Nuts and Volts - February 2018

Mims Circuit 14. Using Range

1 allows the frequency to be adjusted within a useful audio range. Keep in mind that with the PIC emulating a one-shot, the pulse width — or period — is the parameter being controlled. This means that a small change in control voltage at the high frequency end (lower voltages) will yield a large change in frequency.

The narrowest pulse width is about 30 µs, giving a period of about 60 µs ( 16. 66 kHz), with a resolution of 10 µs. The next lower frequency is about 12 kHz, period = about 82 µs. Note that the period resolution is double the pulse width resolution.

Another solution for a PIC based audio oscillator is the NCO discussed in Part 3 of this series (and shown here in Schematic 5). Although the schematic is for a toy organ (discussed next), you can replace the resistor network on pin 5 with a simple divider or potentiometer.

Mims Circuit 15

Circuit 15 (Schematic 4) shows the 555 being utilized as a toy organ using different value capacitors to develop the different frequencies. With all switches open, the output will be high. As soon as one (or more) of the switches is closed, the 555 will start to oscillate.

Schematic 5 shows an easy way to implement a toy organ using a PIC NCO with the program NCO_Toy_Organ.asm (available with the downloads). The purpose of the diodes is to enable the output by grounding the gate when the associated switch is closed. Range 0 for the NCO is the most appropriate for an audio application: 7. 5 Hz to about 7. 5 kHz. If you want a higher maximum frequency but still stay within the audio range, you can modify the program so that the A/D value is doubled. This will make the lowest frequency and step size about 15 Hz, and the highest frequency about 15 kHz.