Channel selection and effects/loop control voltages

Hi again.  I am using 2 double switch footswitches to control the channel selection on one and the effects and loop on the other.  These are passive and work perfectly, but they don't have indicators to show you what is selected.  I know the channel switches on the amp light up to show channel selection but the effects and loop don't, and it would be much better to see the state of play on both of the footswitches too.  This will mean adding power to the units to drive the LEDs and the logic and driver circuitry but as they are in a pedal board that is no issue.

The effects and loop side is a doddle.  One is on the tip and the other on the ring and they work independently.  A simple LED and driver for both is all that is needed.  The channel selection, while it isn't rocket science, isn't a totally trivial task as the logic of the two is connected.  It's kind of:  [The Clean light will be on] OR [either the Crunch OR Lead light].  If you just drive the LEDs from the control voltages as they are fed to the amp you will get either a Crunch or Lead LED showing even when the Clean is selected.  So I have come up with a nice cheap little control circuit to take care of that.  Now I obviously need to know what the selection voltages represent on the switches.

The Clean/Overdrive switch and the Crunch/Lead switch will be on the tip and ring of the stereo jack plug and each will be shorted or not to the ground barrel.  Does anyone know which switch is on which terminal, and what the high and low voltages on the tip and ring represent in terms of selection?

Likewise if anyone has the similar effects and loop switch info it would be useful.

Ok, so no one seems to have done this sort of work and has the info available.  I've opened up the footswitches to play and made the measurements direct.  For reference here is how this area is set up.

Obvious to most but just in case... This is the terminology of the plugs carrying the control voltages:



The control voltages are carried on the tip and ring and the sleeve is the ground. The footswitch either leaves the tip and ring alone or shorts one or both of them to the sleeve. This is the control logic involved:

Channel Select:
Clean/Drive switch :	Tip	Clean - Open (14.5V)	Drive - Shorted (0V)
Crunch/Lead switch:	Ring	Crunch - Open (14.5V)	Lead - Shorted (0V)

.

Reverb & Loop:
Reverb switch :	Tip	On - Shorted (0V)	Off - Open (14.5V)
Loop switch:	Ring	On - Shorted (0V)	Off - Open (14.5V)

.
Now to the more technical bit. I have come across other sites where people working with different makes of amp have driven LEDs and a current limiting resistor directly from these voltages. This is not a good option for the TM36 and I would suspect H&Ks in general. I have tested the voltages supplied from the amp on the tip and ring and they seem to be supplied via a fairly high impedance. In other words there is not a lot of current on tap for the indicator before the amp thinks it must be a short and changes selection. It drops from 14.5V to 6V with a 1k resistor across it while still holding its selection, any resistance less than that and it triggers a change. Doing a bit of arithmetic that is an output impedance of about 1.4k. That's only 6mA before it is unstable and this is not really enough to give you a sensible amount of output from your LEDs. If you have a low current high efficiency LED then maybe, but with normal types not really. And you don't get the right logic!

It may be more complicated but I would recommend setting up a simple driver circuit for each LED and power it externally as any other pedal would be. The sensing can be done at high impedance from the amp lines so as to not draw current. In fact, in the case of the channel select, it would be necessary to follow this approach just to control the logic which should allow only a single LED to be on at a time. With the direct drive approach you will not be able to turn off whichever of the Crunch/Lead LEDs is selected when the Clean channel is selected, and you will end up with the Clean and one of the other LEDs lit at the same time.

It is simple to design a circuit where the control is done via a cheap and cheerful CD4011. That would be fine for LEDs which will work on less than 10mA. You need to add a driver transistor and resistor to each LED to pump out more. This can be built up on veroboard and squeezed into your footswitch. With a Blue, Yellow and Red LED, they give you exactly the same visual indication as the amp front panel does. I have the circuit designed and when I get to that stage I'll post a couple of pics.

Progress!  I got the circuit built up on Veroboard and it worked first time.  As I said, it's really not rocket science.

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I decided to keep mounting it really simple.  I fitted a panel mounting 2.1mm socket on the back of the switch which you can just make out above the number 2. I then soldered the veroboard +9V top lines directly to the +terminal inside.  The ground side of the board was mounted onto a really stiff short wire connected to the ground of the Crunch/Lead switch, while the Yellow and Red LEDs, the ones mounted on the board, are fitted fully into the top panel holes to stabilise it.  It's rock solid and incredibly light anyway.

I left the blue LED on a pair of flying leads and mounted it into it's top panel hole next to the number 1 with a strong tape sticking it down on the back.  A slit to go around the legs was all that was needed. That will hold it until I get round to filling the front holes in with a dab of clear tinted nail varnish.

It all works perfectly.  Now I get exactly the same indication on the pedal as the amp.  The LEDs are all extinguished in the picture, I used clear higher efficiency types but any type would do. The only real features are the swapping of the Yellow/Red LEDs with the Crunch/Lead switch and the inhibit circuitry to extinguish the Yellow/Red LEDs when the Blue is lit by the Clean/Drive switch.

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One important point is that there is no effect on the normal action of the switches. I can use it without the 9V connected and everything is as it always was, (except for the LEDs which don't light of course).

The Reverb/Loop switch unit should be a piece of cake in comparison.