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USB footswitch help...

Ben Mathers

May 13, 2015
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Hello!

Just wondering if anyone here would be able to help me. I bought a USB footswitch which I wanted for parts for another USB footswitch I'm making (with a metal momentary switch in a wooden box). I had thought that the switch I bought would have some kind of mechanical switching mechanism, in fact I believe it uses an infrared switch, I have uploaded some pictures. What I am wondering is whether there is a way I can bypass the infrared switch to connect up to my momentary footswitch (this sort of thing: http://goo.gl/5cb556), or will this just fry the usb switch?

Any help would be much appreciated!

Many thanks,

BenIMG_20150513_084706_739.jpg IMG_20150513_084717_668.jpg
 

Harald Kapp

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Welcome to electronicspoint.

This circuit seems to operate by interrupting a light beam, thus creating the signal for the controller chip under the black blob.
One way to substitue a mechanical switch is by using a normally closed switch (opens contacts when actuated):
  • Remove the LED (the clear plastic component near the mounting hole). Note the orientation of the LED as you will have to put it back in correctly later.
  • Bend one wire of the LED to the side (I suggest you use the one that is located away from the edge of the PCB).
  • Connect a wire to the bent connection of the LED.
  • Put the other wire of the LED back into the PCB and re-solder it. Observe the correct alignment to the receiver. You may want to fix the LED with some glue to the PCB.
  • Solder a second wire to the now empty pad in the PCB (where the other leg of the LED used to be).
  • Connect both wires to the normally closed switch.
The way this is supposed to work is that by interrupting the current through the LED you simulate interrupting the light beam.

This should be all.
 

Ben Mathers

May 13, 2015
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Hi Harold,

Thanks for your reply, that makes perfect sense. Only problem is that I can't seem to find any normally closed foot switches... Is there any way to invert a normally open switch without getting over complicated??

Thanks,

Ben
 

Ben Mathers

May 13, 2015
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Just thinking about it I suppose I could use a DPDT momentary footswitch? I've just created a really terrible diagram from a DPDT switch schematic. Would this work or have I missed something really obvious?

Thanks again.

Ben
 

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Harald Kapp

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You can use any of these switches as long as they have at least one normally closed contact.

An alternative is a normally open contact that is connected in parallel to the two LED leads. That works by short-circuiting the LED when the contact is closed. The current is bypassed from the LED, the ED is off, same effect as interrupting the light beam.
The only issue is whether the curent limiting resistor is able to take the additional power - I'd expect no issue there.
 

Ben Mathers

May 13, 2015
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OK great, yes perhaps I will try that then since I already have the right switch to do that. So they are both LEDs then are they? and it doesn't matter which I use to short circuit?

Thanks again.

Ben
 

Harald Kapp

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So they are both LEDs then are they? and it doesn't matter which I use to short circuit?
No, they aren't. The one I told you to de-solder is an LED. You can see that because in your first image this part is between Vcc and GND only current-liited by R8.
The other one, more to the center of the PCB is a photodiode (receiver). It is conectd to the circuit under the blob which wil regenerate a clear digital signal from the weak signal of the receiver.

Therefore it does matter which one you short circuit.
 

Ben Mathers

May 13, 2015
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Ah ha, sorry yes I see, great, thanks very much for your help. I will give it a go and let you know how I get on.

Best wishes,

Ben
 
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