Instructables

How do i determine what voltage the leds are that are scrapped from toys?

And what other stuff should I scrap b4 tossing? New to LEDs!! I have 4 kids, & therfore lots of broken toys, game controllers, etc... I don't wanna toss anything and then once I get more advanced regret throwin it all away..... like all the resistors and circuit boards themselves.... please advise and recccommend the ibles I need please! Thanks in advance!!

westfw5 years ago
One issues is that the voltage of an LED is relatively unimportant; LEDs are current-driven devices. Most LEDs operate at their typical brightness with a current of about 20mA, and almost all will operate from 4.5V or less. So if you use a 9V battery with a 470 ohm resistor, the current will be somewhat less than 20mA if you had a 0V LED, and about 10mA if you had a 4V LED, and the LED should light pretty well in either case. Once you have the circuit set up (9V battery, 470ohm resistor, LED) and working, you can MEASURE the LED voltage with a voltmeter by putting one probe on each side of the LED, and this will give you some indication of how few batteries you can get away with, and how to get a more exact resistor value for maximum output. But in general, one uses a voltage significantly higher than the LED voltage and a current limiting resistor, rather than trying to set up a power supply at the right voltage. And as UziMonkey says, there are a fairly limited set of values for "typical" LED voltages. About 1.7V for read, 2.2V for yellow and yellow/green, and about 3.6 for white, blue, and modern "exotic" colors...
miqt westfw5 years ago
so if i find some random white LED's in an external harddrive casing and i take them out, would it be safe to assume that i can power them with 1.5 volts each? and if so, then what do i do to figure out what kind of resister i need....i'm sorry if i sound ignorant, i really need someone to fully explain it, i've looked at hundreds of websites trying to explain it, and tons of instructables, but i still can't seem to grasp it...i just don't get how much power to use and how to figure out resisters.....anyone help?
westfw miqt5 years ago
I said white/etc were about 3.6V? Where did you get the idea that 1.5V each would work?

The resistor should have a value of:
(powerSupplyVoltage - LEDVoltage) / (desiredCurrent)

So for a white LED, a 9V battery, and 20mA, you get
R = (9 - 3.6) / 0.02 = 270 ohms

There are several instructables and many web pages that explain this and offer calculators and such...

The PHYSICS actually makes sense. An LED emits light when an electron falls from a high-energy "position" to a low-energy position. A voltage is a measure of an electron's energy, so this drop in voltage is directly related "voltage drop" of the LED, AND to the energy lost in emitting the light. Higher frequencies of light (blue) have higher energies, so the voltage drop is higher for blue leds than for red leds... (and a white LED is usually a blue emitted coated with phosphor...)
Laminarin5 years ago
You don't even need a resistor to find the voltage drop, just a voltmeter, a 9volt (or 6 volt, or 2X 1.5 volts). Just measure the voltage of the battery (rarely are they at exactly the nominal voltage), then put the negative end of the LED (the one with the flat spot usually) on the negative terminal and measure the voltage from the positive terminal of the LED to the positive terminal of the battery and the reading should be lower than the battery voltage (Basically just read the battery voltage through the LED). That difference is the voltage drop. To find the right resistor just take the voltage of your power source minus the voltage drop of the LED, then divide that voltage by the desired current in amps(commonly 0.02 amps or 20 mA ).
(Vbat-Vdrop)/Current=resistance
FrozenStar5 years ago
5v will give enough current for most LEDs
dazzamatazz5 years ago
If you have a multimeter put it on the diode setting and it will read the forward voltage of the LED.
That only works up to about 1.4-1.7V. It won't help for LEDs with a higher forward voltage.
It works on most LED's it depends on the multimeter mine will read up to 4volts forward voltage
UziMonkey5 years ago
The lower LEDs are in the spectrum, the less voltage you need. For example, red LEDs only need 1.7v or so most of the time. Yellow and green need 2v to 2.5v usually. At the opposite end of the spectrum, blue LEDs often require 3.5v or even 4v. Start with a lower voltage (larger resistor). Keep stepping up the voltage until it lights, and go no higher. Many LEDs have a pretty narrow voltage at which they'll light. For example, a red LED might light at 1.7v, but burn out at 1.8v, so be careful in choosing a resistor. It's easier just to order new LEDs since they're so cheap. You can get 50 LEDs with resistors for under $10 on ebay (even with free shipping). I wouldn't waste my time harvesting LEDs or resistors from old circuits. You also get the added benefit of knowing the exact voltage and current requirements of the LED, instead of having to guess.
Sandisk1duo5 years ago
usually all colored LED of the same color, have have the same power requirements