If the Spice definitions aren't clear, you can just look at the gate input triangle to see where it actually switches.
Most cmos schmitts will have their switch limits straddling Vcc/2, maybe a bit less. A simple RC (or R with no obvious C!) should make a pretty symmetric square wave.
I wonder how big R can be. 1 Gohm easily. Maybe 1 Tohm if everything is clean. An open cmos gate input will hang high or low, whichever way you leave it, for seconds at least. I had one floating cmos level last week that took about 15 seconds after powerup to change state. That might roughly correspond to a pA of leakage.
Wrong again! I don't know why you keep doing this.
My sketch didn't specify HC logic. All I did was post a scribble that had a schmitt oscillator in it, followed by a cap+diode charge pump. That was the idea I offered. You offered zero ideas.
I fact, an HC14 would easily charge-pump 5 mA average current into a blue LED. But the HC series is ancient, and most more modern cmos logic has even more drive capability at 3.3 volts.
I offered an idea. For free. I didn't volunteer to do a detailed design.
Sure, less current makes less light, and you can break any circuit if you use sufficiently stupid values. But all of your sims pushed reasonable currents into the LED, so your "doesn't work" and "can't work" and "cockamamie circuit" comments were doubly bizarre. Your own sims are working examples of my circuit; you just didn't realize it.
Why bother? I *know* that it works, and you have already proved that it works.
And you proved that the schmitt+resistor thing *does* oscillate, as another geezer swore it wouldn't.
Name a few whose input switch limits don't straddle Vcc/2. Then I'll name a few that do.
Couple tenths of a volt maybe, at 3.3 supply. Most cmos inputs transition a little below Vcc/2. It's not guaranteed, and there are lots of cmos parts around, but that's the pattern. Hadn't you ever noticed?
Insane. ECL input transitions are at about 1.35 volts below Vcc, independent of Vcc-Vee. And their outputs swing about two junction drops.
The NXP HC14, with 4.5 volts Vcc, has datasheet typical switch levels of 2.38 and 1.40. The mean of those two is 1.89. Vcc/2 is 2.25. So the stuff I said is right.
What don't you understand about "pretty symmetric"? Hint: the pulses you simulated weren't very symmetric.
A ballpark estimate of gate leakage current is useful. More than a ballpark is useless for design purposes. Knowing that the leakage is in the low pA range would allow one to use a 10M or 100M or 1G oscillator resistor with perfect safety. You can't do that with a classic 555.
So, how big a timing resistor is it safe to use with a CMOS 555?
150 mA decaying to near zero over a 4 nanosecond pulse repeated every 10 microseconds: (Did I get that right?)
Suppose the average current during the pulse is 50 mA. Doing that for 4 every 10,000 nanoseconds would make the average current 20 microamps.
Since a usual blue LED is likely to be more efficient at 50 mA than at
20 uA, I would expect it to be brighter with this than with 20 uA steady DC. (Ratio of photometric output to current usually peaks around 1.5-4 mA with these LEDs.) So, I would expect brightness typical of 30-40 microamps steady DC. That sounds to me on the dim side for an indicator LED, but I do expect this to be visible.
If this blue LED is switched to 3.3V through a MOSFET, such as in a CMOS IC, then the full 3.3V is available. A fair number of blue LEDs nowadays on average need no more than 3.3V to push 20 mA through them, and that would make them so bright at 20 mA that some of these come with warnings to not stare into them. So, it sounds easy to me to get half a milliamp or a milliamp through them from 3.3V without a boosting circuit. And at this much current, many blue LEDs get plenty bright.
--
Well, I'm talking "straddle" in the strict sense, which would mean
that the typical switch points would be equally displaced about Vcc/2.
But, OK.
For the Philips case we have:
Vt-typ Vt+max
1.4V 2.38V
| |
0V--------------+--------+--+---------------------4.5V
|
2.25V
Vcc/2
Straddle?
That's almost sidesaddle!
Even more intersting, the minimum and maximum thresholds for HC:
Vcc/2
2.25V
Vt+min | Vt+max
1.7V | 3.15
| | |
0V---------+--------+--+--+--------+--------------4.5V
| |
0.9V 2.0V
Vt-min Vt-max
Which says that for a device with a Vt+min of 1.7V, Vt-min will,
presumably, lie between 1.7V and 0.9V.
Your turn now; let's see even _one_ that straddles Vcc/2
symmetrically.
We use blue LEDs on our VME modules to indicate bus access. I started with Cree SiC parts, driven by two paralleled 74F38 sections, 5 volts,
27 ohms. That was just barely OK.
As the parts kept getting better, my customers started complaining about the blinding blue LEDs. So we keep ratcheting up the resistor. We currently use a 2N7002 or BCX70 and 1K to +5.
This can't go on forever, since the LEDs have to eventually stop at
--
Hmmm...
Since you weren't up front enough to explain why, or if, that's true,
and maybe take some lumps if you were found to be wrong, it sounds to
me like you're trying to set a trap so that, no matter what my
response might be, you'd impugn it with garbage which would then have
to be shown to be garbage and refuted.
Generally a huge PITA since the rate of new garbage in from a
detractor seems to increase as the old garbage in is disposed of.
As usual, it's much easier for a scoundrel to make up charges than it
is for his target to refute them, so unless you reveal what you're
holding in abeyance, I'm outta here...
