Articles Hierarchy

Articles Home » other » LED and LIGHT


until now i only play with pcb LED, or 12VDC LED strips / RGB / and PWM controlled
but now want to try so called
- High Efficiency LED / Super bright : 5mm or 10mm 20mA max 30mA or as 12V version with internal resistor
- power LED over 100mA

but i am also interested in reading the light intensity,
so i combine this with a measuring project:
Arduino with photoresistor

here some links what i just talked about:
ES: photoresistor datasheet

making some calculations for a theoretical calibration:
if the photoresistor is in series with a 5kohm resistor as shunt for Ain
and photoresistor on Vcc 5V the calculation could be

LUX = -1 + exp ( 0.00733* Ain )

with Ain from 0 .. 1023.

i did not find 5k resistor, only 5.67k / 5.72k by 2 ohm meter,
but here first results from the LUX meter ( a variant of PMS2 ):
darkest: 0.2 LUX
can barely see: 4.2 LUX
3W LED lamp 30cm: 1160 LUX
3W LED lamp 20cm: 1400 LUX
3W LED lamp 10cm: 1500 LUX

a way to calibrate? 1 candle 1 meter?

super bright 20mA LED
ES: 5mm LED 11800mcd

??? 12 candles burning in that LED? wiki
the CANDELA, LUMEN and LUX units are tricky,
looks like:
the light energy from a lamp is the LUMINOUS FLUX (luminous power ) LUMEN [lm]
the light intensity CANDELA [cd] is the above LUMEN per steradian angle

so more LUMEN means more light,
but more CANDELA could possibly mean same LUMEN, same efficiency
but stronger lense, so only from the top it appears brighter.
The same light is more focused?
But also it could mean that for same electric power ( 3.xxV (white), 20mA )
get more light or LUMEN because efficiency is better.

and how about the LUX?
here some nice german calc tool
what show us for above LED the relation:

the LUX is the CANDELA in 1 meter distance,
and depends on the distance only,
for 180 the 11.8 candela would be 74 LUMEN,
but this LED has only 15 and 0.6 LUMEN.

next point is how to power a LED?
better supply it by a controlled current or connect it to a voltage source via a resistor.
for the possible circuit and resistor calculation try this resistor circuit tool
and use 12V, 3V, 20mA, 5 leds and get 4 different ways to wire them.
Pls. check on the power consumption and see the 12V ( one ) LED with resistor is the most

how to put it in words,
for the efficiency of a LED must look for highest LUMEN/WATT
( maximum theor. possible efficiency, 683 lm/W ) ( for white LED max 260 ..300lm/W)
a power led with Vf=3.2V and 350mA has 1.12W, with 75 lm it has 67lm/W < 10%
but for the setup with power supply need more,
the voltage supply should be efficient ( like switching power supply higher volt 90%
but at 5V only 82% )
the best match for higher volt will be the serial connection of several LED and a resistor
The resistor can be emulated by a current regulator like LM317.
using the higher peak current of a LED by a PWM control can improve efficiency
even you don't want to use it as a dimmer, because it could eliminate the resistor,
and the PWM mosfet switcher have minimal losses.

using this uncalibrated LUX meter, put LED and Photoresistor in a LIGHT TUBE
at a distance of 15cm,
using 330ohm in series with a 5mm and a 10mm LED
here light and electric data:
VOLT [V] AMP [mA] 5mmLED [LUX] 10mmLED [LUX]
6.17 1.4 100 140
7.55 4.7 300 360
9.0 8.6 390 500
12.2 17.4 500 570

with a voltage drop
of 3.16V at the 5mmLED and 3.5V at the 10mmLED and 5.59 at the Resistor
at 12.25V and 17.4mA.

with looking from the side i would say the 5mm is brighter,
looking inside the LED not recommended, can't read for next 2 minutes.
but the electric data ( same current different dV, and the ?LUX? data show that the 10mm LED appears brigther.

So, even from the normal 20mA operation these super bright LED's
can be used as mini spotlight ( or torchlight applications )
how much i could increase the current i not know without datasheet.

now test the POWER LED (warm white),
the shop where i buy it ( for 105 Baht ) connected it to 3VDC and show 190mA,
but no product info, datasheet at all.
After googling power LED i just assume it might be a 350mA type,
as warm white 60 ..80lm, InGaN,Vf 3.2, view angle 120 ..150, example Vf:

lets check it, on up to 12VDC and 3 different resistors the dV and mA of LED
3 330 2.57 2.1
4.5 330 2.6 6.2
6 330 2.66 10.6
7.5 330 2.7 14.8
9 330 2.7 19.3
12 330 2.74 29
7.5 120 2.78 37
9 120 2.782 49
12 120 2.87 75
7.5 22 3.04 168
9 22 3.17 230
12 22 3.37 375

would have been nice to have a current source for all this tests.

and read LUX and current and diode volt by the computer.
snap the terminal readings to excel *.CSV
LOL and set the current too ??

here the current limiter


Not look good, while with 12VDC i can
run 3 white POWER LED ( with a 7ohm resistor 1W ) at 350mA
i can only run 2 on the LM317 ( if i read datasheet correct)

and that means electric efficiency ( without that of the power supply )
goes down from 0.8 to 0.53
so, current limiter is nice for play, for continuous operation a resistor is better.