I was wondering why a 555 circuits i have seen, send a low pulse as a trigger (-ve) when only 1/3Vcc is needed which could be a +ve 1/3 of course., say if Vcc was +12v.

One would associate a trigger as an event and events are "positive" in that they initiate some re-action (activity) and therefore one might conclude that a trigger pulse would be +ve

I note that pin 2 is connected to base of a PNP so this requires a -ve charge at base?
?

 

I was wondering why a 555 circuits i have seen, send a low pulse as a trigger (-ve) when only 1/3Vcc is needed which could be a +ve 1/3 of course., say if Vcc was +12v.

One would associate a trigger as an event and events are "positive" in that they initiate some re-action (activity) and therefore one might conclude that a trigger pulse would be +ve

I note that pin 2 is connected to base of a PNP so this requires a -ve charge at base?
?

This high-to-low transistion as a trigger may be a random artifact of design (see next paragraph) or it may have been intentional to allow multiple 555 timers to be cascaded and easily trigger a series of events. As the first 555 event ends, the high output (pin 3) transitions from high to low and triggers a subsequent 555 for a next event (and so on). No glue logic bewteeen the 555s - just direct connections.

As you get deeper into Electronics, you'll see that "active low" is a common way to turn something on and other devices are triggered by a transition (from high to low, or, from low to high). The decision from the designer is practicality. Some chips have multiple inputs (or multiple configurations) that allow the user to pick which type of trigger they want. See CD4017 and CD4538 for examples of multiple formats.

Also, an inverter chip is widely available and widely used to invert the signal.

 

One would associate a trigger as an event and events are "positive"

As a blanket statement, that is bigly incorrect.

You are correct *only* for circuits that have "positive-true" input. "Negative-true" is an equally valid logic state. Many flipflops and counters that have a positive-true clock input have a negative-true reset input. The output enable input on many tri-state octal latches is negative-true.

At the gate level, the DeMorgan equivalent of a NAND gate is an inverting input OR gate - a low on any input pin forces a high at the output.

ak

 

I was wondering why a 555 circuits i have seen, send a low pulse as a trigger (-ve) when only 1/3Vcc is needed which could be a +ve 1/3 of course., say if Vcc was +12v.

Because it's easier that way. The one shot will trigger before the trigger voltage gets to 0V, but who cares?

 

One would associate a trigger as an event and events are "positive"

That's apples and oranges.
A positive change in the voltage of an electrical signal is completely unrelated to whether the event it causes has a "positive" quality or attribute.

 

One would associate a trigger as an event and events are "positive" in that they initiate some re-action (activity) and therefore one might conclude that a trigger pulse would be +ve

Your association is like saying a toggle switch must point up to be on. If there was a need (demand) for a 555 timer that is triggered with a high transition, you could order one. There are other parts that can be used for a single shot if needed.

 

Ok chaps, i get it. All perfectly logical responses and having read your replies i understand.

Im quite drawn to the concept of cascade triggers. Indeed i have modelled this in arduino mcu. Silly didn't think of that before posting ah well.

Yes, i agree, my analogy to triggers being positive action, was not the best

Ok. So thank you chaps