In that case, the blue LED will really shine. Even if the current flows through the LED during high current pulses that the LED handles half as efficiently as it handles 5 mA steady DC, the LED will still be plenty bright. Consider how bright most of these LEDs appear with 2-2.5 mA of steady DC. That is usually 12.5-16% of the brightness that they achieve with 20 mA of steady DC. Many of these are characterized at 20 mA and are rated 30 mA maximum average and a fair subset of those come with warnings that staring at them could cauase eye damage.
They are usually "good-and-bright" at 1 mA and they usually achieve typical "indicator LED brightness" at ~~ .5 mA.
OK, you don't know why all of your sims pumped about 5 mA into the LED.
Of course it's because a charge pump outputs an average current
I = C*V*F
where V is how much the voltage across the cap changes every cycle.
Assuming a typical diode and blue LED, when the schmitt is low, there's about 2.7 volts across the cap. When it's high, there's about
0.3 in the other direction. So delta-V is around 3. At 150 KHz and 10 nF, that works out to 4.5 mA, close enough.
It's that simple. As long as the schmitt can get close to the rails by the end of each transition, the internal impedance of the schmitt, and its rise/fall times, don't matter.
What's weird is that you used sims to prove that "my" circuit wouldn't work, and all of your sims showed that it *does* work, but you wouldn't believe it.
The nice Infineon right-angle surface-mount blues look good as panel indicators, backlighting a window in a sticker, at 2.5 mA. Our next gadget will use light pipes to go from a top-firing surfmount LED to just below a sliding cover.
Under ideal conditions, a good Agilent green LED up against my eyeball, dark adapted, I could just detect light at about 700 pA.
I sem to figure the greens may be dropping 2.6 volts. So, I figure about 4 mA, 10% of the time to get an average current of .4 mA. Since these LEDs are probably a little more efficient at 4 mA than at .4 mA, I would expect brightness about typical for .5 mA. And I have some Cree and Nichia green LEDs that are plenty bright at .5 mA.
You or anyone else want LEDs that get nice-and-bright at low current?
I just tested a Cree CP41B-GFS-CM0N0784 that I got from Digi-Key. That is a 4-pin through-hole part having characterization current of 30 mA and maximum average current of at least 35 mA IIRC. I would expect its ratio of light output to current to be maximized somewhere in/near the range of 2-5 mA.
The test that I just did was to measure light intensity at 100 mm from the LED, with a Lutron LX-101Alight meter, with LED current of .5 mA. The reading was 0 lux when the LED was off, and 10 lux when the LED was on. As a result, it appears to me that this LED with nominal viewing angle of 70 degrees achieved 90 millicandela at .5 mA.
Please don't blame me if repeating this test with different instrumentation and LEDs from a different lot being operated at current far from what they are characterized at only deliver 50 or 40 or 30 MCD at half a milliamp (still plenty good-and-bright for indicator LED purposes), especially for rated "viewing angle" wider than 60 degrees!
How about a more extreme one? In the old-fashioned-through-hole 5 mm (T1-3/4) through-hole format? Nichia NSPG520AS! Sadly, only (at least officially) available from Nichia's sales offices, in minimum quantity of
100. IIRC, I got 100 of them for 60 cents each plus shipping. I tested a few (all from same lot) and achieved 25-40 millicandela at .15-.16 milliamp (with the variation mostly noise in my measurement with a rather inexpensive light meter). The nominal viewing angle of that one is 45 degrees. I found more like
48, maybe 50.
There is a 3 mm version - NSPG320CS. I found the viewing angle to be more like 50-55 degrees, hardly narrower than most diffused LEDs rated for
60 degrees. (Viewing angle is usually defined as where intensity is halved by being half that angle off-axis.) My limited testing of a few units in one lot found 25-40 millicandela at .21-.22 milliamp.
And, 10 millicandela appears to me to be somewhat maybe-fairly above median for indicator LEDs, and 15 MCD appears to me to be somewhat to maybe fairly above mean.
How about a white one with an almost-Lambertian-wide viewing angle of
110 degrees (half intensity 55 degrees from axis) and extremely efficient and Digi-Key-available?
That one is Cree C535A-WJN-CU0V0231. It is a 5 mm / T1-3/4 LED that is stubbier than the usual "bullet" shape ones. Minimum order of 1 at 58 cents plus shipping/handling and any under-minimum-order fee.
That one deals with low current extremely well. I just tested one to have voltage drop of 2.6 volts at .5 mA. I found 30-40 millicandela, and it may take 15-25 to appear decently bright for a white LED as opposed to a deep color one that appears similarly decently bright at 10 MCD.
Sampling - 1 unit of 1 lot with current 2.5% of "characterizing current". I disclaim warranty beyond what I got paid to post this if your LEDs fail to do what mine did. :) However, I expect extremely high rate of these "wide angle" white LEDs being "nice-and-bright even for white indicator LEDs" at 1 milliamp. Even after being filtered to yellow by yellow "plexiglas" (to a shade of yellow less orangish than usual of yellow LEDs - maybe pushing to "a hair greenish").
3 milliamps has efficiency or at least "overall luminous efficacy" of these close to maximized at fairly close to 100 lumens/watt. Voltage drop of the one I just now tested is about 2.73 volts at 3 mA. My Lutron light meter says 31 lux at 10 cm - .31 candela, 310 millicandela. Probably around .8 lumen from close to 8.2 milliwatts!
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