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Thread: hooking up a LED to a digital control circuit without affecting function?

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    Old Timer daz's Avatar
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    hooking up a LED to a digital control circuit without affecting function?

    This concerns a digital footswitch and a modeling amp. I posted about this before but what i thought was success in connecting it directly to the 3.5v supply from the footswitch cable was not successful after all. I found that it would work most of the time but at times it would freeze the switching functions. Removing it cured that issue. Is there a way to hook it up so that it's isolated from the control voltage in a way that will allow me to use it w/o messing with the switch's functions??

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    Bent Member Chuck H's Avatar
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    Well you don't really want to hook it up direct anyhow. The LED shouldn't need 3.5V (unless you're using blue?). And the LED doesn't draw much current so you should be able to run it through a high resistance voltage divider (like 470k series, 1M load) and still get it to illuminate. That might mitigate it's interaction with the circuit.

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    "I'm just going to perform a bit more scientific investigation, turn it up to 11 and rip of the knob." überfuzz

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    Old Timer daz's Avatar
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    Thanks, i'll give that a shot and see if it works. I won't necassarily know for a while whether it interferes with the switching functions since that tends to happen only a few % of the time, but if it lights and the switch is working right off the bat i think that should isolate it. I tried several LEDs tho and 2 of them stopped the switch from working art all so that that may happen.

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    Old Timer daz's Avatar
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    wouldn't light, but it;s a flasher.Maybe a non flasher works but not this one? I'll have to try it later when i have time.

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    Bent Member Chuck H's Avatar
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    Well I just looked at an on line LED calculator and it seems that to get an appropriate current through the LED (I'm assuming red) you'll only use a 100R resistor in series and the LED. The voltage divider I suggested above probably won't illuminate the LED, but I don't know. So... Perhaps there is hash in the circuit that is causing the switch function to fail. You might try the 100r resistor in series with the LED and put something like a .1uf capacitor in parallel with the LED.

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    "I'm just going to perform a bit more scientific investigation, turn it up to 11 and rip of the knob." überfuzz

    "...less ear-friendly but handy for jazz." Leo_Gnardo

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    "If you build it, it will hum..." Justin Thomas

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    Lifetime Member Enzo's Avatar
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    It is about the circuit, not the parts. I don't know what you mean by digital footswitch. You actuate it with your toes - your foot digits?

    For example, a switch to ground operates some circuit, so you add an LED. That LED now acts as a pullup for the switch node. That may or may not interfere. Do the switches even reference ground? Not all do in digital control systems.

    Are these stomp switches? As in push on push off. Contrast to momentary switches, which do not latch. The circuit they are in may latch electrically, but not the switches themselves.

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    Old Timer daz's Avatar
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    fender mustang 4 button. You should know those being a fender tech. digital amp, footswitch controller with 4 momentary circuit board mounted momentary switches with metal spring loaded plungers as the foot switches who's functions are are assigned and configured in the amp.

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    Old Timer daz's Avatar
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    Well it works with a 100R in series and so far no switch glitching. But it will take time before i'm sure. Last time it started after a couple hours. I tried it for about 15 minutes so we shall see. I'd think the 100R would at least isolated it a bit and since it only did it now and then maybe thats all it'll take.

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