# led's

G

#### George Herold

Jan 1, 1970
0
Phil is fond of ebay APDs, which are apparently surplus from some

expensive projects. How fast do you need? Pulse or sine wave?

Yeah, all good questions. I'll have to send an email to Cliff. I'm guessing it'll be easier to modulate with a sine wave. So at 100MHz I'd get a full 2*pi phase shift with a path lenght of 3 meters. That seems reasonable. But I guess a bit slower would work too ~50 MHz. (Sometimes I make something, measure it, and then define the spec.)

I'm not sure we can use ebay as a source, unless I can buy a few build it and then get several hundred more. And do I need APD's? I've been reverse biasing all sorts of diodes lately. The optoelectronics PD's I'm using list a maximum reverse bias of 30V, I've had 'em up to 60V and no problem. (Iran out of voltage.) I was wondering if I could make garden variety PD's avalanche.

George H.

D

#### Daniel Pitts

Jan 1, 1970
0

Those are not "3 volt leds." Their typ forward drop is 3.2, but they
are still not shown as being voltage operable; they are spec'd at 20
mA.

---
Well, with no voltage to drive current through the junction they
wouldn't work at all, so they are certainly "voltage operable".

Also, notice that the data sheet shows a range of from 2.8 to 3.8V
across the junction with 20 mA through it, so 3.0V dropped across the
junction with 20mA through it is certainly within the range of
possibilities.

Consequently, for that LED, connecting 3.0V to it directly will force
20mA through it.
You've got cause and effect backwards here. Forcing 20ma through it will
cause ~3.2v drop across the LED. It is not necessarily the case that
3.2v will cause 20ma. Diodes have an exponential curve relating voltage
to current, a slight change in voltage can have a significant change in
current. Which is why you want to have some other device (eg, a
resistor) to help set the current.
Ergo, 3 volt LED.
Ergo, Nope.

M

#### Mr. Man-wai Chang

Jan 1, 1970
0
Someone as dim as you should probably steer clear of sarcasm!

Is this 1mA LED military grade stuff?

--
@~@ Remain silent. Nothing from soldiers and magicians is real!
/ v \ Simplicity is Beauty!
/( _ )\ May the Force and farces be with you!
^ ^ (x86_64 Ubuntu 9.10) Linux 2.6.39.3
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G

#### George Herold

Jan 1, 1970
0
On Thu, 1 Aug 2013 10:30:01 -0700 (PDT), George Herold

A modulated laser, ballpark 1 milliwatt, will make gobs of signal into

an ordinary photodiode, no need to avalanche. If you're using sine

waves, and can use a tuned amp, even better.

Hey! That's interesting. Could I resonate the PD capacitance with some inductor? I could even tune it a bit with the PD reverse bias. (Or were you thinking of a tuned stage after the PD?) (I had this 'crazy' idea in the past about using a T-coil* as part of a PD front end... only to find that Phil H. had already done it.)

George H.

*this was soon after reading about T-coils in one of the Jim Williams' books.

G

#### George Herold

Jan 1, 1970
0
On Thu, 01 Aug 2013 19:35:47 -0500, John Fields <[email protected]>

The way you use it, it's not a law, it's a definition.
Hey this is kinda interesting. (But let's not have a big John vs John confrontation.)

So last week I was running this workshop on noise. I knew I'd have some spare time while the attendees were doing stuff. So I took along a setup to measure the Johnson noise of a light bulb with a DC current going through it. (The measruements were a bit of a pain, I had to abandon the inductor Iwas using as a bias element and go with a simple resistor...anyway that's not important.)
So at some voltage across the light bulb I measured the current. And I tookthat ratio to be the resistance of the bulb. And then I assumed that the bulb would be making Johnson noise given by v^2 = 4kTR*BW. Where I'd seemore noise because of increased temperature of the bulb.
(The idea was to try and measure the temperature.)
Do you think there is something wrong with this 'theory'?
Does the light bulb have resistance?
Does it have Johnson noise?
What's the 'correct' relation between them?

