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I see that the datasheet calls this configuration a pulse position modulator. It's true that the low output time remains almost constant as the voltage on pin 5 is varied.Hero999 said:The duty cycle also changes too.
The correct term is pulse position modulation.
Hi Dean,Dean Huster said:Although the 566 function generator is cute with both triangle and square wave output, it isn't nearly as versatile as the 555/556. There have probably been more DIFFERENT types of circuits designed around the 555 that there have been around the op amp.
You can't ever get exactly 50% duty cycles from a 555 without some external tricks (like the binary divider at the output). But for most purposes, especially if you're varying the frequency, setting Ra to a value of 1K ohm and then selecting the timing capacitor to keep Rb at least 10 times higher than Ra and finally selecting Rb for the desired output frequency will normally give you close enough to a square wave for all but the most demanding applications.
Note that Signetics does the duty cycle "upside down" in their equations. For the 555 output, a 75% duty cycle is high 1/4 the time and low 3/4 the time, just the opposite of what you're normally taught. Not sure why they did that ... it was always confusing to me.
Dean
You can't ever get exactly 50% duty cycles
Yep, you're correct. It's an analog solution, so it will have some error. It could be greater or less than 50%. Of course, if you wanted to get really technical, you could say that a divide-by-two on the output will also not have a 50% duty cycle, because rising and falling propagation delays are not equal.Dean Huster said:But Ron, doesn't that still make the above statement correct, albiet a very slight error from exactly 50%? I was working from the classic/standard 1972 Signetics datasheet configuration: Ra from +V to pin 7; Rb between pins 7 & 6; pins 6 & 2 connected together and to the timing cap (other end to ground). Where you could get 49.99999999999999999999999% but not 50.0000000000000000000000%.
We're going to MN for a wedding in May. Why don't you come up and visit us? I think it's only about a thousand miles.Added seconds later: Hey, I notice you have the metal detector avatar back! Any plans yet for a vacation through here?
Dean
eblc1388 said:When connecting the CMOS 555 output pin as the charging/discharging source for the timing capacitor, one should take care that even a small 10mA loading on the output pin would drop/raise the output(satuation) voltage by one volt and thus one would easily get a LCO (load controlled oscillator) in return.