# [life size] LED Traffic Light

I want to try building an 8" LED traffic light bulb and control it using an arduino.

I came across this schematic,
from this blog

Will this be a great connection for what I have in mind?

But I still don’t know if I should divide my LED in 20 for every 1 transistor if I will be using 132 LED of L35SGD. And what suply should I use?

Be very careful with those LEDs, they run at 10mA (or less) and have a max current of only 11.5mA. Most circuits on the web assume 20mA LEDs.

Each LED needs 4V so that circuit won't work as-is.

WITH MODIFICATION, The "2N3053" part of the circuit is a good way to do it but use 19V instead of 6V, this allows more LEDs. (nb. Many laptop power supplies are 18.5V/19V - try and find one).

Put 4 LEDs per column instead of 2, use 220 Ohm resistors instead of 100 Ohm.

Put 11 strings of LEDs per transistor instead of 10, that gives 44 LEDs per transistor.

132 LEDs can be done with three transistors (44 x 3 = 132).

You can connect three transistors to a single Arduino pin if you use 470 Ohm resistors between the Arduino and the transistor (instead of 220).

Thank you!

If I will use a 12v supply should I modify something?

arduinoTime: Thank you! :)

If I will use a 12v supply should I modify something?

12V is only enough for two LEDs in each column.

I’d use NPN connection on cathode side for all arrays, same as the top left 1/3 of the schematic.
Then a high from Arduino to turn on each transistor individually.

I’d use NPN connection on cathode side for all arrays, same as the top left 1/3 of the schematic.
Then a high from Arduino to turn on each transistor individually.

Oh, I didn’t spot he’d put the triansistors above the resistors.

When I said “The “2N3053” part of the circuit is a good way to do it” I also meant “the other part is a bad way to do it”. The other part inverts the signal so they turn off when the others are on.

As long as we're in sync then OP should be in good shape 8)

fungus:
12V is only enough for two LEDs in each column.

So if I use 12V, is it okay if I put 76 LED in one transistor? How can I compute this? Could you teach me? I don’t want to bother you every time I change anything.

I’d use NPN connection on cathode side for all arrays, same as the top left 1/3 of the schematic.
Then a high from Arduino to turn on each transistor individually.

What do you mean when you said “a high from Arduino”?

76, one transistor, no problem.
This is what High from Arduino to turn on transistor means.

CrossRoads: 76, one transistor, no problem.

Even if the LED max current is 11.5mA?

(Vsource - (#LEDs x Vf) )/10mA = resistor
For Vf, use the lower voltage if a range is given, say 3.8 to 4.2 - use 3.8.

Then (19V - (4 * 3.8 ))/0.01 = 380 ohm

Resistors will have some tolerance - 1%, 5% - plan on it being on the low side:
(19V - (4 * 3.8 ))/(.95 * 380) = 3.8/361 = 10.52mA

Ok? So do a little number crunching, a little planning, keep it safe.

Can also factor in the voltage drop across the transistor.
(19V - (4 * 3.8 ) - Vce)/.01 for an NPN
or for a MOSFET
(19V - (4 * 3.8 ) )/.01 = CurrentLimitResistor + Rds

Thank you! :) How can I compute the max string of LEDs er transistor?

arduinoTime: Thank you! :) How can I compute the max string of LEDs er transistor?

It depends on the transistor. You have to look at the datasheet.

The datasheet for a 2N3053 is: http://www.centralsemi.com/leadedpdf/2n3053.pdf

Under maximum ratings it says "Continuous Collector Current 700mA"

At 11.5mA per LED string that means you can do 60 strings per transistor.

Datasheet ratings are for ideal conditions. If you actually put 60 strings it will probably get quite hot, especially if it's enclosed in a box or whatever. In practice you should do less than that, 50% is a good target for things to be reliable and last a long time. 30 strings per transistor shouldn't cause any problems. Even 40 is probably OK.

arduinoTime: Hooray for CrossRoads and fungus! Thank you! :)

Karma rating are given over on the left.

Another question, what transistor can I use to replace 2N3053? can I use TIP31?

I can then use only 1 transistor with 200 LED because it has 3A collector current.