How would u trigger 555 monostable on both edges?

Thread Starter

Wendy

Joined Mar 24, 2008
23,415
I've been thinking about it, still haven't come up with an answer I like.

One 555 monostable, being feed with a symmetrical square wave. How would you trigger the monostable on both edges?

I think diodes might be the answer, to suppress the positive edge.
 

SgtWookie

Joined Jul 17, 2007
22,230
You could feed a square wave through a differentiator circuit, then through a precision rectifier circuit to invert the negative pulses. You would wind up with a stream of positive pulses.
 

Papabravo

Joined Feb 24, 2006
21,159
If you exclusive OR a square wave with a delayed version of itself you get a frequency doubler that is a string of short positive pulses. You can delay a square wave by using a single pole RC filter or you can use a high speed clock and a flip-flop to produce a delayed version of a square wave.
 

Thread Starter

Wendy

Joined Mar 24, 2008
23,415
A simple square wave, with an inverted version of the same waveform available if needed. I still have diode in the back of my mind as the answer, to allow a quick rush of current one way (negative edge) and a slow charge the other (positive edge).

What has me thinking there might be a problem is a positive pulse is mixed with a negative they could cancel out.

I'm going to draw something out to model it.

*********************************

This might work, I'll have to try it out.



If someone has a different idea I would be interested.
 

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Papabravo

Joined Feb 24, 2006
21,159
Don't know if they still make it but the 8T20 double edge triggered one-shot would fill the bill. We used them a lot to make data separators for FM and MFM encoded disk drives.
 

KL7AJ

Joined Nov 4, 2008
2,229
I've been thinking about it, still haven't come up with an answer I like.

One 555 monostable, being feed with a symmetrical square wave. How would you trigger the monostable on both edges?

I think diodes might be the answer, to suppress the positive edge.
Run the trigger train through a full wave bridge rectifier! I had to do precisely this trick a while back so that i could use T1 timing as a clock source.


Eric
 

Ron H

Joined Apr 14, 2005
7,063
Hello,

A XOR gate with an RC will also work.



The RC will delay the input and give a puls at the output when the two inputs differ.

Bertus
XOR will give a short positive pulse. You can follow it with another XOR configured as an inverter.
Alternately, you could use a single XNOR (still with the RC delay).
 

Thread Starter

Wendy

Joined Mar 24, 2008
23,415
Bill,

Do you think this will fly?

Ken
I think it would. I know why D3 is there, I wonder if I could eliminate it without messing anything up.

Run the trigger train through a full wave bridge rectifier! I had to do precisely this trick a while back so that i could use T1 timing as a clock source.


Eric
A thought similar to that occurred to me, but I couldn't see how to implement it.
 

Thread Starter

Wendy

Joined Mar 24, 2008
23,415
XOR will give a short positive pulse. You can follow it with another XOR configured as an inverter.
Alternately, you could use a single XNOR (still with the RC delay).
That would work too. I'm thinking of a real application, the time is a little critical.

When I first thought of a circuit like the ones show I went into brain freeze. I think I've been staring at this screen too long.
 

KMoffett

Joined Dec 19, 2007
2,918
D3 just provides a rapid discharge across R6.

I still don't know if your input is a monopolar square wave, or a bipolar one. ???

Ken
 

Thread Starter

Wendy

Joined Mar 24, 2008
23,415
Think single power supply, and a digital train from that.

I really like Ron's approach, I would have never thought of using a common base transistor for the positive edge, and the whole arrangement replaces the signal conditioner.
 

Ron H

Joined Apr 14, 2005
7,063
Think single power supply, and a digital train from that.

I really like Ron's approach, I would have never thought of using a common base transistor for the positive edge, and the whole arrangement replaces the signal conditioner.
The common base transistor handles the negative edge.
 

inwo

Joined Nov 7, 2013
2,419
Ron's circuit works perfectly on the bench, with minor changes.
It will not double when triggered in actual circuit using hall sensor.
Rising edge is slow.
Looking for a simple way to square up rising edge.
If I need to add a schmitt trigger gate, maybe I should use another solution.
I'm all about simple.clockdoubler.gif
 
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