From:
         
          MBSetzer   (QualityAna@aol.com)
    Date:
             4/21/2002 5:44 PM
 Subject:
          Re: Making a Footswitch



Looking at the separate footswitch part of the drawing, I can't read the
numbers very well but I think I know what to do.
 
Actually the pedal switches the relay (plus red LED, and further controls the
transistor) and the transistor switches the green LED.
 
When the footswitch makes contact, you are solidly grounding the red LED as
well as the relay coil in series with it, allowing enough current to pass from the
*B+* at the top through the horizontal resistor, through both the relay coil &
the red LED to energize both.
 
The cap is there to quench spikes when the relay coil is de-energized.
 
The red LED & coil were grounded before but it was through the relatively
high resistance pair of series resistors around the base of the transistor, and that
would not let enough current flow to activate the relay or light the red LED.
 
However, when that relatively small current was flowing through the base
resistors, it was serving to bias the transistor on which allowed enough current
to pass through the 470K(?) and the green LED to light it while the red one is
off.
 
When the footswitch makes contact, it is so much easier for the current to go
straight through it to ground, nothing more passes through the biasing resistors
and the transistor shuts off the green light.
 
I say simply try wiring the LED in your pedal so it is in series with the switch,
with cathode (flat side) toward ground.
 
There is definitely enough current limiting to protect your pedal diode, since the
horizontal resistor and relay coil in series are already adequate to protect the
panel-mounted red LED from frying.
 
There could be a problem if adding the new LED in series resulted in them both
running a little dim. In that case you could use a higher brightness LED in the
pedal (keep in mind a more narrow viewing angle) and/or slightly reducing the
value of that horizontal resistor (still can't read its value) to let more current
flow, maybe by just paralleling it with something else. You wouldn't want to let
too much current flow through LED's though, usually no more than 10 or
20milliamps.
 
Before changing anything, you could measure the voltage drop across that
horizontal resistor when the red LED is on, and divide that difference by its
resistance to see how many amps (multiply them by 1000 to get milliamps or
just remember that 0.010amps is 10milliamps) are passing through it at the
time. That same amount of current is naturally also passing through the relay
coil and the red LED on its way through the footswitch to ground. So now you
would know the milliamps that are originally being used to light the LED and
energize the relay, in case you want to compare or adjust things after you have
wired the second LED in series.
 
Hope this helps,
Mike