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  #1  
Old 02-28-2007, 06:43 PM
star star is offline
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Default Applications for the monostable multivibrator

Hi there,

I understand that the monostable multivibrators are used to hold output voltages in its unstable state for a certain period of time regardless of how long the triggering waveform is held high/low, as well as being used for rising/falling edge detection in flip-flops.

However, how can I relate the importance of the monostable multivibrator to applications with more significance/meaning to day-to-day life?

Any ideas would be appreciated.
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Old 02-28-2007, 06:53 PM
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There use was more prevalent in the distant past. Now they are largely forgotten since we can clock logic at much higher speeds then we could in the 1960's. In my 45 years of hardware design experience I've never had occasion to use one.

The main problem with them is that although the leading edge is syncronous after a small and predictable delay from the trigger, the trailing edge is not. It depends on the actual values of an R and a C somewhere. The pulse width is not tightly controlled, but wanders all over the place.

Last edited by Papabravo; 02-28-2007 at 06:55 PM.
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Old 02-28-2007, 06:57 PM
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Oh okay... I didn't realise the technology was so old..
Which technology is in use now for clocking?
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Old 02-28-2007, 07:20 PM
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I think you might misundertand. A clock is derived from an oscillator or astable multivibrator. This device has no stable states and continually switches back and forth between two states.

The flip-flop has two stable states and is also called a bistable multivibrator.

Review:
astable -> no stable states, an oscillator
monostable -> one stable state, a one-shot
bistable -> two stable states, a flip-flop

BTW - what replaces a one-shot is a counter or a shift register which is clocked very much faster than the process being controlled. In this way both the leading and trailing edge can be precisely positioned.
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Old 02-28-2007, 08:02 PM
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Okay, I understand that now.

I'm still confused about the one-shot though.. where is it, or in the present-day, the shift register used, in terms of real world applications?
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Old 02-28-2007, 09:10 PM
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The main place it would be used is to stretch a narrow pulse into something longer. For example if you have a 1 microsecond pulse and it needs to be 4 microseconds long then a one-shot or a shift register can accomplish this function.
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Old 02-28-2007, 11:09 PM
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I actually used one of these in the recent past for a magnet switch debouncer. Once the 555 goes high, the switch could go on and off several times withing that short (milliseconds?) period of time. Without the monostable, it would be pulsing each of those several times. I used the output of the monostable to feed my clock pulse on a counter.

Just my $.02
RJ Vandermate
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Old 02-28-2007, 11:21 PM
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Quote:
Originally Posted by Archiebold View Post
I actually used one of these in the recent past for a magnet switch debouncer. Once the 555 goes high, the switch could go on and off several times withing that short (milliseconds?) period of time. Without the monostable, it would be pulsing each of those several times. I used the output of the monostable to feed my clock pulse on a counter.

Just my $.02
RJ Vandermate
In this case each of the input pulses retriggers the one-shot whose output looks like a single long pulse.
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Old 02-28-2007, 11:25 PM
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The main reason that multivibrators were banded on designs at the company where I work was that while the pinouts for a given device with the same root part number were the same across manufacturers; the equation for computing the pulse width from the resistor and capacitor timing components tended to vary between manufacturers.

This frequently resulted in boards that failed to function properly because some vendor convinced a purchasing employee that the parts were identical. Safeguards were put in place but the occasional incompatible part would make its way into the board production run.

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Old 02-28-2007, 11:34 PM
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Quote:
Originally Posted by hgmjr View Post
.... the equation for computing the pulse width from the resistor and capacitor timing components tended to vary between manufacturers.
...
Not to mention the various environmental factors that effect pulse width like temperature, supply voltage, and the phase of the moon.
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