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# Digital logic question

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#### ethan169

##### New Member
Ok so I have another question for you guys about a simple logic circuit.

All i really need to do is get a single pulse each time the inputted square wave has a rising edge.

This seems like a pretty simple task to complete to me, I just cant think of the logic that is needed. My first thought was something with a counter or Flip-Flop.

And here they are basically using two counters to double the frequency of a supplied square wave by triggering on both rising and falling edge.

I just need something like the Ctr 1 out in this article.

Its basically just configuring a counter to provide a pulse at the output on the rising edge of the input. What counter could i use to do this?

I took digital all to long ago and unfortunately didn't pay much attention.

Thanks for any help guys!

-Ethan

an edge triggered monostable....a 555 timer chip would do..

Number 8 in the sequence..

Last edited:
The circuit is an "edge detector". Attached using CMOS version. R2/C1 sets the output pulse width. SW1 just simulates the input square wave.

ken

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• Edge Detector.gif
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The 555 timer circuit looks like it defiantly does the job that i need.

But Heres a question on the edge detector circuit.
The circuit is an "edge detector". Attached using CMOS version. R2/C1 sets the output pulse width. SW1 just simulates the input square wave.

ken

This will give me a pulse on both the rising edge and the falling edge. How would i modify this to be just rising edge?

-Ethan

btw thanks for the quick responses!

74HC123 can be set for rising or falling edge but will not respond to both. Time is set by RC.

a small capacitor coupled to a pull down resistor on the target might be adequate, I've used it to latch a 74xx573 in the past.

The attached gives a very short positive pulse on the output for each rising edge on the input.

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• Mono.png
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