Voltage Controlled Oscillator (VCO) Project

The Voltage Controlled Oscillator (VCO) project was initiated by Dr. Dan Boye for use when demonstrating the properties of sound to his students.  I built 2 separate prototypes over the Summer of 2003 and finished the final version shown here in October of 2003.

Dr. Boye's requirement for the VCO was that it produced 5 distinct types of audio signal waveforms:  square, sawtooth, triangle, sine and a pulsed sine wave that had a duty cycle of 3 seconds on and 3 seconds off.  Additionally, he required a variable frequency adjustment of 500 Hz to 5Khz with a center frequency of 1 KHz.  An external induction coil was also specified so that he could demonstrate the effects of using a magnet, with the induction coil, to produce a voltage that would translate to a frequency shift resulting in a change in the audio output out of the speaker.

The main component is a MAX038 chip that is made by Dallas Semiconductor.  A W172DIP-141 reed relay, LM555 timer and LM386 audio amplifier were also used.

             VCO with external induction coil and magnet.

The principle of operation for the VCO is simple.  The VCO is powered off of two nine volt batteries connected in series.  Circuit common is tapped off of the junction between the two batteries which results in +9VDC and +9VDC supplies for the VCO.  The power supplies are regulated to +5VDC and -5VDC supplies that are used by the MAX038 chip.  The unregulated +9VDC supply is used by LM386 audio amplifier and LM555 timer chip.  The W172DIP-141 reed relay chip also uses the +5VDC supply for its coil voltage.

By means of the 5 position rotary switch, the proper audio signal waveform is produced by the MAX038.  This is done by toggling either a +5VDC or common to two inputs on the MAX038.  The signal from the MAX038 passes through the reed relay chip, then to the LM386 where it is amplified and finally sent to the speaker for output.

If the pulsed sine wave option is selected (3 second on and 3 second off audio output) the LM555 is enabled which produces a timing pulse that is used to toggle the reed relay. The timing pulse from the LM555 controls a switching transistor which in turn toggles the reed relay coil on and off (allowing the audio to pass at 3 second intervals).

VCO control panel close-up.

VCO inside view showing location of components.

The VCO also has a connection for an external induction coil for use in showing how magnetically induced voltages can vary the audio frequency output.  The audio output of the VCO can also be varied between 500 Hz and 5 KHz by means of a potentiometer.  To view schematics of the VCO in Adobe Acrobat format click on the links below. 

VCO Schematic 1 (MAX038 Chip & Circuitry)
VCO Schematic 2 (LM555 Timer, LM386 Audio Amp & Circuitry)
VCO Rotary Switch Schematic