Example Applications (continued)

6 of 7

ELM711

ELM711DSA

Elm Electronics – Circuits for the Hobbyist

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resistor (R) will have a voltage across it, and the total

current in the string will be determined by the voltage

across the resistor, and the value of that resistance.

Choosing a resistance value may require a little

experimentation, but to get you started:

If you plan to use N LEDs in the string, each with a

forward voltage drop of Vf, then choose Vp such that:

Vp

≥ (N+1) x Vf

Then, given the LED current (I), determine the

value of R from:

R = (Vp - NxVf) ÷ I

For example, if you wish to have 4 green LEDs in

a string, then you need to choose:

Vp

≥ (4+1) x 2.0

Vp

≥ 10.0 volts

Since 12V supplies are very common, we will use

that. Then, you need to choose R, which requires a

knowledge of the LED current. A good starting point is

to use 9 or 10 mA for red, yellow and green leds, and

a smaller current (4 or 5mA) for the blue and white

LEDs. As this calculation is for a green LED, we

choose I = 9 mA (or 0.009 A). R is then given by:

R = (12 - 4x2.0) ÷ 0.009

R = 444

Ω

While 444

Ω is not a standard value, you could

select either 430

Ω or 470Ω for the resistor and the

current would still be close to the desired 9mA.

Continue in a similar fashion for the other colours. You

may need to try a few resistor values for each colour in

order to make all the LEDs appear to be of the same

brightness, as your eye is more sensitive to some

wavelengths, and some LEDs are simply more

efficient than others.

In this way, all of the component values can be

chosen and a circuit such as the one in Figure 4 can

be designed. Note that Figure 4 does not show a

transistor driving the white LED, as the wires to it are

short, and the resistor helps to limit currents should

some be induced. If you are considering very long wire

runs, the White LED should also have a transistor

buffer added in the same way as the others.

Figure 4 also shows a supply bypass capacitor

connected across the ELM711 power supply pins. This

is to prevent problems due to the transients, or

‘spikes’, that are inherent in all digital circuit’s current

flow (due to the internal switching process). By adding

Figure 3. Driving LED Strings

a capacitor very close to the IC, much of the current

for these spikes can be supplied by the capacitor

rather than through the (inductive) leads of the supply.

The net result is that there will be less noise generated

at the supply by the ELM711. A value of 0.01µF works

well.

We will leave the wiring details to you, but to get

you started, we have found that the wires for a small (2

to 3 foot high) tree should be about 5 feet long. Space

the LEDs about 12 inches apart, beginning at the end,

and install them by cutting a 2 inch piece from one of

the two conductors in the wire. When you bring the two

ends of the cutout together to solder them to the LED,

the other conductor is forced to form a loop, which

allows it to hang from a tree branch.

Enjoy the light show!

4.7K

Ω

2N3904

Vp

R

N LEDs