keytar neck. I had to trim about 1/8” ( 3. 2 mm) off each
side of the softpot to fit on the top half of the body.
Be careful not to cut into the traces of the pot if you
do elect to trim it. The back of the softpot comes with an
adhesive to mount it. I routed the connector ribbon into
the body by cutting a thin slot perpendicular to the upper
case half with a hack saw (Figure 6).
The width of the hacksaw blade was more than
adequate for the connector to slip into.
To wire up the softpot (Figure 7), I connected it to the
two remaining wires from the TRS pedal jack. If you’re
careful, you can snake the wires underneath the keyboard
where they are protected and out of the way. One wire
connects to the ring (ground) and the other wire hooks up
to the sleeve (AD input). Be sure to run the ground to the
minimum side of the softpot. Don’t forget to protect any
exposed terminals with shrink tubing. Also take care to
short the minimum softpot terminal to the middle/wiper
terminal so it operates correctly.
Don’t just hook the AD input to the middle wiper and
the maximum value. The wiper goes open circuit when a
finger is not applied to it. Remember the goofy behavior I
mentioned with the software? By shorting the minimum
terminal and wiper terminals together, you ensure the AD
input always has at least 10K ohms of resistance and
doesn’t see an open circuit.
I should mention the softpot has a minimum
resistance of 100 ohms, which takes care of the AD input
not liking to see less than 100 ohms on its input.
Figure 8 shows the normal softpot operation and how
January 2016 33
■ FIGURE 6. Softpot mounted on the keyboard neck.
Note the hacksaw cut made to accommodate the
softpot's flat ribbon cable terminals.
■ FIGURE 7. Softpot wiring detail. Note shrink tubing
applied to the terminals to prevent short circuits.
Also note the new red sustain wire connection to the
neck PCB at the 'over_d' location.
RB3 Keyboard Controls:
Velocity note data
MIDI program up
MIDI program down
MIDI panic (three finger
Mod wheel (pitch bend
MIDI sustain (jack input)
MIDI foot controller,
expression control (jack
MIDI start, stop, continue
1. Add a toggle switch to
select ribbon mod or pitch
2. Convert flat neck switch
into a sustain switch.
3. Add a pot on the neck
for foot controller/
■ FIGURE 8. Softpot operation. The top picture shows
how the circuit appears without a finger pressing on it:
The wiper is open circuit. The middle picture shows
that the softpot behaves like a normal potentiometer
with a finger applying pressure to it. The bottom picture shows how we short the wiper to the maximum
value softpot terminal to ensure the keyboard's AD
input always sees at least 10K ohm of resistance —
even when the finger is removed.