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555 one shot

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Rogue5

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I'm trying to use a 555 as a one shot pulse generator, I have it working, but the trick is that I want to drive it with a square wave output from a PLL, so that I have a pulse train that is locked in phase with the square wave.

The circuit works fine when I use a button on the input, tied to ground when the button is pressed, and I can adjust the delay with a potentiometer, it works slick.

the problem is that when I try to use a square wave on the input, the smallest I can get the pulse width is the pulse width of the high portion of the input wave.

I have an input square wave at 60 Hz, and I'm trying to get a pulse width of about 1ms. but the best I can get is about 8.3ms, so the output is high when the input is low.

I googled this for a couple hours yesterday to no real effect, most seem to say that it's possible to do, but nothing I've tried has been able to decrease the pulse width below the input width.

Hmmmm, If I could decrease the duty cycle to like 10% that would probably work. I'll give that a try through google.

I'm not restricted to using a 555, but the lab does not have all that much variety for components.

any help would be great,
Thanks
 
Here is a little tutor about the 555, if you looked on google chances are you seen this page.
 
I often generate short pulses from levels going high or low by using just a inverter or two and a RC constant. The position of the resistor R1 can be connected to GND depending on which edge you want to trigger.
 

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Connect the PLL output to pin 2 through a 1nF capacitor, and connect 100k from pin 2 to VCC. This is a differentiator that will give you a short pulse on pin 2. It is sometimes necessary to add a diode (1N4148) from pin 2 to VCC, cathode to VCC, to protect pin 2 from overcurrent on the rising edge if the impedance of your source is low.
 
Thanks Ron, that worked like a charm.

that's a cool way to do it with the inverter too though, I'll have to put that in my bag of tricks.

Thanks guys
 
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