Help with simple 555 timer

[pranmala]

Hello, I am having trouble getting a very simple 555 timer circuit to
work. I have studied hard the documentation for the various pins and am
trying this simple circuit, but the o/p
PIN3 stays high for just a fraction of second when I take trigger input
2 low. The values of
R or C in the RC combination seem to have no bearing on how long the
pulse width should be


The circuit I am working on is the first part of the '2 stage Time
delay circuit'.
I am giving the link to the image of the cicuit that I am using (with
due credit to rpaisley)

http://home.cogeco.ca/~rpaisley4/LM555.html#24

Instead of PIN3 of the first chip feeding the second chip, I have an
LED (with 1K resistor in serial going to -ve ). I have assembled the
circuit on a bread board. Thinking the chip is faulty, I had replaced
it twice. I also tried switching the capacitor from 100microfarad to
..001 microfarad. The resistors from 220K to 1 K. The time constant of
1.1 RC seem to have no bearing on my circuit.

Any help will be greatly appreciated.
Thanks !
pranmala

 

[John B]

pranmala scrobe on the papyrus:

Hello, I am having trouble getting a very simple 555 timer circuit to
work. I have studied hard the documentation for the various pins and
am trying this simple circuit, but the o/p
PIN3 stays high for just a fraction of second when I take trigger
input 2 low. The values of
R or C in the RC combination seem to have no bearing on how long the
pulse width should be


The circuit I am working on is the first part of the '2 stage Time
delay circuit'.
I am giving the link to the image of the cicuit that I am using (with
due credit to rpaisley)

http://home.cogeco.ca/~rpaisley4/LM555.html#24

Instead of PIN3 of the first chip feeding the second chip, I have an
LED (with 1K resistor in serial going to -ve ). I have assembled the
circuit on a bread board. Thinking the chip is faulty, I had replaced
it twice. I also tried switching the capacitor from 100microfarad to
.001 microfarad. The resistors from 220K to 1 K. The time constant of
1.1 RC seem to have no bearing on my circuit.

Any help will be greatly appreciated.
Thanks !
pranmala

'sci.electronics.basics' is the correct place to ask.

 

[[email protected]]

Hello, I am having trouble getting a very simple 555 timer circuit to
work. I have studied hard the documentation for the various pins and am
trying this simple circuit, but the o/p
PIN3 stays high for just a fraction of second when I take trigger input
2 low. The values of
R or C in the RC combination seem to have no bearing on how long the
pulse width should be


The circuit I am working on is the first part of the '2 stage Time
delay circuit'.
I am giving the link to the image of the cicuit that I am using (with
due credit to rpaisley)

http://home.cogeco.ca/~rpaisley4/LM555.html#24

Instead of PIN3 of the first chip feeding the second chip, I have an
LED (with 1K resistor in serial going to -ve ). I have assembled the
circuit on a bread board. Thinking the chip is faulty, I had replaced
it twice. I also tried switching the capacitor from 100microfarad to
.001 microfarad. The resistors from 220K to 1 K. The time constant of
1.1 RC seem to have no bearing on my circuit.

Any help will be greatly appreciated.

The 555 isn't exactly my favourite chip, but I note that you have
omitted the 10nF capacitor from pin 5 to ground that the manufacturers'
data sheets shows on most application circuits, and you have left the
reset pin (pin 4) open circuit as well - again, in the example circuits
this is returned to the positve rail if the reset function isn't being
used.

I've not played with the 555 since sometime around 1974, when I found
it a pretty unatisfactory, so I don't know if either of these omissions
could be the source of your problem, but I suspect that the open reset
pin is a bad idea.

 

[mc]

The 555 isn't exactly my favourite chip, but I note that you have

omitted the 10nF capacitor from pin 5 to ground that the manufacturers'
data sheets shows on most application circuits,

But usually unnecessary in practice. Its only purpose is to give slightly
greater precision by removing noise pickup.

and you have left the
reset pin (pin 4) open circuit as well - again, in the example circuits
this is returned to the positve rail if the reset function isn't being
used.

THAT's the problem. The 555 will not work unless Reset is connected to V+.
I'm surprised there was any output at all.

 

[Spehro Pefhany]

But usually unnecessary in practice. Its only purpose is to give slightly
greater precision by removing noise pickup.


THAT's the problem. The 555 will not work unless Reset is connected to V+.
I'm surprised there was any output at all.

It's the base of a (presumably low-gain) PNP transistor in the
original bipolar LM555 so it will function (typical reset current is
something like 100uA).

In the modern CMOS variants it's very high impedance, so it might
drift in and out of operation, or stop/start based on a touch.



Best regards,
Spehro Pefhany

 

[Brian]

Any help will be greatly appreciated.
Thanks !
pranmala

Isn't this what the tiny PICS are for?