I have 12V 2.5s duration pulses arriving at intervals of about 100s. What would be the simplest way to reduce these pulses to say 250ms? AIUI 555s cannot be used in monostable mode when the trigger pulse is longer than the desired output pulse so it seems I need some solution that doesn't require 555s.
Of course you can always shorten the pulse first, using a differentiating network (series-capacitor, resistor to ground), and then use your favorite timer.
Edge-triggered monostable multivibrators exist in the wellknown logic families (74XX series, CD4000 series), but it depends on your other requirements of these can be easily dropped in instead of a 555.
Of course this is liable to misbehave if your input isn't what you expect.
You can also do it with a CD4013B flip-flop: clock 1 into it on the rising edge of your pulse, and use the RC delay on the reset pin. That's liable to be less well behaved than the Schmitt method, though.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
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Indeed, those are usable types. The CD4538 was of course the CD4000 series predecessor of the 74HC4538.
However, all of those have much less output drive capability than a 555. When you want to drive something, it may be necessary to add something like a BS170.
Anyway, these small MOSFETS are really convenient as drivers. Source pin interfaces directly to the logic output (no resistor required), switches very fast, almost no saturation voltage hence little loss and dissipation.
May not be the simplest way, but I would consider using an ARDUINO. Use one pin to sense the incoming pulse and another pin for the output pulse. You can make the output pulse pretty much what you want with one millisecond resolution.
This kind of assumes you have some other things you want and can use the arduino to accomplish those additional tasks.
It does - only too well! According to the simulations I've run, the shrinkage is *too* effective and the output pulses too skinny. Which components on your schematic determine the output pulse duration, please? I shall need to tweak them. Many thanks.
Actually, on the simulations I've run with different values (over several decades) for these components, I can't detect any change in the input/ output pulse duration ratio. But the 2n2222 input device is useful in itself; didn't think of trimming the pulse that way.
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On the trigger side it's actually C1 which is doing the work by
differentiating the negative-going pulse out of Q1 into a spike with
way less ON time than the 555's output pulse.
John Fields
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