led – Marco Gonçalves

Tune-up a Ikea Expedit shelve with a LED Strip

I was tired of looking into my plain, dull, boring, tedious, monstrous Ikea Expedit shelve in the living room. And, it seems that the new trend at the independent furniture shops (ok, the Pakistani at Lisbon downtown) is to apply LED strips to all kind of furnishings: beds, tables, chairs, stools, sinks, whatever… and boom!!! Eureka, the aha moment, why not leave the bed and table alone, but cheer up my shelve! Also it made sense in a functional way, since at mid-light room I can barely see the stuff that i get from the inner guts of it.

Next day, was shopping day for LED strips in Ebay the global marketplace. Now bear with me, the shopping process is crucial for a good development of your project. There are several decisions you must take. First, the length of strip that you need, for the full IKEA Expedit we are talking about 1.47cm * 5 = 7.35m (and give yourself some cut margin and error margin). Next the LED type, the protection rating and color. For the LED types there are two mainstream options 3528 and 5050. They get their numbers because the dimensions of the chip, 3.5mm * 2.8mm for the 3528 and 5.0mm x 5.0mm for the 5050. So the 5050 is bigger than the 3528, are substantially brighter, with an average of 18 lumen Vs 6 lumen, but also more power angry 0.24W vs .08W. So, for decorative lighting your best bet should be the 3528 and for functional lighting or bright environments you should go for the 5050. Also, take care of the number of LEDs per meter, it should be clear now that a meter of strip with 60 3528s should output the same amount of light that of a meter with 20 5050s (but the first will look much more smooth)… now for the protection rating, you can check the full spec here, but i will break it down for you. The first number is for solid particle resistance and the second for the liquid protection. The common IP20, stands for 2 (protection against fingers or similar?!) and 0 (not protected against water), other common available IP in LEDS strips is IP65 (6 – No ingress of dust; complete protection against contact and 5 – Water projected by a nozzle (6.3 mm) against enclosure from any direction shall have no harmful effects). Usually higher the IP, higher the cost of the strip. For me, with 2 young, active and curious cats in the house, the choice was obvious (IP65)… finally, the color is mainly a question of personal taste, but if you want to change the color on the fly, the way to go is a RGB 5050 LEDs strip as each chip has the 3 main colors that combine into the color that you want (or some psychedelic effects).

This guide is for the 4×4 IKEA Expedit shelve, but is easy adaptable to other piece of furniture.

You will start with:
– 2 rolls of 3028 IP65 warm white
– a transformer (more on this later)
– wire for the connections, don’t worry too much about the thickness as the flow of power will be limited, but worry to get a flexible wire, the flexible the better
– electric block connector (for parallel connections, if series you don’t need this but a little more solder..)
– insulation tape
– a virgin IKEA expedit shelve

Tools:
– a multimeter to check continuity
– a soldering iron and some solder
– a wire stripping tool, or a knife or scissor or use or teeth (don’t use the teeth)
– a screwdriver for the electric block connectors
– a X-Acto knife
– a ruler to check out the dimensions is not a bad idea

Other stuff needed:
– a bottle of beer, so when you get thirsty don’t leave the workstation

You start to cut down five 1.47m segments out of the LEDs rolls. The LEDs strips have a scissor mark that repeats itself along the way after some number of LEDs (mine was between each 3 LED). So look for the nearest mark and cut it with precision on the middle. It’s important that you cut cleanly at the middle as will be easer in the soldering step.

Now, you must decide what kind of connection you should do. You can go parallel or series. For parallel you have the advantage of less soldering to do, if one section stops to work the others will continue to, but you will use more wire, block connectors, and it will be a pain to hide the electric circuit if the fitment is in the middle of the room. The series will use less wire, no block connector, it will be easier to hide completely in the back side of the fitment, but much more soldering to do, and if one of the strips go bad (or the connection between) the upstream will also not work.

