In this chapter we have described the operating principle of the Voltage-controlled Oscillator (VCO). The VCO produces a pulse-train having a frequency that is proportional to the voltage applied to its input. By measuring the frequency (or period as we did) the voltage/frequency relationship can be used to generate a measurement of voltage. In this way the VCO can be used as a simple and inexpensive alternative to an analog-to-digital converter.
A new object class named VCO was developed using the ParallelPort as the base class. Software methods have been described to continuously check the level of an incoming digital signal while incrementing a counter, and thereby measure the period of the waveform. Graphics programming was introduced to display the resulting waveform, followed by the development of a program that uses the thermistor on the interface board with the VCO to measure the actual temperature.