the PWM 555 as shown on AAC (http://forum.allaboutcircuits.com/showpost.php?p=117641&postcount=6) doesnt use the disc pin 7, why (the left one in fig 5.2)? should it not have disc connected (line in red)? if not, how does it cycle?
ah, i totally missed it, i didnt see that line from R2 to pin3 was not tied to Vcc (no trace dot, etc). i was scratching my head on a few things there, but now it's all good. my R7 and pin3 are wrong in my pic, etc. always good to check and dbl check the schematic, etc......This configuration of an Astable uses the Output as the charging/discharging source voltage, so the Output is very close to 50:50 high/low
EDIT.
Which is required if you are using the 555 for a PWM App.
The pot will vary the mark/space ratio, but the total period will remain constant
The discharge and output pins are exactly in phase - you can use 1 less resistor in the timing CR network and get the fastest possible astable configuration by driving the CR timing by (both source and sink) the output pin.the PWM 555 as shown on AAC (http://forum.allaboutcircuits.com/showpost.php?p=117641&postcount=6) doesnt use the disc pin 7, why (the left one in fig 5.2)? should it not have disc connected (line in red)? if not, how does it cycle?
thats crappy for me, i need a 10-90% range.Simulation shows the duty cycle range is ~1%-30%
yeah, i am leaning that way, but i also need adjustable freq, and i started with the basic 555 1-99% duty cycle and then added a 8 position rotary to switch in different caps to change the freq, which might be all i need, etc. i might also add in a switch to add some extra uF to each selection so i can have two ranges of freq, etc. i would rather not add more IC's if i dont have to, etc.Then use the schematic I posted as is.
sure, i saw it, it needs an additional 393 IC, etc. i need adjustable freq in the 1-200Hz range, with duty-cycle in the 10-90% range. the 555 output is nice because it can sink and source thus allowing me to drive a pair of opto's in flip-flop fashion (one on while the other off, a pair of led's if you will).Lets back up, what are your requirements?
This circuit (also in that chapter)...
Provides both excellent freq stability and PWM stability. You can also use the spare LM393 gate to make a hysteric oscillator, to eliminate the 555. If I knew what you were after I could draw something up.
String the pot across the tops of the 2 diodes and connect pin 7 to the wiper, you need the usual maximum frequency limiting resistor from pin7/wiper to Vcc.thats crappy for me, i need a 10-90% range.
i am moving to the 555/393 configuration as this will allow me more functionality.String the pot across the tops of the 2 diodes and connect pin 7 to the wiper, you need the usual maximum frequency limiting resistor from pin7/wiper to Vcc.
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