Elenco 130-in-1 Electronics Playground - Manual (Page 11)

-11-
I. PLAYGROUND OF ELECTRONIC CIRCUITS
-150-
This oscillator circuit produces a sine wave signal. A
sine wave (or sinusoid) is a wave of pure single-
frequency tone. As an example, a 400Hz sine wave
is a wave that oscillates 400 cycles in one second
and contains no other frequency contents. Non-sine
waves (such as square wave or triangular wave
signals) have harmonics - waves with frequencies
that are multiples of the single-frequency
fundamental wave. A non-sine 400Hz wave can
include the 400Hz wave (its fundamental wave)
along with an 800Hz wave (its 2nd harmonic wave)
and a 1200Hz wave (its third harmonic wave). A
square wave signal contains many harmonics.
Experienced technicians can test a circuit by putting
a sine wave into it and listening to its output - and
you can too. If you put in a sine wave, and something
else comes out, the undesired harmonic frequencies
must have been generated somewhere in the circuit.
The key parts of this circuit that produce a 400Hz
sine are:
• A 0.1
μF capacitor connected across terminals
3 and 5 of the transformer. This forms a tank
circuit that resonates at about 600Hz.
• A 470k
Ω resistor to turn on the base of the
transistor only a little.
• An adjustable feedback circuit that includes the
control and the 0.05
μF capacitor.
• A 100
Ω resistor connected to the emitter. This
helps to stabilize the circuit and keep the sound
from being distorted.
Connect the earphone to terminals 1 and 2 of the
transformer. Start with the control on maximum (10
on the dial) and slowly decrease the control setting
while listening to the tone quality of the output. Before
the oscillations stop, you will reach a point where you
hear only one tone. This last clear-sounding tone is
the sine wave. Repeat these control adjustments until
you have no trouble distinguishing between a sine
wave and a distorted wave.
Notes:
EXPERIMENT #128: SINE WAVE OSCILLATOR
Schematic
Wiring Sequence:
o 1-EARPHONE
o 2-EARPHONE
o 3-28-109
o 4-94-106-124
o 5-41-110
o 26-40-93
o 27-105
o 42-71
o 72-119
o 121-122