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, 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:

ohm_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, input the resistance and get the color codes (and vice-versa) or the mens method trough a 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://
cd wiringPi
git pull origin
cd wiringPi

Then run

gpio -v

Raspberry Pinout DiagramAnd 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:

Raspberry First Led

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:

raspberrypi_input_schemWe 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

while True:
    input = GPIO.input(buttonPin)
    if input == 0:
        print("Button pressed")

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: