Square wave generator for Exact frequency !!

Status
Not open for further replies.

nikhilsigma

Full Member level 2
Joined
Jun 19, 2010
Messages
142
Helped
17
Reputation
34
Reaction score
16
Trophy points
1,298
Location
Delhi, India
Activity points
2,584
hi guys...

I want to make a square wave generator with :
1. Exact 50% duty cycle
2. Exact .2s time period....

i used a 555 calculator n made a circuit according to it.....but it didn't worked...!! :-?
i think that must be due to the variation in the values of capacitors n resistors...because of there tolerances...but i am not sure....


so Is it possible to generate an Exactly accurate wave as i want....
please help... ;-)
 

How exact is exact?

Without adjustment, you should expect a few percent duty cycle deviations due to imperfect 555 threshold voltages. The period will be also affected by threshold voltage variations and capacitor and resistor tolerances. The datasheet says about 2 % initial accuracy for astable period. 5 or 10 % would be a standard capacitor tolerance. Voltage and temperature dependency is specified in the datasheet.
 

look i am using image processing...
and my camera is taking pics at a frame rate of 5fps(i.e. one pic after .2sec).
now if i use a led to glow for .2secs and then turnoff for next .2 secs(by using a square wave of 50%duty cycle n pulse width=.2s and thus time period= .4s)
then in one frame led will be glowing and in second frame i will be off, then in third it will glow and this continues...
so for this to happen my square wave should be exact as i stated above...

so what should i do ?? :-?
 

as FvM says how exact is exact? You could use
1. microcontroller clock freqiency generated by a crystal (which can be very accurate)
2. have a timer interrupting at half the period
3. invert a digital output on each interrupt

I have generated sine waves using similar techniques, e.g. timer interrupts every 1/360 of period - output sin(x) on each interrupt where x is 1 degree, 2 degrees, etc
 

using micro-controller only for this will inc. the cost unnecessarily....
is it not possible by any circuit...or ic.....

in my opinion 555 is best.....but need ideas from u guys to make it output more exact....


and regarding "how much exact" i have told in my last reply about what i need to do with it....n if that work is successfully done...then it's exact according to me... ;-)
 

Run the 555 at a higher frequency, or better still, use a crystal clock oscillator with a decade counter to divide the frequency down to something useful, feed that output into the clock input of a toggling flipflop to give you your exact 50% duty cycle, not forgeting that every flipflop used will divide the frequency by 2.

Now all you have to do is sync it with your camera. Not an easy task to be that precise unless...

If your camera has a flash accessory socket, then an easier way would be to drive the toggling flipflops clock directly from that and all your problems would be solved at a single stroke.
 

using a microcontroller the square wave would be as accurate as the crystal (assuming you calculate the timer values correctly)
you could use a low cost development kit like the Microchip microstick
**broken link removed**

it is self contained with on board USB programmer, header so you can attach itto your own breadboard, free compilers etc etc
we use lots of these as a basis for student projects (undergradate and postgraduate)
 

and regarding "how much exact" i have told in my last reply about what i need to do with it
Click to expand...
I told you about the expectable 555 accuracy, but you didn't refer to it. It seems like you don't know the requirements yourself.

I completely agree, that a microcontroller with a crystal will give best accuracy. I also guess, that adjusting a 555 timer will be sufficient for your applications, but as you didn't tell about the actual requirement, it's only a speculation.
 

I agree about the accuracy of the 555, if you're trying to keep it in sync with anything else not physically connected to it then forget it. componant % and temperature will make the frequency drift.

Forget the 555, use flash photography and clock a flipflop in toggle mode with a photo transistor. It'll cost about £3.00, if that. Though you'll need a really good flash to keep up with that frame rate, or maybe a strobe would do the trick.
 
Last edited:

Status
Not open for further replies.

Similar threads

Menu
Log in
Register
Cookies are required to use this site. You must accept them to continue using the site. Learn more…