Good Old Fading LEDs

millwood

Joined Dec 31, 1969
0
The two 555 timer circuits are identical, except that U1 has an LED load on it's output.
and that made them NOT identical to each other, and NOT comparable to the circuit we discussed.

the output pin in the 555 has its output impedance: the data sheet gives you some hint about how big it is but for precision timing, you shouldn't be driving more than 1k load.

in our design, we are delivering 44ma in one direction, and 37ma in another direction.

in your design, one 555 is pulling about 40ma, and the other 555 is seeing no DC load. Hello?!

the load is affecting the output voltage on the output pin, thus the charging up / discharging of the rc timing network.

try to have your other 555 deliver a comparable current and then compare the two.

again, the "voltage" from your load means nothing. What does is the current.

simulation doesn't mean anything if you don't know what you are simulating.
 

millwood

Joined Dec 31, 1969
0
this is my sim using your file, with an added led for the 555 on the right, to provide a similar but not identical profile.

the capture is done after 9 seconds (0ms here = 9 seconds of run time).

as you can see, the current differential going through the two diodes (one LED and another just a plain silicone diode) is quite different: high 30ma vs. high 20ma, or 1/3 - 1/4 off. yet, timing is remarkably close.
 

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millwood

Joined Dec 31, 1969
0
same simulation, except one side has 3 LEDs (total forward voltage drop of 11v), vs. another side has Vf of 0v - D1 is shorted.

see much timing difference between this one and the one before?

not really, :).

hopefully you will have figured out why by now.
 

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SgtWookie

Joined Jul 17, 2007
22,230
and that made them NOT identical to each other, and NOT comparable to the circuit we discussed.
But they ARE comparable.

What do you think happens to the timing in your circuit when one of the outputs is high and the other is low (ie: load condition) vs. when they're both either high or low (ie: no load condition)? There will be a considerable amount of jitter in your base frequencies due to the variable loads the timers inflict on each other.

the output pin in the 555 has its output impedance: the data sheet gives you some hint about how big it is but for precision timing, you shouldn't be driving more than 1k load.
One can't say 555 timer and precision timing in the same sentence seriously. :)

in our design, we are delivering 44ma in one direction, and 37ma in another direction.

in your design, one 555 is pulling about 40ma, and the other 555 is seeing no DC load. Hello?!
And in your design, your load varies between 44mA + the charge/discharge current, 37mA + the charge/discharge current, and just the charge/discharge current.

Bueller? Bueller?

the load is affecting the output voltage on the output pin, thus the charging up / discharging of the rc timing network.
That's exactly my point.

try to have your other 555 deliver a comparable current and then compare the two.
Realize that you have more than just the two states.
If both of the timers are either high or low, there will be nearly zero current flowing through the LEDs.

again, the "voltage" from your load means nothing. What does is the current.
Right. The current through the load will cause a corresponding voltage drop across the Darlington voltage follower in the 555 output stage, which regrettably the Linear Technology model does not replicate well.

simulation doesn't mean anything if you don't know what you are simulating.
Correct. Please understand that you are completely forgetting about the difference in output source/sink current when one output is high & the other is low, vs when both are in the same state.

This will make roughly a 0.4v difference on the timing resistor, which will impact timing considerably - as the threshold voltage will not have changed.
 

SgtWookie

Joined Jul 17, 2007
22,230
this is my sim using your file, with an added led for the 555 on the right, to provide a similar but not identical profile.

the capture is done after 9 seconds (0ms here = 9 seconds of run time).

as you can see, the current differential going through the two diodes (one LED and another just a plain silicone diode) is quite different: high 30ma vs. high 20ma, or 1/3 - 1/4 off. yet, timing is remarkably close.
There is a very noticeable shift in just 70mS.
 

SgtWookie

Joined Jul 17, 2007
22,230
same simulation, except one side has 3 LEDs (total forward voltage drop of 11v), vs. another side has Vf of 0v - D1 is shorted.

see much timing difference between this one and the one before?

not really, :).

hopefully you will have figured out why by now.
Changing the loads on the outputs in that simulation does not mean that you are interpreting the results correctly from your simulation.

Perform a FFT on the waveform of your timing cap, or the output, of your original simulation. You'll see the jitter then.
 

millwood

Joined Dec 31, 1969
0
Perform a FFT on the waveform of your timing cap, or the output, of your original simulation. You'll see the jitter then.
just what exactly such a fruitless "analytics" will achieve for you?

I am done for this one, :).
 

SgtWookie

Joined Jul 17, 2007
22,230
Anyway, back to the original train of thought (a miracle, it only took three pages!)

Bill, I mentioned using the 20k resistor on pin 5 to ground to correct for the Darlington output drop; this seems to work fairly well with Vcc=10v. Attached is a simulation using a different model for a 555 timer that more accurately models a real-world bjt 555. Note that with just a 0.24mA load on the output (10v across a 42k resistor) the output drops to nearly 8.7v, or Vcc-1.3v. I used a 17k resistor on the 2nd 555 timer just to speed up the beat frequency.

Note that at the output of the 4070 XOR gate, the PWM (cyan trace) goes from nearly 0% to nearly 100%. Since the library in this sim didn't have a ULN2x04, a MPSA14 Darlington with a 10k base resistor was a reasonable substitute.

But if I were really going to build something like this, I'd use a microcontroller instead.
 

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fanie

Joined Jan 20, 2007
63
PDubya,
Us a 12F683 pic and it's PWM output pin 5. You can even set the dimming rate with it as well as add a switch to set them on if she wants to get up.
 

Wendy

Joined Mar 24, 2008
23,415
Really, an XOR gate is just one component (and pretty cheap, too)
Ah, but then you have to have drivers, which come with a 555. I understand your point about 555 Hysteretic Oscillator, the basic design isn't too stable, though predictable. So then there is always the fall back 555 design, though I might try the hysteretic oscillator approach. It really does need a 50% duty cycle, which the 555 versions of the hysteretic oscillator doesn't do.

Here is how I would do it.




Remember, the bridge can be one component too, as can the two 555's.


.
 

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