Simple 555 problem: Input high=>Output constantly high

Thread Starter

Selportion

Joined Mar 21, 2011
8
Stand-alone 555 problem (monostable)
((I bet this question has been answered before and sorry for that, but all previous titles are like "555 help" and i cannot find the answer to that specific problem, i ask for your understanding))

I am using 555 monostable timers for a project, and all of them have output constantly high (1) when the input is constantly high(1), which I think is against the expected responses. If I change the input to (0) then the output lowers to (0) after the expected time and remains so.


Using:
  • "CHN NE555N KA6735"
  • all with capacitors non-electrolytic (i think ceramic) C=0,01uF
  • and some of them with R=270k and C=10uF electrolytic
  • and the others with R=3,18k and capacitors non-electrolytic (i think ceramic) C=0,01uF
Schematic as

http://www.google.gr/imgres?imgurl=...D4gby98DgCA&page=1&ndsp=30&ved=1t:429,r:1,s:0

From your experience is it broken/burnt or some sort of wrong use of capacitor types/values?
Any help appreciated!:)
 

R!f@@

Joined Apr 2, 2009
9,918
Hey Welcome to AAC

A monostable has only one stable state. A trigger will cause it to switch and after the time constant it will revert back to it's stable state
Recheck ur wiring
 

DerStrom8

Joined Feb 20, 2011
2,390
I suppose when you say "it remains so," you expect it to go high again (on-off-on-etc.)? That would be astable operation which requires this circuit:



I hope this helps :)
Der Strom
 

Thread Starter

Selportion

Joined Mar 21, 2011
8
Thanks for welcoming, I've been reading around for a while but never bothered registering till now!

Hm, I've messed up my initial description, and i see i cannot edit it, I got it wrong. So here's more clear:
  • I need monostable operation, like LowLowLow...-High for X secs-LowLowLow...
  • I get for Input HIGH => Output LOW
  • Input HIGH->LOW->HIGH => Ouput HIGH for x secs
  • Input steadily LOW => Output steadily HIGH <<Unwanted response
So could it be something like too small/big capacitors or not/yes electrolytic capacitors or destroyed 555? The wiring seems fine.
PS>Vcc 5.2V
 

Audioguru

Joined Dec 20, 2007
11,248
When pin 2 of the 555 goes lower than 1/3rd of the power supply voltage then the timer starts and the output goes high. Pin 2 must become high again or the timer will never time out. When the timer times out the output goes low.
 

Thread Starter

Selportion

Joined Mar 21, 2011
8
Pin 2 must become high again or the timer will never time out.
I didn't know that was true..
So if i want: 50ms after hitting a switch, one led to get light up for 3s, i cannot use 2 monostables (50ms / 3s) in row? Cause you are saying that after the 1st monostables finishes its flash (=>output low), the second one (input low) will never stop (output constantly high) ??
Pin 2 must become high again or the timer will never time out.
I see the whole edge-triggering idea, but doesn't it require a manual switch on the input? Or is it just a schematic?
Thank you all for your help!
 

Thread Starter

Selportion

Joined Mar 21, 2011
8
Thank you very much guys, edge triggering works and it's easy if you understand it.

So, to anyone with the same problem,
put any small capacitor to your 555 input, 2 reletively big resistors from the capacitor legs to +Vcc and connect your input before the capacitor, just like the image on previous post.

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