Author Topic: Need help understanding my footswitch  (Read 6025 times)

Explorer45

Need help understanding my footswitch
« on: December 29, 2012, 04:50:13 PM »
Hi all,

New member needing advice. :) I've been looking for a neat stompbox that can switch between two pedal loops and simultaneously switch channel on my Blackstar HT-5R amp. Since I haven't found a pedal matching my needs I decided to make one myself. I began by opening up the HT-5R LED footswitch to understand how it changed the channel. Apperantly it has a SPDT switch and a LED in parrallell.

Pic:


From what I can tell the switch short curcuits the LED when the clean channel is on and when the switch is off the current flows through the LED and the lead channel engages. I measured the voltage coming from the amp to 15.16V. My first thought was that the LED must have a built in resistor but when I tired to measure the resistance I got nothing. How can it not have a current limiter? Could it be that the amp has an internal resistor?
I want my stompbox to have two switches, one to bypass the effects loops, and one to both switch between the two loops and switch channel on the amp. Both switches will be accompanied with LEDs and I will have a 9V adapter providing them with juice.

I made a quick schematic in EAGLE (I apologize in advance):


I'm still not entirely sure on how the channel switching work. Does the amp tell the difference in current when the LED is short circuited and switch based on that?

Any input is highly appreciated!

earthtonesaudio

Re: Need help understanding my footswitch
« Reply #1 on: December 29, 2012, 09:32:56 PM »
First, your amp almost certainly has a resistor on its output.  And when you measured 15 volts that was with the footswitch unplugged.  Normally the amp sees either zero volts (led off) or about 1.5 volts (led on), and those two voltages are most likely what it is sensing.

The amp could alternatively be sensing the led versus short circuit current, but in any case you should leave the led for the amp channel as-is. 

Your diagram looks fine, but keep the original led in there.  You can tuck it inside the enclosure if you want, but my gut says it needs to be there electrically.

Explorer45

Re: Need help understanding my footswitch
« Reply #2 on: December 30, 2012, 05:29:07 AM »
Thanks, that makes a lot of sense! Would it be safe to just try a LED I have lying around (Vf 1.8-2.4V) and see if it works, or could I damage the amp? I guess I could email Blackstar and get the Vf but I'm not sure they can/will tell me.

slacker

Re: Need help understanding my footswitch
« Reply #3 on: December 30, 2012, 06:45:26 AM »
It should be safe, worst that can happen is if the Vf of the LED is too different than what it expects it won't switch. If you've got a meter you can measure the voltage across the original LED when it is on, that will tell you what its Vf is.

Explorer45

Re: Need help understanding my footswitch
« Reply #4 on: December 30, 2012, 07:03:09 AM »
Lol, how did I not think of that?  :P It was 1.824V over the LED and I'm guessing it's limited to 20mA. I've reworked the schematics and it is now a lot less wires. Ordered some stuff from ebay so now the wait begins. I'll post some pics of the build progress when they show up. Huge thanks to you both!

Explorer45

Re: Need help understanding my footswitch
« Reply #5 on: December 30, 2012, 07:54:56 PM »
I realized I wanted to add a third button which will engage both loops so I can use the OD on the clean channel without having to rewire the pedalboard. This wasn't quite as easy as I thought since I want another LED for that switch. I would have wanted a 4pdt to completely isolate the LED but couldn't find any. Will the DC source interfere with the signal in the node I've pointed to in the schematic below?



Is there an easier way to do this or will this work?

earthtonesaudio

Re: Need help understanding my footswitch
« Reply #6 on: December 30, 2012, 08:57:15 PM »
The DC will interfere, yes.  But beyond that there are errors.  In the current state, guitar is going nowhere (muted).  Is that what you mean by bypass?  I'm assuming not.
You also have the inputs and outputs of your clean loop shorting together.


Could you draw what you want to accomplish as clearly as possible?  Maybe then a schematic will be easier to make.

Explorer45

Re: Need help understanding my footswitch
« Reply #7 on: December 31, 2012, 06:23:56 AM »
Hm, will be tricky to get a LED in there then. Why would the guitar be muted? With the first switch in it's current position it would continue to the loops and if switched, it would go straight through to the amp input, no? I want the bypass to send the signal straight from the guitar to the amp.
I took away the LED section and it looks a lot cleaner now. What I was thinking was that when the L1+L2 switch is in down position (engaging the L1/L2 switch) the wire going down from the clean input will be disconnected by the L1+L2 switch and therefore the signal will continue through the selected loop and to the amp input, no?



