The Audio Theory
Audio signals are no real mystery. They've been around and understood for far longer than VGA has so I'll keep the theory on it short and sweet. All we will be doing is sending out a single frequency tone. We'll be able to vary the tone from the lower end of audio frequencies 70 Hz to the higher end at 14 KHz using a trimpot. The signal that will be generated should maintain a duty cycle of 40-60% in order for the tone to be constant and audible.
555 Timer Signal Generation
Creating monostable output with the 555 Timer is as simple as choosing the proper R1, R2 and C1 seen in the above schematic. The formula seen below determines the final frequency of the monostable output that we'll use for the audio output signal. Try plugging in some numbers and see what frequency you get out.T1 represents the time the signal is at +0v and T2 when the signal is at +5v (unless I have that backwards..?).
Closer Look At Single Tone Audio
The duty cycle (on/off) of the signal is also going to determine how audible the signal is and will become a concern for us because, one look at the graph above and you can see how the output signal generated moves out of the 40-60% desireable range as the 100k trimpot is varied. While it does approach 90% and above, this only occurs at the frequency range above 5 KHz which you won't want to hear consistently for very long anyway, it's ear piercing and annoying.
Audio signals are no real mystery. They've been around and understood for far longer than VGA has so I'll keep the theory on it short and sweet. All we will be doing is sending out a single frequency tone. We'll be able to vary the tone from the lower end of audio frequencies 70 Hz to the higher end at 14 KHz using a trimpot. The signal that will be generated should maintain a duty cycle of 40-60% in order for the tone to be constant and audible.
Creating monostable output with the 555 Timer is as simple as choosing the proper R1, R2 and C1 seen in the above schematic. The formula seen below determines the final frequency of the monostable output that we'll use for the audio output signal. Try plugging in some numbers and see what frequency you get out.T1 represents the time the signal is at +0v and T2 when the signal is at +5v (unless I have that backwards..?).
The duty cycle (on/off) of the signal is also going to determine how audible the signal is and will become a concern for us because, one look at the graph above and you can see how the output signal generated moves out of the 40-60% desireable range as the 100k trimpot is varied. While it does approach 90% and above, this only occurs at the frequency range above 5 KHz which you won't want to hear consistently for very long anyway, it's ear piercing and annoying.