Re: If I'm to connect about 20 LED's together, how would I do that?

Beam me up, Scotty!

As I'm sure you found out, LED's come in a large variety of colors, sizes 
and brightnesses - but, they all share some common characteristics such as 
maximum voltage and current limits that must not be exceeded or they will 
be destroyed, maximumm operating temperatures, and optimum operating 
voltage/current ranges; outside of which performance either decreases or 
does not significantly increase. By performance, I'm referring to light 
output (luminence) vs. power consumption and increased heat production.

To get more specific, an average LED operates in the range of about 1.9-
2.02 volts and 10-40 milliamperes, with a MAXIMUM of about 2.4 volts and 
50-70 milliamps. Therefore, using a 9-volt battery (or any power source 
above about 2.35 volts), you must supply a series resistor to keep the 
voltage and current within maximum operating parameters.

POWER SOURCE:
While it makes ecological sense to use solar power, you will find that the 
Cadmium Sulfide photocell (CdS) is unsuitable for a variety of reasons. 
Firstly, they typically don't produce enough voltage or current to power 
anything like an LED, and secondly, the output varies considerably with 
the amount of incident light, making it virtually impossible to maintain a 
stable light output. A silicon photocell would be better because it has 
much higher current output with the same amount of light but again, 
control would be difficult. The 9-volt battery is a better choice with 
its' more or less constant output, but it would not be able to power 
multiple LED's for a very long period due to its' limited ampere hour 
capacity. A much better choice would be 2-"D" cells in series, thus 
producing 3 volts at a fairly high current capacity. You would still need 
to use resistors to reduce the voltage to under the maximum rating of the 
LED(s).

RESISTOR SELECTION:
Using Ohm's Law (I=E/R), you can calculate the size of the resistor 
required. Assuming you want to operate an LED at 30 milliamps and you're 
using 3 volts as your power source, you can plug them into the equation to 
come up with the resistor size. In this case, E(3 volts)/I(.03 amperes)= 
100 ohms. Experiment with different values of resistors until you get the 
desired brightness, but remember - some LEDs produce a lot of light and 
some produce only a little with same voltage and current. Use VOM in 
series with the battery and LED to make sure you don't exceed about 50 
milliamps! I would suggest you use a 1k potentiometer, adjust it for the 
desired brightness and current draw, then disconnect it from the circuit 
and measure what resistance it is set at (using your VOM). Use that for 
your fixed resistor for that specific LED. Repeat the procedure for all of 
them. The human eye is more sensitive to green light, about 532 nanometers 
(nm) wavelength, then red light, about 628 nm, so different colors will 
require more or less "drive" to get the same apparent light output; which 
bring us to the next topic-

SERIES or PARALLEL?

If you were using the same LED for all 20 or so (like the Hewlett Packard 
HLMP-EG15-PS000), you could theoretically use either a series or parallel 
hook-up because every LED within that batch is matched (electrically) to 
every other LED within that batch. For a series hook-up, the supply 
voltage would have to be increased to about 48 volts because the voltage 
drop accross each LED is about 2.4 volts(MAX.), and no resistor would be 
required. Because you are using many different LEDs, the series hook-up is 
not practical nor would it work properly due to the differing electrical 
and luminance characteristics. The parallel hook-up is much more 
complicated because the resistance of an LED(or any diode, for that 
matter) varies with the voltage applied across it. Figuring the value of 
the required resistor requires a special formula which takes into account 
a parallel resistor formula, LED threshold voltage, temperature 
coefficients, etc., and is beyond the scope of this reply.

CONCLUSION:

Use the above mentioned potentiometer/VOM method and use a seperate 
resistor for each LED. Remeber also, that LEDs are POLARITY SENSITIVE. If 
you reverse the voltage they will not light at all or, at worst, they will 
be destroyed if the "maximum reverse voltage" is exceeded.

Go to this address for more info on LEDs. Agilent is the semiconductor 
division of Hewlett Packard.
 http://www.semiconductor.agile
nt.com/


If you have need any further clarification, contact me directly at:

karlkolbus@ameritech.net

May you future be "bright"!

Your not-so-mad scientist,

Karl