I will also go through an LED example to show a little better the considerations that go into sourcing an LED when you're not just buying the grab-bag at radioshack.
I'm trying to be clear, but if I throw out a term that you're not familiar with I recommend doing a wiki search.
I am not affiliated with any of the websites or places I'm going to list... They are where I go when I need parts. Also I live on the East Coast of the US, Hobby stores and other outlets are available in other geographical regions.
I'm sure there will be a lot of feedback listing other great resources that I'm leaving out and might not know about... That feedback is great!
Step 1: Electronic Parts Sources
Online sources: they stock very similar things but have different on hand stock. It sometimes takes searching all of them to find a specific part that you can order single quantities of
For Audio Related Products:
Step 2: Determining What LED Is Right for You: Color
Reading through these will help (at least a little) to make sense of ALL the menu options that come up from the searches.]
First and most obvious, what color do you want? ROYGBIV
All are available, most at RadioShack. ROYG have been around for decades. BIV are more recent developments. RadioShack is 10x more expensive in most cases, but you can get in your car and go purchase one any day of the week during store hours. Pictured is a violet LED that I purchased from radioshack. It's "turn on" voltage is 3.4v and it's peak curren/intensity is 20mA.
I'm going to use a green LED from digikey as an example.
On all the electronic parts websites use the search feature.
on digikey.com use the search to find "green LED"
this pops up a long, potentially daunting, list... We are looking for "Discrete" LED's, Discrete is the term for stand alone electrical components. For indicator purpose we are interested in low power... High-power is awesome and bright and will deplete a battery in a hurry.
So from the optoelectronics section lets pick LEDs - <75mA, Discrete (2,735 items)
From here we want to specify green (again) and "through hole" for breadboarding types. Surface mount is what is used for mass production. From here, there are still 600+ options... But it only takes 1... onto the next part of picking the power!
Step 3: Picking an LED: Brightness/power
Brightness is a function of current efficiency (device physics of bandgap efficiency)
The picture is of 2 green LED's with a turn on of roughly 2.5v running on a 9v supply
The one on the left is getting:
V/R = I
(Vsupply - Vforward)/limit_resistor = Current
(9v-2.5v)/1k = 6.5mA
V*I = P
(the resistor is consuming: 6.5v*6.5mA = 1/24w)
and is about 10x (1 order of magnitude) brighter than the one on the right
The one on the right is getting:
(9v-2.5v) /10k = 0.65mA
(the resistor is consuming 6.5v*0.65mA = 1/240w)
This is where power consumption should be talked about.
If you’re asking “why does this matter?”.
If your circuit is plugged into a wall, 10's of mili-Amps are virtually negligible.
If your circuit is battery powered: Battery life is specified in amp-hours.
A typical 9v battery can supply 300mAh
if you are using up 20mA through your LED, your battery will die in 300/20=15hours vs 150hours for 2mA.
Typically the indicator light should not be the dominant consumer of the sources current. Lets say our indicator is on something that will be battery powered and we want to go with a cheap 2mA solution.
Filtering for 2mA and finding a $0.09 version gives us
As you can see, there are a LOT of options... but pretty much any of them will work. There are lots of resources (wiki, manufacturers' websites) that will detail ALL the specifications... but for hobby cost/size are the best drivers for selection.
Step 4: Important Parts of the Data Sheet (if There Is One)
So the digikey part that we chose has a data sheet available on digikey
All the information in the data sheet is valuable to various engineers. For our immediate purpose, page 3 has the 2 important graphs we care about. Pictured below, they inform us of the Current to Voltage response (IV curve) of the Light Emitting Diode and the intensity versus current.
A resistor follows ohms law V = IR, a diode doesn't.
A diode will have a relatively fixed voltage across it once it's turned on. This is the "forward voltage" also referred to as the "Turn on voltage". This is where the current limiting resistor calculation takes form.
(Vsupply - Vforward) / Max current = Current Limiting resistor value.
9v battery powered, 1.9v Forward Voltage, Forward Current = 2mA,
(9v - 1.9v)/0.002A = 3550ohm resistor.
If operated at or below the recommended operating current the device will last many many years. It's ok to use a value up to a decade(10x) larger than the Current Limiting Resistor Value that the formula gives.
This works for ANY supply voltage, The value of the current stays fixed so the value of the resistor just needs to increase to keep the formula balanced.
Example: If you wanted to plug it into a wall (not recommended!)
(170v - 1.9v)/0.002A = 84,050ohms (84k)
BUT: 168.1v X 0.002A = 0.3362W (slightly greater than 1/3W).
Through hole resistors come in 1/16, 1/8, 1/4, 1/2, 1w, 10w variety's.
So in order for this to work without burning out the resistor you need to use a 1/2W, 84k Resistor (or larger)
The device will function at higher currents, and it will get brighter, but it will burn out! The graph on the right shows the increase in brightness is linear to current. If you want a brighter LED and you want it to last, buy one for the brightness you want or be prepared to replace it.
Step 5: Order It and Enjoy
I didn't show how to build anything specific, but ALL electronics are built with building blocks. Knowing how to pick and where to get your building blocks is very important information.
Through the formal education channels that I went through this parts selection was never covered. Isn't the internet awesome?
For more tips and links visit: http://sites.google.com/site/0123icdesign/