George H.

G

#### George Herold

Jan 1, 1970
0
Sure, any resistor has Johnson noise.

If you were to drive the filament through an inductor or a noiseless current

source, and couple a small AC signal into it with a capacitor, at higher

frequencies it would look like a resistor of some ohms, and it would havethe

corresponding Johnson noise. If the filament is incandescent, the Johnsonnoise

will be high.

Yup, that's what I tried (inductor from voltage source) (Driving a light bulb with a current source is asking for trouble. IMHO)
Problem was I didn't have a big enough toroid and the coil inductor picked up magnetic noise big time. (I already knew this, but sometimes I need my face rubbed into something a few times till I remember.)

So I got some data with a resistor as bias, but at the highest temperaturesthe bias resistance was much smaller than the DC bulb resistance and it shunted most of the noise to ground.
That equivalent Johnson resistance would not be the DC E/I and will (probably)

be close to the slope, dE/dI at the operating point. Thermal mass makes dE/dI

frequency dependant. The exact math is beyond my pay grade.

Hmm.. OK I'll have to think some more... why dE/dI?

RE the thermal mass: The time constant for a bulb is something like a second maybe 10mS at the fastest. So only at low frequency is that going to be an issue.
I bet a filament has mountains of low frequency noise. Might be interesting to

measure.

Well I had the thing perched on bubble wrap to try and keep the building shake out of it. Better might be to suspend it. I would sometimes see a bunch of low frequency 'crude'. Which I assumed was adjacent filament loops bumping into each other. But there were long periods of relative quite. (Ialso could filter out the LF stuff.)

One issue is that the filament loops were also an inductor and they would pickup local magnetic field interference. (Mostly from the room lights, which I couldn't turn off because those in the workshop would have objected.) The pickup would increase at higher bulb currents.. which still has me a bit confused. So before I try this again I need some big torodial inductors and some mu metal to shield the bulb.

But this resistance question is great! What does resistance really mean? (Should I be thinking in terms of damping or energy?)

George H.

J

#### Jasen Betts

Jan 1, 1970
0
The filament is a heated thermistor that makes its own turbulence in
the gas. Gotta be noisy.

Flashlight bulbs used to be "filled" with a vacuum, I think they're
using krypton in some now.

probably magnetic fields were moving the filament, changing is emission
pattern and therefore measured output.

P

#### Phil Hobbs

Jan 1, 1970
0
Phil is fond of ebay APDs, which are apparently surplus from some
expensive projects. How fast do you need? Pulse or sine wave?
There was that few hundred pieces of the 1.3 um APDs surplused from
Terabeam that went for about 1 cent on the dollar three or four years
ago, but nothing interesting since.

We should still figure out something useful to do with those.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

Briarcliff Manor NY 10510 USA
+1 845 480 2058

hobbs at electrooptical dot net
http://electrooptical.net

G

#### George Herold

Jan 1, 1970
0
Johnson noise is pretty small in amplitude, so it's only sensitive to
the differential resistance. On time scales long compared with the
thermal time constant of the filament, you'd have to worry about the
nonlinearity, but at short time scales it would just look like a 2500K
resistor, I'd expect.

So there's some energy dissipated in the bulb given by the DC V*I.
But that doesn't come into the Johnson noise.
Then I was thinking of the derivation of Johnson noise that starts with a parallel RC and the equipartition theorem. (the capacitor holds 1/2 kT of energy = 1/2 CV^2) and then you integrate over the RC bandwidth to get the Johnson noise formula, So 'R' in this case is, as you say, the dynamic R.. And that's pretty cool!

So the data for my first crack at this was pretty crappy. (not much signal, since it was all lost in the small bias resistor.. big error bars.) But I got a number for the temperature of about 1500K for a bulb that was a little past orange and getting to yellow. And that number seemed small. ButI was using a DC resistance value. The dynamical resistance looks to be ~50% bigger..

