They are pretty simple in design, just a AAA battery holder (w/ 2 AAA's), an On - Off button, and a 5 mm LED (no resistors or anything else). Problem is, I'm having a little trouble figuring out just WHY they are working...sounds odd right?...allow me to explain:
Let me start by saying I didn't really plan these out all that well...I had an LED kicking around from an old toy, so I basically just kept hooking AAA batteries up to it until it worked (came out with needing 2 AAAs). Eventually, I went and bought the LEDs you see in the final project (from Radio Shack), which have these specs:
- Forward Voltage: 3.7 typ 4.5 max
- Forward Current: 20mA (30mA max)
So, that being said, I decided to see if I could figure out how to determine how much voltage I actually need for the LED to work (I wanted to stop putting the cart before the horse, so to speak, and this isn't going to be my last project, not by a long shot, so I wanted to get more knowledgeable before I moved forward). After reading the LED tutorial here http://www.ladyada.net/learn/arduino/LEDs.html, I realized it was pretty easy to figure out the power requirements for an LED, per the article:
If you are making up a power supply, by adding up AA's in a pack, try to have about half or one volt minimum 'headroom' above the highest forward voltage, so that you can have a small resistor, around 100 or 200 ohms. Going lower than that isn't suggested because the forward voltage can vary, and resistors can vary, and the battery can vary and all these little variances of 0.2 Volts or so add up and you won't get the brightness you want.
So that got me to thinking...the current power supply I'm using is only 3 volts (2x AAAs @ 1.5v each), but my LED has a Forward Voltage of 3.7v...which, at least if I understand things correctly, means that my 2 AAAs shouldn't be able to light up my LED (which I also confirmed with this calculator: http://led.linear1.org/1led.wiz).
Not to be deterred, I told myself "Self, this isn't the real world, and electricity isn't perfect, so you should measure the batteries to be sure they are in fact putting out 3v, as you expect." So I busted out the voltmeter, hooked it up to the 2 AAAs, and determined that they were in fact putting out 1.6v each, rather than 1.5v, but that still didn't work out to be enough voltage...cue pulling out hair session.
So, I suppose, my question (or rather, questions) is, why does my circuit work? Is there still variance to be considered in the Foward Voltage of the LED, and maybe things are meeting up just enough for it to work? Am I reading the LED tutorial too literally? Is there something else I'm not taking into account?
One of the reasons I'm asking is because I'm about to make some more of them (some other kids in my family were infatuated with them, and would like some of their own), and I was looking at other LEDs on mouser, and was going to purchase ones that had a Forward Voltage of 2.1v typ and I wanted to make sure that I would be able to properly supply power to them. My guess is I'll need to use a resistor with those LEDs, but I don't want to guess at something until I figure out why what I have now works!