hallo !
is there any circuit that can narrowed a pulse?
for example, if there's a 50% duty cycle, it means that, 50% high and 50% low, isn't it?
the problem is, i wanted to, for example, make a 10% high pulse and 90% low pulse. Is that possible? with what? A PWM? how

would be the circuit look like?

The application that i want to make is, a rpm counter that will be interfaced to a computer which display the rpm's speed.
SO, i wanted to change the single pulse's width that the optocoupler produced to ACK pin of parallel.

thanks...

 

From a hardware standpoint, the simplest setup would be a basic microcontroller. All you would have is 3 wires. Power, Ground, and the PWM output. Timing is adjusted in software and is very easy to do.

 

What range (speed) is this going to run? The parallel port is not super fast.

Maybe a micro like palanx said, and have the microcontroller send the count to the serial port on the PC.

 

A 555 timer would do and it's far simpler than a complex microcontroller.

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A PIC10F or 12F can do everything he needs with no external components other than wires and will be far more versatile than a 555 timer. The software is simple enough to be a good project for a novice programmer.

 

Depends on your experiance, it'd take me less than 5 miniutes to put together a 555 timer circuit on vero board, but it'd take me a lot longer to learn to program a PIC also I'd have to purchase a programmer and the componants will cost less too.

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As part of a larger project I am putting together a pulse width modulator circuit. It will consist of a 555 timer to generate a ramp or sawtooth voltage that will run between 2 volts and 4 volts. The ramp goes to an LM339 comparator that is also fed with a reference voltage. The comparator output will change (either on or off) as the ramp voltage rises above or drops below the reference voltage. If you visualize a ramp or sawtooth voltage then draw a horizontal line that cuts thru it (representing the reference voltage) you can then visualize adjusting the ramp voltage up or down cutting thru longer or shorter periods of the ramp/sawtooth.

This is only one way - as others have suggested there are many other ways to do it. Note that a general purpose (low cost) op amp can be configured as a comparator.

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stevez

 

I realize that I made an error here - "you can then visualize adjusting the ramp voltage up or down cutting thru longer or shorter periods of the ramp/sawtooth" - this should say " you can then visualize adjusting the reference voltage up or down cutting thrur longer or shorter periods of the ramp/sawtooth"

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stevez

 

an LM555 can do thet better then any other described scheme trust me on that. if you are doing modulation look at pin 5 as a souce of input also usea current source to linearize your ramp rc time is not linear. An LM555 can be one shot nonresatble ,astable and modulator. of course you could use a PC to do that and don't forget the kitchen sink. take care

 

hyedenny

How about using a couple inverter gates and a transistor? You can get pulse widths of just a few nanoseconds (not necessarily 10% duty) quite easily this way. If you want adjustability, the easiest way is probably with the 555.

Attached Images

pulse_209.jpg (13.1 KB, 405 views)

 

hyedenny

There are 2 ways to measure RPM: counting the number of pulses during a certain time interval, or measuring the period of the pulse.... Which are you doing - frequency or period??

 

in this reply, i added an attachment of the block diagram/schematic...
hope that'll helps

Attached Images

iwa2.gif (22.5 KB, 342 views)

 

I don't really understand what you want at all?, why not just input the pulses directly? - the pulse width is pretty irrelevent. Assuming you do want to do it (perhaps you'd explain why?) you just need a monostable - 555's are commonly used in rev counters fed from pulses like this.

A simple google find loads of examples, you might find this one interesting? http://www.fs2000.org.uk/Revcountercircuits.htm.

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I have implemented the the triangle wave into a comparator idea recently, and have found it to work very well indeed. It has a full 0 - 100% adjustment, is very linear in operation and has the nice feature that its voltage controllable. With 555 PWM, I always find that as you alter it, the frequency drifts aswell, sometimes quite a lot. Using it as a variable duration monostable following the oscillator can work, but then it is poor with a varying frequency input.

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Nigel Goodwin:
The pulse width is actually to speed up the respon of the holes of the disc, that's why i have to change the pulses from the optocoupler.
To change it to down pulses, an inverter could done it, isn't it?
By the way... thank's for the link that you provided, it's a good reference....

Dr.EM:
Impelementing triangle wave into a comparator? How?

Hydenny:
Between frequency and period? I'm doing period.

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