problem with duty cycle for given diagram

[rajaram04]

Hello sir
After reffering to many areas i got a circuit for 555 as astable with this diagram below


here with R1=1.2k , R2=1k , C= .01 mf , im getting results with online calculator as 45 Khz
but the duty cycle is 68.75 % as it is showing here

So if i want to get 50 % with same frequency out , what measures i ve to take please responce
i completed to many practicals but still having problem in that
thanks

 

[watersky]

produce 90KHz, a D trigger.

 

[BradtheRad]

Link to website which tells about installing a diode across one resistor, to obtain 50% duty cycle. (Scroll just over halfway down.)

http://www.kpsec.freeuk.com/555timer.htm

 

[crutschow]

If you want an exact 50% duty-cycle then operate the 555 at double the desired frequency and run it through a ÷2 flip-flop.

 

[rajaram04]

sir would you please let me have a circuit diagram or circuitry idea about how to run it through a ÷2 flip-flop. please . . . thanks

 

[FvM]

The circuit in post #1 can't achieve 50% duty cycle by nature. There's apparently a problem with the said online calculator or a misunderstanding. A 50% duty cycle circuit is shown in the 555 datasheet. Another popular circuit uses a single resistor between output and capacitor.

 

[rajaram04]

kkk sir in that case is there any resistor between +ve & pin 7 of the i.c. as we do in general ?
please comment

 

[crutschow]

sir would you please let me have a circuit diagram or circuitry idea about how to run it through a ÷2 flip-flop. please . . . thanks

Use a D type flip-flop such as a CD4013. Connect the /Q (Q not) output to the D input of one of the flip-flops. Connect the 555 output to the FF clock input. The Q output will be 1/2 the 555 frequency with exactly 50% duty-cycle.

You can also use a JK FF such as the CD4027. Just tie the J and K inputs to the V+ voltage and the 555 to the clock input. Again, the output will be 1/2 the 555 frequency.

Note: Make sure you tie all unused inputs of both FFs to ground.

 

[rajaram04]

hmm okk i got it but sir tell me please about the frequency responcing limits for this 4027 etc
cause i hv listened about 555 that it has good responce upto 100 KHgz
so if i need a 50 Khz 50% duty cycle out using filp flops then i ll have to make 100 Khz on 555 timer

& too a problem is here that if i ve to make final output of flip flop to 100 Khz then i hv to make frequency of 200Khz on 555 timer which is practically i guess not possible

so please help

 

[crutschow]

hmm okk i got it but sir tell me please about the frequency responcing limits for this 4027 etc
cause i hv listened about 555 that it has good responce upto 100 KHgz
so if i need a 50 Khz 50% duty cycle out using filp flops then i ll have to make 100 Khz on 555 timer

& too a problem is here that if i ve to make final output of flip flop to 100 Khz then i hv to make frequency of 200Khz on 555 timer which is practically i guess not possible

According to this (pg. 7, FAQ 2), you can use a 555 up to about 500kHz. The CMOS circuits will work above that frequency.

 

[rajaram04]

ok sir tell me its perfect nmber code so that i may choose from the given

currently we have NE555 in market & now a days only this series is avaliable for 555 so please help me so that i can place order there

 

[crutschow]

ok sir tell me its perfect nmber code so that i may choose from the given

currently we have NE555 in market & now a days only this series is avaliable for 555 so please help me so that i can place order there

I don't understand your question. Are you asking about where and how to order a 555? Since I don't know where on "earth" you live, I can't really help you there. (Now if you were on Mars, I may know a place. :wink

 

[rajaram04]

hahaha no no actually i am asking the proper number or say a code including the 555 labelled on tha i.c. . . Are all 555 i.c.s available in market relating CMOS family ?