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Vintage LED's

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CatalinaWOW:
You have some real human factors challenges in your project.  I have some very early red LEDs and by today's standards they are nearly invisible.  If any modern LEDs are on the eyes will adapt to make them invisible in reality.  As I recall the instantaneous dynamic range of the eye is about 40 dB which probably definitely doesn't cover the brightness difference between high power illumination LEDs and those early units.

cdev:
I have a bunch of old LEDs in a bunch of different colors/shapes that I got from the estate sale of an engineer neighbor/silent key.

One thing thats useful about some of them, especially the 7 segment displays, some make pretty good varactors when back biased, they act like variable capacitors. (not emitting any light)

This property is not consistent from one to the next, some are substantially better than other identical displays.

zingzang:

--- Quote from: spec on January 10, 2019, 04:55:26 am ---
--- Quote from: zingzang on January 10, 2019, 01:46:27 am ---
--- Quote from: spec on January 09, 2019, 10:04:49 pm ---Not sure if you already know this, but the best and most reliable way to drive a LED is with a constant current, rather than a constant voltage.
--- End quote ---
I'm not sure if I know that - you mean DC verse AC but I was under the impression that DC current is constant and that the only issue in my situation would be not to feed too much voltage into them. I'm a novice with electronics so feel free to teach me what I'm missing! 
--- End quote ---
OK No problem.

No not DC versus AC- just DC.

The forward voltage of a LED for a given current is indeterminate and varies from sample to sample even of the same LED type and from the same batch. The forward voltage is also dependent on color, age, and temperature, especially with the earlier LEDs.

The main objective with a LED is to have it look as you like, in terms of brightness etc. But once again, there are more variables in this because the efficiency of LEDs also varies quite a bit. With early LEDs the variation in brightness is quite large as is the beam angle.

So the best way to illuminate a LED is to feed a defined DC current through it (in electronics this is called a constant current). The LED will then be able to have whatever voltage across its pins that suits it at a particular brightness and temperature.

You can generate a semi constant current by putting a resistor in series with the LED and connecting the circuit to a DC voltage, but this is not very accurate unless the voltage is relatively high. Or you can use a simple circuit to generate an accurate constant current. Note that although the current is constant, it can be set to any value that you like, within reason that is. But once set the current stays constant irrespective of the LED forward voltage and any changes in the LED forward voltage with temperature etc.


--- Quote from: zingzang on January 10, 2019, 01:46:27 am ---Adjusting the input voltage to be in exactly the right spot on on the diode's I/V curve would be much more complicated.
--- End quote ---
It would be harder than that. :)


--- Quote from: zingzang on January 10, 2019, 01:46:27 am ---A 3V coin cell has an internal resistance high enough to limit the current.

--- End quote ---
Afraid this is not true. :)

--- End quote ---

Excellent write up! Thank you. I'm not terribly concerned with getting the exact voltage for maximum brightness so I'll likely error on the side of caution. The early LED's are still pathetically weak compared to modern (or even late 70's) LED's so they'll look underpowered either way.


--- Quote from: spec on January 10, 2019, 05:31:19 am ---+ zingzang

Just to get the scope of this project:

* Can you say what the maximum number of LEDs on display will be?
* Will you want the LEDs to be illuminated one at a time or simply all on, or perhaps any number of LEDs on and any number off in any combination?
* Will you want to vary the brightness of the LEDs. If so, all together, or individually in any combination?
* Will you be displaying any modern high-power LEDs (say 1W) as a contrast with the older LEDs?
* What will be the power source: battery or mains?
--- End quote ---

* Aiming for somewhere between 10-20 but it could end up being fewer than 10 depending on what display case I go with
* Unknown - I've thought about keeping it simple and having them all power on at once. But with the abilities that come with arduino I have been thinking of having a delay between LEDs.
* Probably not for most of them. There is one LED that is yellow at peak voltage but turns red as the voltage decreases - I'd like to have that bad boy cycling to show off the feature. The other LED's that change are either 3 wire (so just alternating between one die and the other or they have an IC built in (a slow blinking LED I had.)
* Probably not anything that powerful. I have a few early white LED's but they aren't that bright. I'm still deciding what post-2000 LED's deserve to be included. I'm focusing on 3mm and 5mm LED's so I'd like to avoid the SMD super bright LED's.
* Most likely battery but that could change if I need more powerI'll be consulting with everyone here as the build progresses so stay tuned. Probably won't even begin to play around with the circuits until next month.

--- Quote from: CatalinaWOW on January 10, 2019, 06:45:29 am ---You have some real human factors challenges in your project.  I have some very early red LEDs and by today's standards they are nearly invisible.  If any modern LEDs are on the eyes will adapt to make them invisible in reality.  As I recall the instantaneous dynamic range of the eye is about 40 dB which probably definitely doesn't cover the brightness difference between high power illumination LEDs and those early units.

--- End quote ---
Agreed. Thats why I'll probably leave out the high powered LED's. I did find an individually addressable 5mm from one of those neopixel websites that I feel fit's in with the T1's (the classic "bullet" shaped LED) If the brighter LED's wash out the dimmer one's then I'll likely resort to having the series of LED's light-up on a delay.

--- Quote from: cdev on January 10, 2019, 03:26:32 pm ---I have a bunch of old LEDs in a bunch of different colors/shapes that I got from the estate sale of an engineer neighbor/silent key.

One thing thats useful about some of them, especially the 7 segment displays, some make pretty good varactors when back biased, they act like variable capacitors. (not emitting any light)

This property is not consistent from one to the next, some are substantially better than other identical displays.


--- End quote ---
Interesting effect. I've not messed around with the 7-segments much but I have seen first hand the unusual qualities some of these LED's have.

zingzang:
Forgot to mention that I was going through some new arrivals and found a bi-polar red LED. Both dies are red (the anvils are a cool design) but I'm wondering why go through the trouble of designing it?

I get if one of the dies were a different color - that way reversing polarity would produce a different color. But why have two identical dies? Was this an attempt to power LED's using AC?

james_s:
Yes, there have been a number of LEDs made for use in AC that have two dies in them. It solves two problems at once, producing light on both half cycles, and protecting each other from reverse voltage. Many early LEDs are easily damaged by reverse voltage. They are also not very bright so need all the help they can get.

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