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Help with simple 555 timer

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)

formatting link

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

Reply to
pranmala
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pranmala scrobe on the papyrus:

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

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John B
Reply to
John B

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.

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Bill Sloman, Nijmegen
Reply to
bill.sloman

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.

Reply to
mc

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

--
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Reply to
Spehro Pefhany

Isn't this what the tiny PICS are for?

Reply to
Brian

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