LED's for the footswitch

[dobbhill]

Is there a simple way to add LED's to my 2-button footswitch that I use to actuate the OD and PAB relays in my clone? I have a 5vdc regulated supply tapped from the 6.3vac filament circuit that runs the relays. I switch them on by grounding through the footswitch. I know nothing about how to wire up LED's at all.
Thanks in advance.
D

[ElectronAvalanche]

You can easily just put the LED (right orientation though) between the wire going to the switch and the switch. If your switch grounds the relay (thus activates it), then the LED will illuminate.

Make sure to use a low mA LED and maybe you might need some sort of series resistor.

Maybe the LED will rob some voltage and your Relays will not activate. Then you will have to increase your supply voltage a bit.

Hope this helps!

Electron

[dobbhill]

Thanks. If I put the LED in series with the switch as you mentioned, what voltage LED should I use? Low "ma" makes sense. My supply is regulated 5vdc, so I probably have some headroom there.
D

[Luthierwnc]

I have done or seen this done a couple of ways. There was a Marshall channel switching link that is still around that shows the LED going to ground in parallel with a small resistor -- I think it was 47 ohms. That way the LED only pulls as much juice as it needs. Use Ohm's law to get the right resistance for your diodes. LEDs are pretty touchy if you give them too much voltage and you also don't want them to be the voltage regulator. With two of them on at once and only 5 volts to play with, you need to make sure the relays get enough volts. They pull some voltage too.

Glasman posted a footswitch on his website that uses a zener diode at the end of the chain. I haven't tried this but intend to.

Finally, you can just use a double throw switch and hook the LED up to a 9 volt battery in the footswitch box. Check the Fulltone site for his switching box to get an idea of the resistors to use with the LEDs. If you use a powered pedal board you can also add a 9v jack to the box. Mouser and Radio Shack sell a 2.1mm shorting jack that works fine. You have the battery hot go to the shorting jack blade so when you are plugged in to the pedal board supply, it saves your battery.

Skip

[Bob-I]

For a 6V supply use a 220 ohm resistor in series. No need for anything more complex.

[BobW]

LEDs (red) typically drop about 2 volts, Green about 3 volts, Yellow about 2.2 volts, (Vf) the forward voltage required to turn it on. Typically 10 to 15mA (15mA preferred) is what is needed to drive an LED into sufficient illumination to overcome stage lights bleedover, etc. If you place an led in series with a coil, make sure the coil can turn on with as little as 15mA, and can stand the Vf voltage drop of the LED (2, 2,2 or 3 V), otherwise the relay coil won't have enough current or voltage to work. Preferably placing an LED in parallel with the coil is a wiser choice. To determine the series current limiting resistor required to turn on the LED using any given voltage, use the following formula (ohms law:

R = [DC supply voltage- (Vforward voltage of the LED, 2,2.2, or 3)] / 15mA

Once you have the resistor calculated, determine the power rating required for the resistor by the following:

Pwr resistor rating = (15ma)squared x resistor value (ohms).

I find Digikey has a lot of LEDs available and the provide the Vf value for each LED. If you want to use a white LED, then just look up ther Vf value and plug into the formula above.
hth and wasn't too trivial.

[dobbhill]

Thanks, Bob. That's exactly the info I needed. I can build tube amps, but I can't design LED light bulbs!!!
D

[Luthierwnc]

Without adding too much confusion, here is a link to the circuit I mentioned above. This is some very cool information on Marshall tweaking so it has a lot of merit beyond the pedal idea. If you look at the footswitch, you see that the relay is grounded through an LED across a small resistor. The resistance is calculated to make the LED draw enough to light up but still let the relay ground.

I am a little nervous about leaving the overdrive (and/or whatever else is switchable) in series with something as fragile as a commercial LED. Let's face it: a lot of the builds on these pages weigh 30 pounds with triple bullet-proofing. Having it all rely on a 78 cent part used to make clock radios doesn't hold; especially considering how perilous a stage floor can be. Even if the led fails, the switch will work.

Good luck, Skip

http://www.lynx.bc.ca/~jc/marshallAmps.html