84 Nicolas Collins
cassette/CD, and confirm that audio passes through. Take the whole rat’s nest
into a dark place. The sound should get much quieter. Turn on the flashlight
and pass the beam across the photocell -- the sound should get louder when the
cell is lit, quieter when the cell is dark. This circuit won’t shut off the sound
completely, but you should hear a significant volume difference between light
and dark. The back side of a photocell is usually translucent, so total darkness
can only be achieved if you fully enclose the cell – in your hand for example, or
you can cover the back with black electrical tape. Make sure you don’t let the
leads short out (you should spread them apart before applying the tape.)
You can increase the dynamic range of this circuit (the difference in loudness
between “on” and “off”) by adding a resistor of about 10kOhm between the
output signal and ground, as shown below. Without getting into unnecessary
technical detail, the resistor “clamps” the output to ground when the circuit is
“off”, minimizing bleed-through of the input signal, and increasing the depth of
the muting.
Our second photocell circuit, shown below, can be used in two ways:
Connect one audio source to points “A” and “B” (use clip leads); connect “C”
to the input of one amplifier, and “D” to the input of another. When you pass a
flashlight across the two photocells, the sound should pan from one speaker to
the other.
Connect one audio source (ie, a CD) to jack “A,” and a different one (cassette)
to jack “B;” jumper “C” and “D” together to the input of one amplifier. When
you pass a flashlight across the two photocells you should be able to mix
between the two sources (like cutting between turntables.)