less LEDs = more Battery-lifetime
Moderators: adafruit_support_bill, adafruit

less LEDs = more Battery-lifetime

by lu1s on Thu Apr 26, 2012 4:43 pm

Hi!
I want to desolder 2 LEDs to expand battery-lifetime. Does this increase the power to the other LEDs or is the power limited to a specific level? (It seemed to me the NPN would do that). I dont want to "burn-out" the 2 remaining LEDs.

Another thing:
On v1.2 when setting the EU-only-Flag, the code wont compile. I had to add the code_naXXXTimes-codes 004,009,005,031,021,000,022 to the second code-part at "#ifdef EU_CODES" in the file WORLDcodes.c
lu1s
 
Posts: 2
Joined: Thu Apr 26, 2012 4:32 pm

Re: less LEDs = more Battery-lifetime

by john444 on Sun Apr 29, 2012 2:18 pm

Hi Lu1s,
Your concern about "burn-out" of the LEDs is justified.
From the schematic (http://www.ladyada.net/make/tvbgone/design.html)
each of the 4 LEDs has its own drive transistor with no current limiting resister.
This circuit relies on the internal resistance of the battery to limit current to the LEDs.
If you remove some LEDs then the other LEDs will draw more current since all the LEDs are in parallel.

This would result in little increase in battery life and even though IR LEDs are pretty rugged,
current though the remaining LEDs may become excessive reducing their life.

It will be necessary to add a resister in series with the remaining LEDs to limit their current.
It is difficult to calculate the value needed since we don't know the battery resistance.
But, you could start by trying values in the 10-ohm range or less.
john444
 
Posts: 443
Joined: Sun Mar 04, 2012 2:42 pm
Location: Claremore, Ok

Re: less LEDs = more Battery-lifetime

by lu1s on Tue May 01, 2012 5:55 am

ok thx, it seems there is not enough space left on the board. I'll leave it way it was.
lu1s
 
Posts: 2
Joined: Thu Apr 26, 2012 4:32 pm