[Ronja] some questions.. plz help

[Saad Shakeel]

Thanks J.D Bakker, for your detailed reply, i'll start searching for something in the nanosecond range.

"J.D. Bakker" <jdb at lartmaker.nl> wrote:
  >Similarly , though i wasn't able to find any of the HPWT series LEDs 
>here in Pakistan, i saw alot of seemingly unbranded leds with 
>ratings of 15,000 mcds!! Is that possible to achieve? the shopkeeper 
>told me these were chinese leds and are generally used in traffic 
>signals... any idea if they might work out??

There are four issues involved here:

1) Many of those LEDs achieve high mcd ratings by radiating all their 
energy in a narrower beam than other LEDs. This may make it harder to 
couple the LED's energy to the optical system (ie: the lens), and 
alignment becomes more critical.

2) Most Ronja systems are on 24 hours a day. Cheaper LED chips tend 
to have shorter lifetimes, so you'll need to replace your LED sooner 
(which is not easy, as it requires dismantling the entire transmitter 
head).

3) Related to (2): ultimately the amount of power a LED can emit 
depends on how well it can get rid of its heat. The HPWT-LEDs have a 
special package with more pins, designed to handle heat better than 
normal 5mm LEDs.

4) The LED needs to be switched on and off very fast (~40ns max 
rise/fall time for Ronja, I'd say); not all LEDs can do this. This 
depends on the materials used in the LED and the size of the LED 
chip. Unfortunate trade-off: larger LED chips (needed for more power) 
have larger capacitance and are thus slower to switch on/off.

Then again, you can always try, although I would suggest trying to 
find a LED in a medium-power enclosure like the HPWT. Search for 
'Superflux LED', 'Spider LED' or 'Piranha LED'.

>And can we use an array of leds instead of just to get our required 
>signal strength? like maybe arrange them in a circle or so.

They all need to be focused on the same point (the receiver); this is 
not possible when you use only one lens for all LEDs. In theory you 
could have multiple synchronized transmitter heads, but I don't 
believe anyone has ever done that.

>i also came across siemens ld271
>%C2%A0%C2%A0>http://www.datasheetarchive.com/LD271-datasheet.html> 
>and another by the name led 55c
> 
>http://www.datasheetarchive.com/search.php?t=0&q=led+55c&manystr
> any one with some idea about these? Both are infrared.

Both are too slow (~1us rise/fall times), and their power output 
isn't that great.

Good luck,

JDB.
-- 
LART. 250 MIPS under one Watt. Free hardware design files.
http://www.lartmaker.nl/

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