Oscilloscope + LED problem

[he11fire]

First I need to say I am a beginner in electronics but I'm trying a lot to understand it.
In this experiment i built an oscilloscope which works fine and the idea of the flashing LED popped up in my mind ...So I connected it to the Output of the 555 Timer but the LED works fine only when the signal is 1 .When the signal is 0 it flashes very fast ...Can any1 explain why does this happen ??

**broken link removed**

 

[audioguru]

There is no "1 or 0 signal" in your circuit.
R1, R2 and C2 make the frequency of the flashing about 90Hz which is very fast.

 

[Reloadron]

Also, in real life, unless your LED is designed for 5 volt operation you would have a resistor in series with it to limit the current through it. But that would only be in real life.

Ron

 

[audioguru]

Also, in real life, unless your LED is designed for 5 volt operation you would have a resistor in series with it to limit the current through it. But that would only be in real life.

Ron

No. An LMC555 is a low current Cmos 555.
"Real Life" didn't look at the datasheet of the Cmos 555. With a 5V supply its typical output high current to a 2V red LED is only 5mA which is already current-limited without a series resistor. A series resistor would just reduce the current even more and reduce the already dim brightness.

 

[Reloadron]

Real Life just screwed up. Never saw it and never caught it. Damn, I don't believe I missed that.

*Wanders off to sulk*

Thanks AG
Ron

 

[he11fire]

thanks for the reply ...what do you mean there is not 1 and 0 signal ...the waveform is clear ...
the frequency doesn't matter at this experiment because if you noticed the time passed is only 180 ms ...so I'm saying that the program runs the circuit very slowly and I should be able to see the LED light for a while and than not.
This is not a practical experiment...since i would not see any difference in the LED in real life.

 

[audioguru]

thanks for the reply ...what do you mean there is not 1 and 0 signal ...the waveform is clear ...

Your 'scope shows the output high of the Cmos 555 at +5V which is impossible. The LED forward voltage is 1.7V to 3.5V depending on its colour.

the frequency doesn't matter at this experiment because if you noticed the time passed is only 180 ms ...

The 'scope is set to 10ms/division so the 5 pulses that are shown are in a total time of about 50ms, not 180ms.

so I'm saying that the program runs the circuit very slowly and I should be able to see the LED light for a while and than not.

The LMC555 has a typical output high current of only 5mA into a 2V red LED. The duty cycle will make the LED appear to have an average current of only about 3mA which is dim.
The 90Hz will make the LED appear to be turned on all the time.

This is not a practical experiment...since i would not see any difference in the LED in real life.

What difference? The LED is continuously "blinking" at 90Hz.

 

[he11fire]

Than can you suggest what to do so the LED flashes at say every 1 second , in this experiment ? ...thanks for your time

 

[audioguru]

Than can you suggest what to do so the LED flashes at say every 1 second , in this experiment ?

Don't you have the datasheet for the LMC555? It shows an accurate formula for its output frequency and shows a graph of the output frequency with different timing parts.
The graph shows an output frequency of about 90Hz when the timing parts are 5k ohms, 5k ohms and 1uF like in your circuit. It shows 1 second pulses if the resistor values are increased to 500k ohms (use 470k) or if the capacitor is 100uF.

 

[he11fire]

I did exactly what you said but now the graph shows the tension constantly 5V and the LED stays on.
With the values before the LED stayed on for the Duty Cycle Time and flashed fast for the LOW time.
I am very confused ...

 

[audioguru]

Show on your schematic what you did and show the polarity of C2.

 

[he11fire]

Show on your schematic what you did and show the polarity of C2.

Here it is :
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Types of caps shown don't show any trace of polarity ...don't know if it matters...

 

[audioguru]

Multisim is stupid and does not know that most electrolytic capacitors are polarized.

You forgot to calculate the new frequency or see it on the graph. Since you increased the resistors and the capacitor then the frequency is about one pulse every 94 seconds.

I said, "It shows 1 second pulses if the resistor values are increased to 500k ohms (use 470k) OR if the capacitor is 100uF (not both).

 

[he11fire]

OK I see ...the problem was fixed ...
Still the main problem that the LED flashes very fast when the signal is low is not fixed ...
By the way do you use any software for your schemes and if yes what is the name ...

 

[audioguru]

When the output of the Cmos 555 is low then the LED should not light. It lights only when the output is high.
when both resistors are 470k ohms or when the timing capacitor is 100uF then the lED should blink at about one flash per second.
Maybe your fast flashing problem is caused because your circuit is missing a supply bypass capacitor. Try 10uF.

I use Microsoft Paint program to make most schematics by copying and pasting parts and entire parts of schematics from datasheets or from other schematics.

 

[he11fire]

I tried your suggestion but again the problem persists ...I think it might be a multisim application error ...I don't see any other reason.

 

[he11fire]

It seems that the 20ohm resistor after the LED fixed the problem :
**broken link removed**

 

[audioguru]

I am glad that you changed the timing resistors and capacitor so it flashes slow enough with about 7 flashes per second to see.

Didn't I say that Multisim is stupid?
It shows an output of +5V which is impossible. The datasheet for a Cmos 555 shows that its typical output is only 5mA into a 2.0V red LED without a resistor. Your 20 ohm series resistor will have only 0.1V across it so the output of the Cmos 555 should be only +2.1V, not +5V.