Elenco Snap Circuits SOUND &reg - Manual (Page 61)

The color LED actually contains separate red, green,
and blue LED controlled by a microcircuit. It is designed
for use with a stable voltage (like the batteries); when
used with the keyboard output (a changing voltage
intended to produce sound on the speaker), it gets
confused and blurs its pattern. The result appears white
because mixing equal amounts of red, green, and blue
light makes white light.
Project 114 Red to White
RV3 controls the voltage
to the color LED, through
transistor Q2. When the
voltage is low, the color
LED only produces red,
since that is the easiest
color for it to produce. As
the voltage increases,
green light is added, then
blue.
Build the circuit and turn the slide switch (S1). Press any key on the
keyboard (U26), but just one key at a time. The color LED appears
white, and isn’t changing colors like it normally does. If you look closely
at the color LED you can see separate red, green, and blue lights on it,
which combine to produce white. This is best seen in a dark room. You
can also view it with the egg attachment on the color LED, which helps
to blend the LED colors together.
Use the preceding circuit, but replace the 5.1k W resistor (R3) with
the 500k W adjustable resistor (RV3). Press any key on the keyboard
(U26), but just one key at a time. Slowly turn the knob on RV3 from
right to left across its range while watching the color LED (D8)
closely. Notice how first red gets bright, then green too, then also
blue. This is best seen in a dark room. You can also try it with the
egg attachment on the color LED.
Build the circuit with the black jumper wire connected as shown, and turn it on.
Nothing happens. Disconnect the jumper wire and an alarm sounds.
Project 113 White Light
Project 115 Alarm
You could replace
the jumper wire with
a longer wire and
run it across a
doorway to signal an
alarm when some-
one enters.
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