Footswitch w/ LED indicator for a H&K amp

[fentom_lord]

Hello,
I was wondering whether anyone has ideas on how to build a footswitch with an LED indicator and no battery for a Hughes and Kettner Warp 7 amp. I can't find any FS1 schematics on the net
The idea of switching channels on this one is very simple - you just short out the two leads of a mono jack. The problems start when it comes to adding an LED - when the clean channel's on, there's about 12V on the leads. When you short them out, there's no voltage, and all the descriptions for a H&K FS1 say it's phantom powered.
This switch works with other H&K amps, like the Vortex 80 whose schematics you can find here if it's by any chance useful
http://www.schematicx.com/schematic/hughes-kettner-vortex-80-schematic/
I'd much appreciate any help.
Thanks in advance.

[anchovie]

Use an LED in series with the SPST footswitch when you short the jack leads. You may find that it and the corresponding panel LED are dimmer than the panel with no footswitch but hopefully it'll still be bright enough to use. On the Vortex schematic, R77 provides the current limiting so don't worry that you'll be putting a full 12V through it!

[fentom_lord]

Use an LED in series with the SPST footswitch when you short the jack leads.

You mean something like this? (I'm using a dpdt because I couldn't get an spst...)
http://img692.imageshack.us/img692/407/dpdtled.png

On the Vortex schematic, R77 provides the current limiting so don't worry that you'll be putting a full 12V through it!

So does this mean I can go without a resistor for the LED? Can I put something like 1k, just in case?

[anchovie]

You mean something like this?

That'll work, or you could just run the blue wire straight from the cathode to the return. The second pole of the switch isn't needed at all.

So does this mean I can go without a resistor for the LED?

Yes. Current's already being limited for the panel LED, so it'll certainly be limited for what will be an extra LED in series. Adding an extra resistor could make them far too dim, plus a certain voltage level will need to be maintained for the logic to switch.

(I am working from the Vortex schematic here, and assuming that the Warp 7 LEDs are hooked up the same way).

[PRR]

> When you short them out, there's no voltage

If your short is a SHORT, there CAN'T be voltage.

> say it's phantom powered.

That's mis-use of the term; but we know what they mean.

If not clear yet: here's a generic plan:



When any switch is open, the 1K resistor pulls-up point "?" to essentially +12V.

When switching is closed, point "?" is pulled-down to LED voltage. This may be 1.6V or 5V, depending how-many what-color LEDs are in the chain. But whatever, it is still clearly less than 12V. The "?" point has a voltage checker, and an appropriate decision-level might be 10V. Definitely less than the 12V supply, yet higher than any likely LED voltage(s).

The 1K resistor limits current to 12mA with no LEDs (so you don't burn a switch or pop the 12V power). With two red LEDs, about 3V or 4V LED drop, you get 8mA or 9mA LED current.

The exact plan may be more complicated. There may be radio-suppression caps. A current source can force constant current so adding the pedal does not cause a 10% drop in brightness on the panel LED. Protection diodes so when you plug another amplifier output where the pedal should go, nothing dies easy. There may be some slack in the "10V?": it might have to drop to 9V to go ON and to 11V to turn OFF, so that some 10.00V input does not leave it undecided and maybe wobbly.

EDIT: OK, I see I was close. H&K uses +15V and 1.5K, no big difference. The "? 10V?" decision is taken by a CMOS switch, fed through 220K+150K and eating +7.5V; the threshold is nominally 8V to 10V. There is an extra transistor which only drives an LED with threshold near 5V or 6V.... not sure why.

[fentom_lord]

Ok, I put the LED in series with the switch and plugged it into the amp. The diode turns on, fine, but it also seems to switch BOTH of the amp's channels on...

[fentom_lord]

Any ideas on what I could do? With the LED in series it seems that the leads are shorted (the distortion channel on) and not shorted (the clean channel on) at the same time.

[fentom_lord]

Could it be like that because I used a blue LED?

[anchovie]

Jumper the LED leads to confirm that it switches as normal without the extra diode. If that's fine, try a red one as it has a quite different forward voltage.

I've got the PCB from a dead Warp 7 at home that I've been ransacking for parts - if I get the time I'll try to trace from the footswitch jack and see if there's any other switching logic apart from the 4053.

[fentom_lord]

It does switch on without an additional LED. I'd tried that before.
I used a green diode today and it worked perfectly well. As far as I know, blue LEDs have around 4-5V forward voltage so it must have reduced the voltage too much. Thanks for the help guys! I owe you one
Funny stuff is that shops sell the H&K FS1 for like 20 euros while you can make yourself a similar one for far less than that.
Anyway, thanks again.

[anchovie]

Awesome, glad to hear it works!

[clund]

Found this link in a search...  I'm doing the same thing for my H&K Trilogy, but I am adding an additional footswitch w/ LED to bypass the Effect Loop on my current FS4.   Just wondering the exact setup that worked for you fentom_lord?!?   I am ordering a DPDT and 5mm Green LED.   So hear is what I have gathered so far:

According the the LED wizard (http://led.linear1.org/1led.wiz) - I should be using a 680ohm 1/2w resistor



From these values:
15 - Source voltage
2.2 - diode forward voltage
20 - diode forward current (mA)

Here is the Amp's Schematic for the FS1 connection:



So here is my diagram...  Does this look right?!?!   

[fentom_lord]

Hi, I connected the LED in my homemade FS1 the same as on the drawing. But, as anchovie wrote

That'll work, or you could just run the blue wire straight from the cathode to the return. The second pole of the switch isn't needed at all.

I just didn't use any resistor as it seemed to decrease the voltage too much which resulted in both channels being on. I saw the original FS1's inside in a German forum and there's no resistor there.
Regards,
Matt

[fentom_lord]

Hi, I connected the LED in my homemade FS1 the same as on the drawing. But, as anchovie wrote with reference to this drawing

That'll work, or you could just run the blue wire straight from the cathode to the return. The second pole of the switch isn't needed at all.

I didn't use any resistor as it seemed to decrease the voltage too much which resulted in both channels being on. I saw the original FS1's inside in a German forum and there's no resistor there.
Hope I was helpful.
Regards,
Matt

[clund]

Very helpful thank you...   I will try it without a resistor first, if I have issues, I will try a few different resistors I have here.

Thanks again!

- Corey

[clund]

Wanted to post back that I got all my parts and build my FX Loop Switch pedal last night.   After putting it together the LED light was BRIGHT!!!  Blinding almost...  So I tried to put in a 47k Resistor.  After that, it didn't work.    Have to live with bright! -- I actually used the silver sharpie I wrote "LOOP" on the pedal with the paint the LED and it's not as bright now.