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40
Pitch
OBJECTIVE: To show how to change the pitch of a sound.
Build the circuit on the left, turn it on, and vary the adjustable resistor
(RV). The frequency or pitch of the sound is changed. Pitch is the
musical profession’s word for frequency. If you’ve had music lessons, you
may remember the music scale using chords such as A3, F5, and D2 to
express the pitch of a sound. Electronics prefers the term frequency, as
in when you adjust the frequency on your radio.
Project #105
You need an AM radio for this project. Build the circuit shown but do
not turn on the slide switch (S1). Place it within a foot of your AM radio
and tune the radio frequency to the middle of the AM band (around 1000
kHz), where no other station is transmitting. Turn the volume up so
you can hear the static. Set the adjustable resistor (RV) control to the
middle setting. Turn on the slide switch and slowly tune the adjustable
capacitor (CV) until the static on the radio becomes quiet. You may
hear a whistle as you approach the proper tuning. In some cases you
may also need to set the adjustable resistor slightly off-center.
When the radio static is gone, tap on the speaker (SP) with your nger
and you should hear the sound of tapping on the radio. Now talk loudly
into the speaker (used here as a microphone) and you will hear your
voice on the radio. Set the adjustable resistor for best sound quality at
the radio.
Radio Announcer
OBJECTIVE: To hear your voice on the radio.
Project #106
Project #108 Pitch (III)
We’ve seen we can adjust the frequency by varying the resistance in the
adjustable resistor. You can also change frequency by changing the capacitance
of the circuit. Place the 0.1μF capacitor (C2) on top of the 0.02μF capacitor
(C1); notice how the sound has changed.
Remove the 0.1μF (C2) capacitor and replace the 100kΩ resistor (R5) with
the photoresistor (RP). Wave your hand up and down over the photoresistor to
change the sound. Changing the light on the photoresistor changes the circuit
resistance just like varying the adjustable resistance does.
Project #107
Pitch (II)
Note: If you have the adjustable resistor (RV) set to the right and light shining on the
photoresistor, then you may not get any sound because the total resistance is too
low for the circuit to operate.
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