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Short pulses from an oscillator

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Dr.EM

New Member
How would you go about turning an oscillator output like the one shown at the top into the one beneath it? I would have thought it would be some kind of monostable, but the 555 one at least certainly doesn't work. Any ideas?
 

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Styx

Active Member
Get a monostable to make the 50% duty.

Feed it into a high-pass filter and make sure the resistor to GND has a diode in parallel with it (cathode connected to the R-C star)

The output of the RC+D filter feed into a 74HC14 or any kind of SCHMITT trigger (and again to re-invert)

The resulting pulse-width with be dependant of hte time-constant of the RC
 

mstechca

New Member
an easy way to change the duty-cycle of the 555 is to adjust one of the resistors connected to it.

Just make an astable multivibrator using the 555, and make one resistor a small value (say about 4.7K), and the other resistor a large value (say about 470K).

An easy way to test for this kind of waveform is to connect the output of the 555 to another 555 that produces sound, and connect that output to a speaker (through a coupling capacitor).

You are on the right track when you hear the sound for a very short time, and no sound for a long time.

Because you did not specify any frequency or timing requirements, I cannot suggest any resistors to use.

I also suggest using a small value capacitor (10nF and under), because large values take longer to charge and discharge, and could potentially corrupt your waveform.
 

Dr.EM

New Member
I know that you can change the duty cycle of the 555 in that way, what I have is an oscillator already running with 50% duty and wanted a short output regardless of frequency. Styx suggestion has worked perfectly, I tried it earlier. I used a 2.2nF and 22k which gave around 0.5ms pulses after my schmitt (I have a scope now which makes everything so much easier).

I might as well show what I want to use this for. Its going to be part of my LFO project, which I want an S&H output included. I tried a couple of analogue circuits to no avail, but have came up with this which has some advantages aswell as some disadvantages (it can hold a value with no drift but is quantised)

The sim naturally didn't work, but i'll explain. The 6-bit counter is supposed to cycle through on one clock, giving 64 sequential voltage levels at the summing amp output when the D-types clock input is high. The clock inputs are controlled by my other oscillator, through the high pass filter circuit (this is so you can't hear it changing). So, when they are momentarily pulsed, they take on the D values and give a an output voltage. Because the two clocks are not linked, I would think the output values to be more or less random. I don't have the parts to test this, but is it likely to work. Possibly overcomplicated, but i'm not bothered by that.
 

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Papabravo

Well-Known Member
Feed your clock signal to one input of an exclusive OR gate. Feed a delayed version of the clock signal to the other input of the exclusive OR gate.

The delayed versionof the clock can be created with a simple one-pole RC filter
 
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