Thanks for the advice guys. I read in one of my textbooks that the control voltage affects the reference voltages of the IC's internal comparators (normally 2/3 and 1/3 of Vcc), but it doesn't say exactly what this effect is.
Here's what is said about pin 5 on Tony van Roon's tutorial page:
"Pin 5 (Control Voltage): This pin allows direct access to the 2/3 V+ voltage-divider point, the reference level for the upper comparator. It also allows indirect access to the lower comparator, as there is a 2:1 divider (R8 - R9) from this point to the lower-comparator reference input, Q13. Use of this terminal is the option of the user, but it does allow extreme flexibility by permitting modification of the timing period, resetting of the comparator, etc. When the 555 timer is used in a voltage-controlled mode, its voltage-controlled operation ranges from about 1 volt less than V+ down to within 2 volts of ground (although this is not guaranteed). Voltages can be safely applied outside these limits, but they should be confined within the limits of V+ and ground for reliability. By applying a voltage to this pin, it is possible to vary the timing of the device independently of the RC network. The control voltage may be varied from 45 to 90% of the Vcc in the monostable mode, making it possible to control the width of the output pulse independently of RC. When it is used in the astable mode, the control voltage can be varied from 1.7V to the full Vcc. Varying the voltage in the astable mode will produce a frequency modulated (FM) output. In the event the control-voltage pin is not used, it is recommended that it be bypassed, to ground, with a capacitor of about 0.01uF (10nF) for immunity to noise, since it is a comparator input. This fact is not obvious in many 555 circuits since I have seen many circuits with 'no-pin-5' connected to anything, but this is the proper procedure. The small ceramic cap may eliminate false triggering."
I may take a look at the 555 internals and see if I can derive timing formulas that include the control voltage. If I figure it out, I'll post it. It seems like it would be useful information. After all, the "standard formulas" are what I use to select "reasonable" values of resistors and caps. Otherwise, I'm selecting them by trial and error.
I want to use PWM to send pulses to an optoisolated triac driver to control a heater (you guys have already answered many of my questions on this topic). I have a reference voltage set by a potentiometer and another voltage from a heat sensor. If there is a large differnence b/n these 2 voltages, I want to sent longer pulses (or more frequent pulses). If there is a small difference, I want to send shorter pulses (or less frequent pulses). If my heat sensor has a higher voltage than reference, I don't want to send any pulses (or at least only short, infrequent pulses). I don't necessarily have to use a 555 if there is a better option.
Thanks for the feedback. This group is quickly becoming my best source of information.
Darrin