You should keep in mind a very important detail about LED strips. When you cut a piece in the pre-designated cutting zones, there is a transistor right before the connectors. This transistor will automatically close the circuit for you if there is no upstream continuity. So if you connect in series and connect a 12v transformer (in the right polarity) to the first LED segment it should light up, you don’t need to close the circuit in the other side. If you connect another segment to the first one (again check the polarity, the LEDs strip should have plus and minus markings) it behaves the same, lights up without the need of closing the circuit at the far end.

Expedit Wiring Diagram

Now, for the “funny” stuff, to solder the circuit. With the X-Acto knife remove the protection material (epoxy or silicone) above the solder points. Then with the hot iron melt way the remaining of the protection material. Drop a bit of solder into the solder points, then drop another bit into the end of the stripped wire. Then press and hold the wire against the solder point, and heat it up until both bits of solder melt and fuse. A quick demonstration video:

When it’s all done and fitted, means that is time to calculate the power consumption, and transformer requirements. Each LED uses 0.08W and one meter has 60 LEDs, for the sake of simplicity we will round the 1.47m segments to 1.5m. So each segment has 90 LEDs, 0,08W * 90 = 7.2W per segment. At 12v it gives (7.2 / 12) 0.6amps per segment. For 5 strips in parallel, the total output is 3amps.

To be safe the transformer should output 12v and 3amps (if it’s rated in Watts should be 36W). If you use a less powerfull transformer (let’s say a 1 amp) it can heat up and meltdown. If you feed the circuit with less voltage it will the lights will dim, and as you step down the voltage, eventually it will not light at all. The cost per hour of usage at full throttle is pretty cheap, as 36W/h at 0.20 cents the Kw/h sums at a cost of 0.0072 cents hour….

If all goes well the final result should be this:

expedit_leds

How to calculate resistors for leds

For the, what resistor should i put in the circuit so my led don’t fry question. There is a very easy way, go to a on-line calculator site like http://ledcalculator.net/, input the values and bazinga you get an instant answer.

But that’s for sissies, the only way a real man does it, is calculating (preferably by head) applying Ohm’s Law:

(behold, and bow twice, please)

Current (amps) equals to the proportion between potential difference (volts) and the resistance (ohms). Let’s say for a common red led (voltage is 2volts and current rating is 20mA), and a 9v battery power supply, the math is this:

(9v – 2v) / (20mA/1000A) = x Ohms
7 / 0.02 = 350 Ohms

But with these values in a led calculator, the result is 360 Ohms, what’s wrong with your math? Nothing. This is because of the so called “nearest preferred values resistors”. Resistors are available in a number of standard ranges (being the most common E24) in predefined scale. So you should round up to the next higher value. And from 350 Ohms the preferred resistor value is 360 Ohms.

With the resistance calculation done, you must find the right resistor. Again, there is the sissies method, go to a on-line resistor color code calculator like http://www.electronics2000.co.uk/calc/resistor-code-calculator.php, input the resistance and get the color codes (and vice-versa) or the mens method trough a resistor table:

resistor_table

So, a 360 Ohms resistor is the one in your resistors packs with a Orange (3), Blue (6) and Brown (36 * 10 = 360) band. The last band, that stands for tolerance is probably gold (with my very little experience in electronics never saw a silver resistor).

Disclaimer: remember that (as always) i decline every liability if you burn your LEDs, your head or the whole world because of this post.

Raspberry GPIO

Finally the fun stuff. Messing around with the General Purpose Input Output of the PI. So, first things first. First you must identify what revision PI you are working. Visually, looking at it, if there are 2 mounting holes is a revision 2, if there are no mounting holes it’s a revision 1. A much more scientific approach is to install WiringPi.

sudo bash
apt-get install git-core
apt-get update
apt-get upgrade
git clone git://git.drogon.net/wiringPi
cd wiringPi
git pull origin
cd wiringPi
./build

Then run

gpio -v

And there you got the answer in the last line of the output. “This Raspberry Pi is a revision 2 board.”. So, first thing is to pull from the Internets a wiring diagram.

