Help with basic 555 timer circuit

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 the the pulse width.

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)

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

Sounds like you have the wrong R and C values. Changing the C value from 100uF to .001uF will make the output time very short. Look closely at the parts your using, and be sure they are the right value. 100uF cap and 220K resistor should give you an output time of around 24 seconds.The 220K resistor will have color markings of red,red, and yellow. The 100uF capacitor will have polarity markings of + and -.

-Bill

If this is your first 555 circuit, I recommend using the 555 Timer Lite program to simulate the results of different component values.

On to your problem. First of all, I'm not quite clear on what you are doing with the circuit. It sounds like you are trying to use just the left half of the schematic to blink and LED. Am I correct in this assumption? If your purpose is to simply blink an LED, why not start from a schematic designed to do such? There are many available online.

This page shows photos of a breadboard with a simple 555 timer circuit:

Perhaps that will get you on your way.

Good luck, Tavys

pranmala wrote:

--- Try this: (View in Courier)

+V>----+-----------+------+-------+--------+---------------+ | | |K | |8 | [10K] [100K] [1N4148] [100K] +---+---+ | | | | | 2|_ Vcc |3 | +--[0.1µF]--+------+-------|---O|T OUT|----+ | | | 6| | | | | +----|TH | [R] |C1 | O | 7|_ 555| | [0.1µF] S1-->| +---O|D | [LED] | | O | | GND | |K | | |+ +---+---+ | | | [10µF] |1 | | | | | | | GND>---+--------------------------+--------+--------+------+

C1 should be wired directly across pins 1 and 8 and as close to the package as possible.

Every time you close s1 (a normally open switch) the LED should light for about one second and then go off. Closing S1 multiple times while the LED is on will have no effect on the timeout.

If you close S1 and keep it closed until the LED goes out, the LED may go on again for about a second when you open S1 because of contact bounce.

-- John Fields Professional Circuit Designer

Hello !, Thanks for the response JOhn, I do not follow what 1N4148 is. Is it a diode? I shall try the ckt again as soon as I can find the component Tavy, I will try the link you suggested and get back

Thanks again. Pranmala

--
Yes.

Great response!!!!

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I'm wondering how important the 1N4148 diode is that clamps pin 2 to

+V. I understand it prevents the capacitor voltage from moving pin 2 above the supply voltage, but if the capacitor is small, and the supply voltage is low, is it really necessary? What conditions would you think the diode is necessary, and what conditions not?

-Bill

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behaves exactly the same way Hello again John and Tavy, I had mixed results. John , I tried your circuit( By the way great technique with the courier font) and it behaves exactly the same way (pin 3 is high , when trigger is low, but as soon as trigger is high, pin 3 goes low. As I said before, its like the RC has no bearing whatsoever. I had some success with Tavy's ckt. (the astable vibrator) , but again the frequency of blinking was not affected by RC .

As one of you was asking, my main goal is the

'2 stage Time delay circuit'.

I want a second pulse to be timed based on the end of the first pulse.So basicaly a time delay based on a variable RC. The o/p will drive a buzzer in a ball machine ( to warn the player of a due ball). I want the user to be able to decide how much advance notice they want, with a variable potentiometer.

I am sort of desperate now and will try even a different chip if needed. I have bought the 555 double timer, but not sure if that wil make any difference.

Thanks once again folks for spending some time on this Pranmala

--
For a bipolar 555 it\'s generally not needed, but I stuck it in there
just to make the thing somewhat bulletproof.  The path to ground
from pin 2 is through the base-to-collector junction of Q7, (title
page of http://cache.national.com/ds/LM/LM555.pdf )so if the supply
voltage is high it might PIV that junction.  I don\'t know for sure,
but I couldn\'t find any spec\'s on the maximum voltage allowed, so I
figured the easiest way out would be to shunt the spike to Vcc.

I suspect It\'s all been thought out and with a worst-case supply of
18V the innards can stand a 36V spike, but with no spec\'s, who
knows?

There\'s also the internal path to Vcc, which seems to be through Q7,
Q8, Q11, and Q22, but again the lack of spec made me put the diode
in there for insurance. 

For the CMOS 7555 it\'s almost certainly not needed because the spike
will be diverted to Vcc through the ESD diodes, and as long as the
spike doesn\'t last long or send a lot of current through the diodes
long enough for the chip to latch up, everything ought to be OK.
Still, I almost always stick it in there, just in case.  A Schottky
if I\'m really concerned about it. :-)

--
Other than a bad chip, from your description the only reason I can
think of that the circuit doesn\'t work properly is that there\'s a
wiring error somewhere.  More than likely you have the switch wired
from pin 2 to ground and the output pulse is lasting for less time
than it takes you to press and release the switch.

Hello John, I had the same concen with a bad chip , so went back to Radio Schack and got 2 more chips and tried them with same result ( I am handling them with my hand, I assume it doesn't destroy them). Also the trigger (Pin 2) is held high with the 100K resistor , so I am making sure its not grounded, when not triggered. Also my time constant (1.1 RC , with 100micro farad and 100K resistor is over 10 seconds, but o/p led just blinks for (I guess) about less than half a second. I will look for some means to get an image out as clearly as I can , so you can see it. Thanks again for your help Pranmala

Ok, well it has been solved. But I didn't expect this one bit. it was my power supply!!! The multimeter showed a clean 9V DC, but when a friend suggested I replace with a (V battery , everything worked hunky dory!!!. I placed a

100 microfarad across the power supply and now even that works!!!. I guess I was geting too much ripple and variation in supply. Thanks once again to John and Tavy for the attention !!! Great support!

--
Congratulations!

Next time, an easy way to check your supply\'s ripple is to set your
multimeter to read AC volts and to connect 0.1µF or so in series
with your multimeter and the supply\'s output while the supply\'s
under load.