5.6V Zeners and 100K to +15V. The nonstandard way way M-B draws JFETs makes me uncertain how this works.

[PRR]

Since this custom-built box is opaque to me, may not have power on the 1/4" jacks, and the core logic of the bizzare Channel switching scheme is documented, I threw-away the custom box and went right to the DIN plug.

According to page "Switching Matrix", the LM3914 will enable these channels when it sees these voltages:

Referenced to amplifier ground:
CH1 -2.6V
CH2 1.3V (or 0V in no-pedal mode)
CH3 -1.6V

Amp ground is not available inside the pedal; instead everything is referenced to the line which is -3V at the amplifier. Transposing by 3V:
CH1 0.4V
CH2 4.3V (or 3V in no-pedal mode)
CH3 1.4V

The amp's internal 3-way switch uses 9.1K pull-up and 5.6K or 1.8K pull-down (open for CH2) to get these voltages. 1.8K is equivalent to 5.6K||2.7K.

All that can be done with SPDT switches. Open SW1 for default CH2, close to connect SW2 which selects CH1 or CH3.

With DPDT switches you can do LEDs also.



I threw in a REV switch too. The same plan will work for SOLO on DIN5 except omit the 7406 inversion (I think). It would work for EQ (no inverter?) but I don't know where EQ is on DIN.

[PRR]

In fact only one 470r resistor is needed for all three CHannel LEDs. DIN2 to 470, to top of all 3 LEDs. LED bottoms to switches to DIN1.

The 470 to DIN3 CH control is simply pointless; I don't know what I was thinking. SW1a to DIN3.

That saves $0.36; moreover, less stuff to jam in a box.

EDIT: That one-sixth of a 7406 for effect switching is annoying me. It is not needed. Saves $0.39 and a lot of tedious pin-work.



EQ the same way, if you can find the pin. I think the wiring is like SOLO. If that works backward, wire like REV.

[the recluse]

I truly appreciate you taking the time to give this detailed of an explanation, but I fear you may not be seeing the trees for the forest. 
I didn't include link to the schematics etc. because I don't think they matter in this case.

The external box from Mesa takes the switching and logic circuitry from their stock footswitch and replaces the 6 switches of the footswitch with 5 1/4" jacks, that are triggered with a single latching switch each. 

Jack 1/ Channel 1 and 2 = Switch open is channel 2,  Switch closed is channel one
Jack 2/ Channel 3 = Switch closed is Channel 3, overriding Jack 1 regardless of whether the switch is open or closed.
Jacks 3-5 Reverb, EQ, and Solo functions.  Disregard these, they are not being used.

The switchbox is basically doing the work that you have laid out in your schematics, so that heavy lifting is already done. I can easily wire a switch to handle the functions I need using SPDT (or even SPST) switches.  I am attempting to use 2 DPDT switches so that I can use the second set of poles to operate LED indicators.   I could wire this up with 2 LEDs and 2 DPDT switches easily,  I understand how to do that.  The crux of what I am attempting is adding a 3rd LED without using an additional power source.

To try and re-state this more simply:

I am building a footswitch with 2 DPDT switches and three LEDs.  It will connect to the amp via a Y-cable.   1 1/4" TRS at the footswitch end and 2 1/4" TS at the amp end. 

The S of the TRS is common ground. 

At the TRS end of things

The T of TS 1 = T of the TRS
The T of TS 2  = R of the TRS

Assume voltage is provided by the T and the R of the TRS. 


When R is shorted to S LED 1 lights up.

When T is shorted to S LED 2 lights up. 

The question:

Is there a way to light LED 3 without shorting both T or R to S?
Is there some way to get voltage from T (or R) through LED 3 to ground so that the amp does not see T shorted to S?
Would a diode bridge be a method of effectively reversing the polarity on LED 3 so this is possible, or am I misunderstanding how that works?

[the recluse]

Maybe this will help clarify what I'm asking?  It's my first with DIYLC so forgive the sloppiness.

[PRR]

> I fear you may not be seeing the trees for the forest.

I live in the woods, so I know what you are saying. Sorry for the distraction.

[slacker]

When R is shorted to S LED 1 lights up.

When T is shorted to S LED 2 lights up. 

Does it actually do this, or is that what you want to happen? I would have expected it to do the opposite, when they are shorted the LED would be off. You probably have voltage on R and T, which goes through the resistor and LED to S, so the LED will be on. When you short T or R to S you also short out the LED and resistor turning it off. This is how a lot of simple amp footswitches work. If this is how it works then I don't think you can do what you want without power from somewhere else.

[the recluse]

This is the desired outcome.  I will not be able to dig in and test the pin outs and voltage levels for a few more days, unfortunately.

When I say shorting T or R to ground I mean through the LED by closing the switch.  Otherwise, the LED circuit is open so I don't understand how it could light. 

That's really the problem I think.  I have two voltage sources and one ground. Lighting 2 LEDs is not an issue, but lighting a 3rd while still maintaining an open switch seems un-doable(at least at my level of knowledge).

I'm guessing my options are add an external power source or just scale back to 2LEDs.  Neither of these is a dealbreaker, but I was hoping to skirt either of those options.

[the recluse]

FWIW I opted to build the footswitch with an external 9V DC jack since I have a daisy chain spot open for it. 

Works like a charm.  1 side of the DPDTs work the channels and the other sides work the LEDs.