Now, for the actual cabling from the PI, in my opinion the best route is to get a set of male to female jumper cables. You can also find for sale some ribbon cables specific for the PI. In my opinion, unless you want to connect some device or accessory already wired to match the PI GPIO, the best option are the jumper cables, it’s very easy to visually match the PI pin to a breadboard location, you only use the number of cables needed and they are easily routed trough the venting holes of the average PI case and easy to solder on a protoboard.

I ordered my jumper cables from Ebay, but has i waited for them, the inner MacGyver took over… with a old floppy drive flat cable is not so difficult to improvise. Just cut a segment of the cable with a sharp knife, so in one end you have the plastic connector and in the other you have the wires with a clean cut and no short circuits between them. One problem, is the floppy cable 34 pins versus 26 in the PI. What i did is simple to cut out of the way the first 8 cables of the ribbon (and just forget encasing the PI, the connector protrudes almost the same length as the SD card). Then with cable connected in the right way (not bad idea to test first at the end of the cable with a multimeter) the first cable of the ribbon corresponds to pin 1 of the PI and so on. Some pictures of this setup:

Just another note, i have been messing with the GPIO, juggling and testing different setups without powering off the PI between rewiring, without no problem at all, but please use good judgment and common sense.

1 – Lighting up a LED with the PI (simple output example)

Connect physical pin 11 of the PI to the positive bus of the breadboard and physical pin 6 (ground) to the negative bus of the breadboard, and connect a led and >= 68Ω resistor (why?) closing the circuit between positive and negative bus strips (remember that a led is one way device, so long leg on the positive side and small leg to the negative). The diagram to make even easier:

And now in the PI command line, the command to put the pin in output mode, switch off and on the LED:

gpio mode 0 out
gpio write 0 0
gpio write 0 1

Wait! But isn’t the cable connected to pin 11? What is this stuff with pin 0?
Well.. the logic in wiringPI is to abstract pin numbers in software so it remains “immune” to any hardware changes. As the pin diagram above (can be checked running gpio readall also), physical pin #11 (third column of gpio readall output) corresponds to wiringPI pin #0 (first column of gpio readall) and to the GPIO #17 (the internal pin number used in the chip). So, regarding the logic of hardware abstraction with the gpio command, we can call a pin by the wiringPI numbering or the GPIO numbering (with the -g switch, ex: gpio -g write 17 0) but not by physical pin-out number…

2 – Reading state (simple input example)

To read state from a pin, you must devise a circuit that pulls up or pulls down the pin that you are going to read, then change the state of the circuit with a pushbutton (or any other suitable mean). This is a fucking bad explanation… so let’s look to the schematic:

We have physical pin #1, the 3.3v output (that’s the one that should be used for input reading) going to a 10kΩ resistor then split by a) a pushbutton that connects to physical pin #6 (ground) and b) a 1kΩ resistor that connects to physical pin #11. When the pushbutton is not pressed, no current passes trough there to ground, so the current goes to the 1kΩ resistor and pulls up pin #11. In this state, all the readings from pin #11 will be High. When the pushbutton is pressed the current will flow freely to ground (pin #6) and the reading from pin #11 will be Low. This is a bit counter intuitive, reading High (or 1) when the pushbutton is NOT pressed and Low (or 0) when the pushbutton IS pressed, but is very easy to take care in the software stage.

The diagram on the RaspberryPI and breadboard:

And a very simple Python script to output when the push button is pressed:

import RPi.GPIO as GPIO
import time
buttonPin = 11
GPIO.setmode(GPIO.BOARD)
GPIO.setup(buttonPin,GPIO.IN)

while True:
    input = GPIO.input(buttonPin)
    if input == 0:
        print("Button pressed")
        exit();
time.sleep(0.1)

Save and run as “python script_name”. When the pushbutton is pressed it should output “Button pressed”. Note that with the RPi.GPIO module in the GPIO.BOARD mode, we are addressing the physical number of the pin in the board.

There! Raspberry PI, welcome to the real world. Just baby steps… laying the foundations for further developments.

Some references:
http://elinux.org/RPi_Low-level_peripherals
http://wiringpi.com/