The filament is obviously not in thermodynamic equilibrium, so the usual

math underlying the Johnson noise formula doesn't quite work, but with

DC excitation I'd expect it to be very close. It would certainly work

if the filament were at equilibrium in an isothermal 2500K oven, and

metal conduction is pretty well exactly ohmic, so I wouldn't expect the

comparatively small net drift velocity to make much difference at all.

Ah OK the drift velocity looks pretty small compared to all the other corrections.

George H.

G

#### George Herold

Jan 1, 1970
0
I did once make a photo-optical link that transmitted usable audio,

from a radio. With a flashlight bulb.

The filament is a heated thermistor that makes its own turbulence in

the gas. Gotta be noisy.

Grin, there's not that much gas inside the bulb. It really wasn't too bad..

George H.

G

#### George Herold

Jan 1, 1970
0
---

The filament of an incandescent lamp is made of pure tungsten, which

has a positive temperature coefficient of resistance and is

extensively heat-treated in order to make it ductile enough to survive

the drawing and coiling process required to form it into the shape

required for a particular filament.

Thermistors are an entirely different breed of cat in that they're

constructed by pressing and sintering metal oxides, generally have a

negative temperature coefficient, and can only be enticed to emit

humanly visible photons once, and then only for a very short time.

---

---

Marie and Pierre Curie sifted through a ton of pitchblende, in the

winter and in unheated quarters, and discovered Radium.

You couldn't make your measurements after everyone else left your
comfy lab and you had it all to yourself?
Yeah the workshop was at a university.. not my comfy lab.
Why what? Why try again? why a big toroid?, why shield the bulb?
---

---

It's arguably best to think of resistance as collisions between

electrons which liberate heat.

Well there are other 'kinds' of electrical resistance.. but I agree electron scattering is the most common.

But if we stick to light bulbs, then there is a question of the DC resistance vs the dynamical resistance.

For instance if I was to use a bulb as part of small signal RC low pass filter. (The bulb is the R) Then I think it will be the dynamical resistance (dV/dI)

----

---

With AC coupled into the filament, its wound nature will cause it to

look inductive, so it can't look like a pure resistance regardless of

how lightly it's driven.

Well sure, even without the turns it has inductance and capacitance.

George H.

G

#### George Herold

Jan 1, 1970
0
Last time I checked - the only way you can buy a flashlight bulb is buy a

flashlight with one in it.

There are still one or two discount stores that carry bulb type

flashlights - but increasingly they're dual type with a ring of LEDs round

the reflector.

You can still buy light bulbs from Newark.

George H.

G

#### George Herold

Jan 1, 1970
0
Flashlight bulbs are pretty much extinct in the UK - there may still be a

few long established independent bicycle shops that carry a small assortment

of dynamo bulbs.

As for household lightbulbs; I think anything over 60W has been banned and

anything lower is thin on the ground.

They have introduced halogen capsules enclosed in a regular glass bulb with

a standard (for UK) bayonet cap - the highest I've seen is 70W, but that is

probably as bright as the 100W filament most people would buy given the

choice.

LED house lights are starting to appear in supermarkets - but as yet are

some way short of matching CFLs.

Newark lists 957 incandescent bulbs, isn't element 14 in europe?

http://www.newark.com/jsp/search/br...tt=incandescent+bulb&Ntx=mode+matchallpartial

George H.

T

Jan 1, 1970
0
Never heard of it, so maybe not in the UK or only in cities.

Element 14 was called farnell. They are at least in the UK, Germany,
Austria and the Netherlands. I do not know if they have connections with
Newark.

Draw back is that as private person you can not buy there, atleast not
in Austria and Germany.
It took me a long time to get a Raspberry pi.

G

#### George Herold

Jan 1, 1970
0
Element 14 was called farnell. They are at least in the UK, Germany,

Austria and the Netherlands. I do not know if they have connections with

Newark.

Draw back is that as private person you can not buy there, atleast not

in Austria and Germany.

It took me a long time to get a Raspberry pi.

Wow, that stinks. So where do you get electronics?

