Stec Records - EH16 Switch Page
Electro Harmonix came up with this very clever scheme for reading the
switches on the unit itself and also for a remote foot controller. You
start with a power supply voltage. Each switch is connected to a resistor
connected to a summing node at the input of an A-D converter. The other
side of the switches are connected to ground. As each button is pressed
the voltage at the input to the converter changes relative to it's resistor
value. The clever part is that they worked out the resistances so that
the each switch changed the voltage just enough to toggle a specific bit
of the converter output. It seems like a lot of trouble for nothing but
it allows you to have a bunch of switches on the remote with only a 1/4" guitar cable connecting them. Very cool. No special cables. I like it.
A bitch to troubleshoot though. If you are pressing a switch on the main unit
and the corrisponding AD output is not toggling, chances are the AD is bad (assuming
you have already checked the supplies). Luckily the AD is an ADC0804 which is
fairly easy to find (see Parts).