I have an audio mixer with a mono jack output which enables me to turn off the reverb between songs when I just want to speak to the audience. I have a momentary foot switch plugged in, which just shorts the signal for however long you push down on the switch. It basically toggles between two states internally in the mixer. When the reverb is off, there is an LED indicator that switches on. The problem is that the indicator on the mixer isn't always visible from the stage, so it's difficult to tell if the reverb is on or off. I've been wanting to add an LED or two to the switch itself. I've discovered that there is a 5v signal coming out of the mixer into the switch. When the switch shorts the signal, the reverb goes on or off on the mixer. I have an extra CD40106 hex Schmitt trigger that I thought might be useful, but since it needs a power supply that's always on, I guess I won't be able to use it as a toggle switch in this case. The circuit inside the mixer is able to toggle using a 5v signal that is momentarily shorted. Is there an easy way to do this electronically? It's also a very small foot switch with not a lot of room for electronics and no room for a battery.

 

How much current is drawn from the 5V signal when the switch shorts it to ground? If it's more than enough to light a LED then you might be able to use that signal to charge a capacitor and use as the power source for a latch plus LED. However, keeping the pedal latch in sync with the mixer latch/toggle could be problematical.
Couldn't you use a slave LED fed from the mixer LED? Only one more wire pair to trip over on stage .

 

How much current is drawn from the 5V signal when the switch shorts it to ground? If it's more than enough to light a LED then you might be able to use that signal to charge a capacitor and use as the power source for a latch plus LED. However, keeping the pedal latch in sync with the mixer latch/toggle could be problematical.
Couldn't you use a slave LED fed from the mixer LED? Only one more wire pair to trip over on stage .

I don't think the signal is shorted to ground by the foot switch. I think the 5v is interrupted by the switch until I press it. Then the mixer LED circuit sees a 5v pulse and the signal is interrupted again when the momentary switch is reopened. The 5v signal from the mixer is enough to light both LEDs at the same time. The problem is that there's always 5v going through the switch except for the 1/2 second where the switch is shorted to signal the mixer to turn it's LED on or off. If I just hook up an LED to the switch, it's always on except for the moment when the switch is activated, then it's on again. The mixer LED will be either on or off but the switch LED will practically always be on. I was also thinking about using a capacitor but, I'm afraid that might also cancel the signal that the mixer needs to change it's LED state. I don't think there's an easy way to patch into the mixer LED (after the on or off state is determined by it's electronic circuit). For that, I would have to take the mixer apart, which I don't want to do. I could squeeze one AAA battery into the foot switch, so my idea now is to make joule thief to power the LED in the switch, use a mosfet as a latching switch and include a debouncing circuit. I don't know if all that will fit in. Also, I'd have to always synchronize the on off state with the mixer LED before each concert. I was hoping to find a more practical idea that would save all that extra hassle.

 

I'd have to always synchronize the on off state with the mixer LED before each concert

Does the reverb always come up in the same state when power is applied, or does it come up randomly, or does it come up in the same state as when the power was removed?
If it always comes up in a given state, then you could use power-up to also reset the latch in your foot switch to the same state.