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Thread: LED question

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    Old Timer daz's Avatar
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    LED question

    I just got a new modeler, a katana to be exact. And it has a footswitch jack that can be used with any latching switches to do two functions. It's TRS so the ground is common for both functions and i wanted to use LEDs without having to rig them up with DPDT switched to a battery. So i checked the voltage between the ground and the tip and ring and both are 2.4vdc which should light a standard LED. I have maybe 15 LEDs laying around all used some of which are likely dead but i know some work and most are standard LEDs and few if any would be higher voltage than typical. But while connecting them does indeed cause the switching function to work, they won't light up. One blinking one did, but it didn't cause the amp to switch. But none of the others lit at all and i know most of them are good. Any idea what might work or why it won't?

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    don't forget the joker g1's Avatar
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    If they are latching switches, then any voltage present will be shorted to ground when the switch is engaged.

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    Old Timer daz's Avatar
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    Quote Originally Posted by g1 View Post
    If they are latching switches, then any voltage present will be shorted to ground when the switch is engaged.
    Well, what i meant was the ground lug of the switch which i assume is the negative of the function's PSU. Tip and ring are to the two different functions.

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    Lifetime Member Enzo's Avatar
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    They input to something. You may have 2.4v there , but at what current?

    I suspect when you connect the LED, it drags the voltage down enough to switch the function, but can't draw enough current to light the lamp.

    Monitor the tip or ring voltage and connect an LED, what is the result when it is there?
    I do not know what the input circuit is, but usually a input port to logic has a "pullup resistor". That might be a 10k resistor. At a dead short to ground that would allow half a milliamp from 5v.

    Set your meter to current, and connect it from tip or ring to ground. Read how many milliamps current the thing can source - your meter will look like a short circuit to the system - same as a closed switch.

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    Even if there is enough current to light the LED it still needs a series resistor or the voltage will be clamped at the LED Vf which could prevent the logic from switching.

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    Last edited by Dave H; 03-21-2018 at 11:16 AM. Reason: clarity

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    Supporting Member eschertron's Avatar
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    Just to add: if the input's logic HI is 2.4vdc, then the logic LO is going to be well below the forward voltage drop of any LED. You will most likely need external power for your indicators.

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    Old Timer daz's Avatar
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    Thanks all, but nevermind. I just found out while reading the forums for it that theres a silent delay between patches when switching, so this will be useless for me.

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