555 Calculator

What about a resistor from output to the cap? You may want to use a CMOS

Saves a resistor in any case.

Thomas

put a 100 ohm resitor in series with the hot side of the pot. othewise with it wound all the way down it'll be trying to discharge the powersupply.

Crap repeatability.

Thanks for knocking yourself out on that :) I used Jason Betts suggestion; subbed R1 with a 100 ohm trimmer and used a 22NF. Works good; apx 46K on the trimmer gives 1ms width and pertneer

"budgie"

See " 50 % Duty Cycle Oscillator" - figure 14.

Needs a 14nF cap for 1000Hz square wave output.

Aka " budgie droppings ".

........ Phil

What is your Vcc and what is your load? The half-way point coincides with Vcc/2 across the timing capacitor where the THRESH comparator is just entering complete cutoff and the TRIG comparator is on the edge of linear for low Vcc. If Vcc is small enough, this may reduce the pulldown drive ever so slightly to cause a higher Vce,sat on the output.

Not a standard circuit at all- you end up with odd values and bad sensitivity to component values- difficult to bound against component tolerances and possible to make everything hang. I don't see all that many 14nf caps either.

--- It may work for you, but it's not "poifect" if you're looking for a

-- John Fields Professional Circuit Designer

Forget that idiot- he is clearly wrong about that hokus pokus 50% duty circuit National publishes. I see no practical use for that touchy thing: View in a fixed-width font such as Courier.

. . . . . . Vcc . | . RA TH=069*RA*C . | . DIS-RB--+ RA*RB RB-2RA . | TL= ------- * C *Ln( ------ ) . | RA + RB 2RB-RA . === . | . | . --- substituting RA*C=1.44*TH . . . . TL 1.44*RB RB-2RA . -- = ------- *Ln( ------ ) . TH RA + RB 2RB-RA . . . RA TL 1.44 1-2r . substituting r= -- => -- = -------*Ln( ---- ) . RB TH 1 + r 2-r . . . TL ~ . D=50% => -- = 1 , makes r=2.3 , RA and RB always in same ratio. . TH . . . TL . (--) . TH - r 4.32 . Then S = ----- *( 1 + ------------)= -3.5 . r 1 + r (1-2r)*(2-r) . . . . . . For the case: . . . Vcc . | . RA . | . DIS---+ . | . RB . | . === . | . --- . . TL RB 1 . where RA S = ----- ->0 also . r 1 + r TL . (--) . TH . Example case of C=0.001u RA=10K RB=720K makes S =-0.014 . r . . Looks to be 3.5/0.014=250x better with initial accuracy. . . . The "standard ckt" is patently inferior.

The simple way to ensure a 50% duty cycle is to run it at twice the desired frequency and shove it through a flip-flop. :)

That has already been noted- but I was confining it to just the 555.

Ehhh? What was it that _I_ wrote ?:-)

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

I know but I was confining it to the 555 alone - there is a middle ground between good'nuf and absolute exactitude you know.

doesn't 50% duty cycle imply that the output be on the same time frame as it is off ? i don't see where the rail-to-rail comes into this? since the trigger and threshold are at 33% and 66% of Vcc i don't see where the in ability to effect 50% duty is coming from ?

Fsck you are a priceless piece. I used the CIRCUIT once.