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6. Optocouplers: The optocoupler is the link between the digital control side of the circuit and the
analog signal path. There are two optocouplers for each signal: the standard kit is a mono device
and uses one pair of optocouplers; the stereo upgrade adds an additional pair. Each optocoupler
consists of a light dependent resistor (LDR) and a light emitting diode (LED), sealed together inside
a piece of heat-shrink tube. The LDRs have been hand tested and performance-matched to each
other. This ensures that the volume taper is linear. If you are building the Stereo upgrade, you will
find both pairs packaged in the upgrade kit.
NOTE
: One LDR in each pair has a black mark (50). This LDR goes in OPTO1A. When you
seal this LDR inside its tubing, mark the optocoupler so you know where to install it later!
NOTE
: If you are making the Stereo upgrade, do not mix the pairs together!! Make and
install the optocouplers one pair at a time, so that you keep the matched pairs together.
50
a. First cut the heat-shrink tubing into two equal lengths of about 11/16” (18mm) (51a,b).
51a
51b
b. Insert a green 5mm LED into the end of a piece of heat-shrink tubing (52). Proper placement
of the LED in the tube is important. Slide the LED into the tube until the back of the LED is
about 5mm in from the end. The LED just needs to be far enough into the tube so the end
can be pinched fully closed around the backside of the LED (53). This will keep as much
ambient light out of the tube as possible.
52
53
