Re: [Paddlewise] LEDs was Night Vision

>>
If there is a paddlewiser with circuit design knowledge she may
be able to explain how to design a circuit for multiple green
LEDs that will work with readily availabe power sources.
>>

>From what I have read on the Web, you get more even brightness by
wiring multiple LEDs in series than in parallel. Here's how:

The LEDs should have a forward voltage and a forward current marked
on the package. Add up all the forward voltages and subtract them
from the voltage of your power source. What remains is the voltage
drop across your series resistor. To find the size of the resistor
in ohms, divide the voltage across the resistor by the forward
current in amps.

For instance, suppose you have three LEDs in series with a forward
voltage drop of 2.1 volts each, which adds up to 6.3 volts. Since
your power source must be larger than the total voltage drops across
the LEDs, you choose to use a 9V battery. This makes the voltage drop
across the series resister 9V - 6.3V = 2.7V.

The series resistor controls the current in the LEDs. The current in
a series circuit is the same everywhere, so if the optimum forward
voltage for the LEDs is 30mA (0.030A), that is what you also want in
the resistor. Dividing the voltage across the resistor by the desired
current gives 2.7V / 0.03A = 90 ohms as the size of the resistor. In
practice, anything from about 75 to 110 ohms should work, but since
the voltage in the battery will drop as it ages, I suggest staying
slightly on the small side, say, 82 ohms. The rule of thumb for
adjusting the brightness of the LEDs is to increase the value of the
resistor to decrease the brightness of the LEDs and vice-versa.
However, you should be careful not exceed the maximum forward current
of the LEDs, which is typically around 50mA.

Wire the LEDs together anode to cathode, wire the resistor anywhere
in the circuit, and then connect the anode side of the circuit to the
positive side of the battery and the cathode side of the circuit to
the negative side of the battery.

Tip: If you want to play with the brightness of the LEDs by changing
the size of the resistor, place the resistor close to the battery,
where it should be most accessible, rather than near the LEDs. For
my wife's kayak, I am thinking of running a wire from the LED in the
compass through the deck and forward bulkhead, and fastening the
battery compartment (with switch) to the inside of the cockpit. The
series resistor will be in the battery compartment, so I won't have
to take the compass apart to change it. I might even add a trimpot
in series with the resistor for easy brightness adjustment.

Chuck Holst


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