Have no idea how I'm going to implement a third LED though that activates by the L1+L2 switch though..

slacker

Re: Need help understanding my footswitch
« Reply #8 on: December 31, 2012, 09:12:01 AM »
Your bypass switch is fine, I think you have a mistake with the other switches though. I'm assuming what's suppose to happen as it's drawn is the signal goes through Loop 2 from left to right and then though Loop 1 from right to left, ending up on pin 7 of L1+L2 and then off to the amp, so you have both loops in series, this looks fine. When you switch L1+L2 to the other mode though, with L1/L2 as drawn the signal goes though Loop 2 and then to the amp, this looks fine (the input of Loop 1 is also connected but we won't worry about that for now).
When you try and send the signal through Loop 1 though you're going through it from left to right , so backwards compared to how you did it in the other mode, this won't work, or the other mode is wrong and that won't work.

Apologies if I've missread the schematic, and all that makes no sense.

earthtonesaudio

Re: Need help understanding my footswitch
« Reply #9 on: December 31, 2012, 04:55:57 PM »
Assuming this:
Quote
I realized I wanted to add a third button which will engage both loops so I can use the OD on the clean channel without having to rewire the pedalboard.
...is what you still want, your requirements are:

1. choose amp channel
2. choose either/both/neither loop

And to my mind this means you want 3 switches.  One for the amp channel, that's a given.  But the other two have to be individual loop bypasses in order to get the either/both/neither options.

So then your switch array would look like:

guitarIn---Loop1/bypassLoop1---Loop2/bypassLoop2---ampIn
ampChannelSwitch----(footswitchJack)

Basically that's just two bypass loops in series with your input, and a separate section for your amp's channel switcher.  This could all go in one enclosure and it would have 7 jacks, 3 switches, and 3 LEDs.

slacker

Re: Need help understanding my footswitch
« Reply #10 on: January 01, 2013, 06:55:23 AM »
Hope you don't mind me butchering your schematic, but I think this will do what you want, where lines cross they don't connect.



As drawn it's in both mode, signal goes from guitar >> loop2 >> loop1 >> amp, pressing L1/L2 will change channel on the amp without affecting the loops.
Pressing L1+L2 will put it in either mode, and L1/L2 will then give Loop 2 with one amp channel and Loop 1 with the other.
Bypass just bypasses the whole thing, L1/L2 will still switch the amp.
I can't think of a way to get you an LED on L1+L2 without using a 4PDT, you can buy these as footswitches.

Explorer45

Re: Need help understanding my footswitch
« Reply #11 on: January 01, 2013, 01:07:42 PM »
Thanks guys. I think I managed to solve it without adding a 4pdt though.



Can't wait for the components to arrive so I can start building. :)
« Last Edit: January 01, 2013, 02:11:42 PM by Explorer45 »

slacker

Re: Re: Need help understanding my footswitch
« Reply #12 on: January 01, 2013, 01:25:23 PM »
That's almost there, except in bypass you have the guitar connected to the output of loop 1, this won't work. The output will have a low impedance, which will basically short your guitar to ground, giving little or no signal. If you use the bypass wiring from my idea, that will solve that problem. Evetything else looks good, nice work getting the LED in there.

Edit: actually I think it won't work, just noticed in L1/L2 mode you have the outputs of the loops connected together, you can't do that, for the reason given above.

Explorer45

Re: Need help understanding my footswitch
« Reply #13 on: January 01, 2013, 02:12:46 PM »
Crap. Oh well, back to the drawingboard!

Explorer45

Re: Need help understanding my footswitch
« Reply #14 on: January 08, 2013, 12:50:00 PM »
Haven't had the time to work much on the pedal of late. I started to rewire but no matter how I try I can't get it to work even with a 4pdt switch.



The low impedance on the output you were talking about, wouldn't the signal just see the an open cable and pass right through? Where would the signal go?

teemuk

Re: Need help understanding my footswitch
« Reply #15 on: January 08, 2013, 05:46:41 PM »
The logic is a flip flop that reacts to switching pulse and changes state.

Depending on state, there is no voltage difference or there is a voltage difference between the switch terminals FS1 and FS2.

Outputs S1 - S5 are control nodes for FET switches.


http://oi50.tinypic.com/o9pwf7.jpg
« Last Edit: January 08, 2013, 05:50:40 PM by teemuk »

earthtonesaudio

Re: Need help understanding my footswitch
« Reply #16 on: January 10, 2013, 08:44:15 PM »
I was thinking something like this