Hi,
Scead:
I take it you dont use the 555 much at all
When you say you are "not particularly good with 555's" you may want to note that they just contain a flip flop and two comparators really.
Suraj:
The "CV" terminal of the 555 is there to allow an external signal to modulate the pulse width of the output. This works in either astable or monostable circuit configurations.
The pin connects to the internal voltage divider that sets up the trip points for the internal comparators and is approximately Vcc*2/3 (two thirds of Vcc) without any external connection.
Because the divider voltage is internally set to Vcc*2/3, if you apply an external voltage of Vcc*2/3 then the pulse width is not changed, but if you increase the CV voltage the pulse width gets wider, and if you decrease the CV voltage the pulse width gets shorter. This is the pulse width directly out of the 555 so if you use a transistor inverter the output low state pulse width will change instead.
We could do a more exact analysis of this if you think you'd like to see more detail on how the control voltage really affects the output timing. The main idea is that instead of the cap charging between the two normal voltages of Vcc/3 and Vcc*2/3 it charges between two different levels depending on your CV input voltage.
One drawback to using the CV input voltage is that you loose the ratiometric timing inherent in the basic design of the 555. The 555 output timing is stable with changes to Vcc, but using the CV input changes that because the timing depends not only on Vcc but also the absolute CV voltage level which does not track Vcc, unless of course you care to design it that way. Most CV applications use the CV terminal directly with some modulating voltage that does not track Vcc though.