George H.

T

#### Tom Biasi

Jan 1, 1970
0
You seem to assume that a single shot always takes out every enemy

Ha!

Can you prove that mathematically?
As an NRA certified instructor I must advise against putting any
obstruction in front of the muzzle. As far as accuracy, just a small
nick in the crown at those distances will lead your bullet wide astray.
Just sayin"
Tom

P

#### Phil Hobbs

Jan 1, 1970
0
I could do with a scaled down version of that tracking radar.

In this hot weather with all the windows open, loads of flies get in,
the fly-spray seems inefective unless you score a direct hit (in some
cases you have to literally drown the little fuckers!) - the bastards
come in low & fast so its difficult to draw a bead on them.

I recommend appropriate technology: a screened window with a box fan
sitting in it.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net

F

#### Fred Abse

Jan 1, 1970
0
Element 14 was called farnell.

Still are. Premier Farnell, PLC.

They are at least in the UK, Germany,
Austria and the Netherlands. I do not know if they have connections with
Newark.

Farnell have owned Newark for more than ten years.

Their instruments division (now gone) used to own Wayne Kerr.

Alan Farnell started the company selling components to UK radio repair
shops, out of a suitcase.

P

#### Phil Hobbs

Jan 1, 1970
0
On 08/05/2013 04:30 PM, Ian Field wrote:

On Mon, 5 Aug 2013 20:12:11 +0100, "Ian Field"

On Sat, 03 Aug 2013 17:32:27 -0500, John Fields

On Sat, 3 Aug 2013 20:49:36 +0100, "Ian Field"

message
On Sat, 3 Aug 2013 18:44:58 +0100, "Ian Field"
<[email protected]>
wrote:

message
On Sat, 3 Aug 2013 16:26:06 +0100, "Ian Field"
<[email protected]>
wrote:

On 2013-08-02, John Larkin <[email protected]>
wrote:

The filament is a heated thermistor that makes its own
turbulence
in
the gas. Gotta be noisy.

Flashlight bulbs used to be "filled" with a vacuum, I think
they're
using krypton in some now.

Last time I checked - the only way you can buy a flashlight
bulb is
a
flashlight with one in it.

You can buy spare Maglite bulbs.

I bought a Maglite - and quickly formed the conclusion that they're
overrated.

A 3W LED flashlight is brighter, uses less batteries and the 'bulb'
will
probably outlast me.

The Maglites are physically very rugged, sealed, waterproof. Lots of
LED
lights
are junk and *don't* last long. The Maglite bulbs last because such
lights
are
not used for weeks at a time. They usually have a spare inside, too.

I got stuck in the dark on a cliff-side hiking trail on the north
coast
of
Kauai. The sun goes down fast, splat-sizzle, at that latitude,
and then
the
killer frogs attack.

Have you been licking toads again!!!

(If you're playing sniper in the jungle - you can stick a frog on
the end
of
your rifle to quench the muzzle flash).

---
Quenching the muzzle flash is of no use at all when using a supersonic
projectile traversing a long distance, which is what snipers do, since
the flash will have died out long before the target is rendered meat
and the direction of the shot undiscernable.

You seem to assume that a single shot always takes out every enemy

---
On the contrary, my point was that with the advent of modern smokeless
powders and muzzle flash suppressors, a sniper's position won't be
betrayed whether or not he hits the target with the first round.

More than likely, though, as I understand it, he will. [hit the target
with the first round]
---
There are radar systems that track incoming rounds and fire on the
enemy guns even before the enemy rounds hit the ground.

Why not?

P2, col 1, 80 millisec response time to slew a reply weapon.
Those are actually even cooler than you'd think--they have to use a
bunch of near-field acoustics tricks to reduce the size of the
microphone array and keep the angular resolution. BBN was working on
that sort of stuff five years or so when I visited them in Cambridge MA.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

Briarcliff Manor NY 10510 USA
+1 845 480 2058

hobbs at electrooptical dot net
http://electrooptical.net

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