First Project Showoff, And a Little Clarification Needed
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First Project Showoff, And a Little Clarification Needed

by sunmorgus on Mon Nov 12, 2012 12:02 pm

First, I wanted to show off my first DIY electronic project...homeade Sonic Screwdrivers! (Don't laugh, they were for my 4 & 7 year old boys, both of whom recently became HUGE Doctor Who fans). I'm going to eventually make them look a little nicer, but right now, they are just what the Doctor ordered (pun intended :wink: ).

Image

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!
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Re: First Project Showoff, And a Little Clarification Needed

by adafruit_support_bill on Mon Nov 12, 2012 12:17 pm

Forward voltage varies with current. (See the graph under "LED Electrical and Optical Characteristics" here: http://www.maximintegrated.com/app-note ... vp/id/1883 ) The specified forward voltage for an LED is the forward voltage at the rated current (for most 5mm leds, that is about 20mA).

Many blue and white LEDs will illuminate at around 3v or less, drawing just a few mA. This is fairly safe to do since you are not pushing the current limits of the led.
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Re: First Project Showoff, And a Little Clarification Needed

by sunmorgus on Mon Nov 12, 2012 2:21 pm

adafruit_support wrote:Forward voltage varies with current. (See the graph under "LED Electrical and Optical Characteristics" here: http://www.maximintegrated.com/app-note ... vp/id/1883 ) The specified forward voltage for an LED is the forward voltage at the rated current (for most 5mm leds, that is about 20mA).

Many blue and white LEDs will illuminate at around 3v or less, drawing just a few mA. This is fairly safe to do since you are not pushing the current limits of the led.


So, basically what you are saying is, even though my LED is rated at a Vf of 3.7 typ, it's still getting enough voltage from the batteries to "work", correct? If that's the case, it's kind of "dumb luck" that I managed to get it working with the 2 AAA batteries I suppose, and I should probably try and figure out a better power supply for the next round of Sonic Screwdrivers that allow me to get higher than 3v and add a resistor in (for a more consistent experience).
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Re: First Project Showoff, And a Little Clarification Needed

by adafruit_support_mike on Tue Nov 13, 2012 7:02 pm

sunmorgus wrote:So, basically what you are saying is, even though my LED is rated at a Vf of 3.7 typ, it's still getting enough voltage from the batteries to "work", correct?

Pretty much.

Current flows through diodes based on the mechanics of diffusion, which is more or less the same as heat moving through a bar.

Say you have a metal bar with temperature regulators at each end, the left end set at 100 C and the right end set at 0 C. The bar will transfer heat from the hot end to the cold end, so the device at the right end has to pull heat out of the bar to maintain a temperature of 0 C, and the one at the left has to pump energy into the bar to maintain 100 C. In effect, the device on the left passes energy to the one on the right.

Thing is, the rate of transfer depends on the length of the bar. If the bar is 100 meters long, there's only 1/1000 of a degree difference from one 1 mm section to the next. If the bar is 1 cm long, there's a 10 degree difference from one 1 mm section to the next. The rate of transfer depends on the difference between adjacent sections, so energy will move through the shorter bar a lot faster.

In diodes, there's a zone called the 'depletion region' that works the same way. When you apply forward voltage, you squeeze in narrower, and current flows through faster. There's no strict cutoff line between 'on' and 'off' though, just an exponential curve from 'very little' to 'a whole lot'.

sunmorgus wrote:If that's the case, it's kind of "dumb luck" that I managed to get it working with the 2 AAA batteries I suppose, and I should probably try and figure out a better power supply for the next round of Sonic Screwdrivers that allow me to get higher than 3v and add a resistor in (for a more consistent experience).

It's more a case of "operating at the low end" than "dumb luck", but yeah, adding another battery and a resistor will give you better results. Battery voltage tends to droop as the batteries drain, so the version you have will probably start to fade while the batteries are still good.
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Re: First Project Showoff, And a Little Clarification Needed

by sunmorgus on Thu Nov 15, 2012 8:02 pm

mstone@yawp.com wrote:
sunmorgus wrote:So, basically what you are saying is, even though my LED is rated at a Vf of 3.7 typ, it's still getting enough voltage from the batteries to "work", correct?

Pretty much.

Current flows through diodes based on the mechanics of diffusion, which is more or less the same as heat moving through a bar.

Say you have a metal bar with temperature regulators at each end, the left end set at 100 C and the right end set at 0 C. The bar will transfer heat from the hot end to the cold end, so the device at the right end has to pull heat out of the bar to maintain a temperature of 0 C, and the one at the left has to pump energy into the bar to maintain 100 C. In effect, the device on the left passes energy to the one on the right.

Thing is, the rate of transfer depends on the length of the bar. If the bar is 100 meters long, there's only 1/1000 of a degree difference from one 1 mm section to the next. If the bar is 1 cm long, there's a 10 degree difference from one 1 mm section to the next. The rate of transfer depends on the difference between adjacent sections, so energy will move through the shorter bar a lot faster.

In diodes, there's a zone called the 'depletion region' that works the same way. When you apply forward voltage, you squeeze in narrower, and current flows through faster. There's no strict cutoff line between 'on' and 'off' though, just an exponential curve from 'very little' to 'a whole lot'.

sunmorgus wrote:If that's the case, it's kind of "dumb luck" that I managed to get it working with the 2 AAA batteries I suppose, and I should probably try and figure out a better power supply for the next round of Sonic Screwdrivers that allow me to get higher than 3v and add a resistor in (for a more consistent experience).

It's more a case of "operating at the low end" than "dumb luck", but yeah, adding another battery and a resistor will give you better results. Battery voltage tends to droop as the batteries drain, so the version you have will probably start to fade while the batteries are still good.


Thanks for the further clarification!
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