BY FRANK REISER
the frequency-varying components — C6 and R2. R3 ( 4.7K),
connects the circuitry of this chip to the sine wave circuitry.
Ground is common to all parts of the circuit, of course.
For the sine wave portion, an LM386 operational amplifier is used as a non-inverting (negative-input grounded,
positive input used), integrating-gain amplifier configured to
produce and amplify self-produced sine wave oscillations of
a frequency of about 50 kHz to 100 kHz. Pin 5 is the sine
wave output and pin 6 is the rectified voltage input. Gain
feedback is adjusted via R4, a 5K potentiometer. C8
prevents DC from contaminating the sine wave output.
Feedback after this capacitor goes to pin 3 (+ input) through
R5 and R6, which provide 11K resistance. The amplitude of
the sine wave is varied by R7, a 100K potentiometer, and
fed back to pin 2, which is the negative input. DC isolation
and chip bias are provided by the 0.47 μF capacitor C9.
Under test, I have noticed that the frequency range
varies by about 10% between different circuits that I
have built. The sine wave tends to have a small degree of
distortion. The triangle wave is sharp and crisp, as well
as the square wave. The square wave is susceptible to
distortion when capacitively loaded by an oscilloscope.
I suggest calibrating this function generator with an
oscilloscope and noting the degree of any distortion which,
unfortunately, tends to creep around
the circuit a bit. NV
PARTS
LIST
All resistors are 1/8 unless
otherwise noted
ITEM DESCRIPTION
❑ R1 2K
❑ R2,R6 5K pot
❑ R3 30K
❑ R4 10K
❑ R5 1K
❑ R7 4.7K
❑ R8 1M pot
❑ C1 . 15 μF electrolytic
❑ C2 0.47 μF electrolytic
❑ C3 33 μF electrolytic
❑ C4 0.01 μF
❑ C5 4,700 pF
❑ U1 LM386 op-amp
❑ U2 LM555C timer
❑ D1 Bridge rectifier
(RadioShack 276-1181)
LED
120V/12V transformer
500 uH inductor
1A 115VAC fuse
SPST switch
❑ D2
❑ T1
❑ L1
❑ F1
❑ SW1
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