Why can't I make this work? LED angst inside
August 11, 2008 11:54 PM   Subscribe

Using a AA battery, two pieces of speaker wire and an LED, I am trying to make a simple circuit. I am meeting with complete failure. Help!

I read books for a living. I do not program computers, wire electronics, play with oscilloscopes, or regularly work on projects that require modulating a radio frequency. That said, I'm bored for the summer and looking for new projects. Choice A is a neat LED nightlight, and so I find myself (once again) trying to teach myself simple electronics, and (once again) meeting with middling levels of success.

Basically, I'm trying to wire an LED to a battery. I am following this instructable. I am using a 1.5V AA battery and a 1.7V LED, using a lower-level power supply on the assumption that I don't yet have to learn how to wire a resister. I realize that there are TWO steps to this process - making the connections, and putting the LED in one of two orientations. And yet...my LED fails to light.

I realize that this is possibly the most basic question I can be asking here, but does anyone have thoughts on what I might be doing wrong? Pictures here
posted by puckish to Technology (15 answers total) 8 users marked this as a favorite
A 1.5V battery and a 1.7V LED? Sounds like you need more voltage. Put two batteries in series, then hook the LED up to those. I see that the instructable used a 1.5V battery with a 1.7V LED, but your LED is probably different than theirs, and maybe your battery isn't quite putting out the full 1.5V. It's worth a try, anyway. (Yes, you should use a current limiting resistor.)

Also, be sure to check that you have the negative lead of the LED hooked up to the negative terminal of the battery! If you hook it up the wrong way, nothing will happen (though you shouldn't do any damage at these small voltages). The plastic case of the LED will have a flat spot next to one of the wire leads. The flat spot is next to the negative wire.
posted by samw at 12:13 AM on August 12, 2008

LEDs are a bit tricky. You have to wire them up with the correct polarity and supply them with enough voltage, or they won't draw any current at all and won't light up.

Your 1.7V LED won't run off a 1.5V battery (the instructables instructor was lucky). Think of the LED as having a little step in it that the battery voltage has to be high enough to climb over; a 1.5V battery can't climb over the 1.7V LED's little step.

Of course, you could just wire two batteries in series, giving you 3V. That will get you over the step. But having done that, it will then push way too hard and probably damage the LED. That's why the simplest LED circuit that will work for you is two batteries, and a resistor, and the LED.

What the resistor does is use up an amount of voltage that's proportional to the amount of current flowing through it, and also proportional to its resistance. After climbing over the LED's 1.7V step, your 3V battery has 1.3V left over. If your resistor is 0.1 kilohms (same as 100 ohms), then it will use up that 1.3V when there's 13 milliamps flowing around the circuit, which is about right for a first try.

Get yourself a battery holder that takes two AA cells (soldering reliably onto batteries is harder than it looks). Connect the positive wire (probably red) from that to one end of the resistor. Connect the other end of the resistor to the anode side of the LED (usually the longer wire). Connect the cathode side of the LED (shorter wire) back to the negative wire (probably black) from the battery holder.

If it's not bright enough, try different resistor values. An 82 ohm resistor will run your LED at 1.3V / 0.082 kilohms = 16 milliamps; a 68 ohm will give you 1.3V / 0.068kΩ = 19mA; 56 ohms yields 23mA.

The more current you pump through your LED, the faster you will drain your battery. As a guideline, an AA cell should be good for about an amp-hour. 20 milliamps is a fiftieth of an amp, so you should get about fifty hours operation out of a LED circuit running off a series pair of AA's at 20mA.
posted by flabdablet at 12:21 AM on August 12, 2008 [7 favorites]

You might have accidentally burnt out the LED by not using a resistor.
posted by HaloMan at 12:23 AM on August 12, 2008

Not to get all technical, but look at the i-v graph of a diode. One of the defining characteristics of a diode is that it is nonlinear, unlike say a light bulb or resistor. You have to overcome a certain threshold voltage before anything really starts to happen. Near that voltage there is a sharp elbow in the curve, where it more or less goes from completely off to completely on. This is just a long winded way of saying you can't supply 1.5 volts to a diode with a 1.7 volt turn-on voltage and expect much to happen.
posted by Rhomboid at 12:38 AM on August 12, 2008

Normally, when you put a voltage across an LED, some current flows through it. Increase the voltage, and more current goes through it, like so:
(wikipedia on current vs voltage in a diode)
As the voltage increases a little bit, you get a LOT more current. This is why you need a resistor in series with the the LED to prevent that runaway effect.

Regarding books, I highly recommend
Paul Scherz's 'Practical Electronics for Inventors', 2nd Edition (e.g. without the errata)
along with
Forrest Mims' 'Getting started in electronics'. The latter is more introductory, but good. Try your library.

And pick up a breadboard to build circuits with.
posted by sebastienbailard at 12:40 AM on August 12, 2008

n'thing the resistor.

Try a beginner electronics hobby kit. If you learn to solder, the possibilities are endless, but if you don't have the means or desire, there are breadboard or snap-in circuit options like one of these.
posted by spiderskull at 12:47 AM on August 12, 2008

LEDs are picky about their power supplies and from my experience generally require electronic control circuits to be useful as lights. Many LEDs are designed to be pulse driven, meaning they do not just burn continuously such as an incandescent bulb attached to a DC power source, but rather flash at very high frequencies. Even if you got lucky and lit up your LED straight off a battery you will probably kill it quickly and certainly not run it efficiently unless it is designed for the very variable power coming off a AA (I think it will have to be red). I have been looking into designing a LED system for my bicycle and have found the power control systems to be the most costly part of the design. IA certainly NAEE, please correct me if I'm wrong. But I remember those Forrest Mims radioshack handbooks -- all meticulously hand written!

This page has some good info but a lot of ads to read around.

So far I've ended up just buying dirt cheap lights from china off of ebay and repurposing them but I am interested to hear what you come up with. Oh, look at this.
posted by headless at 12:54 AM on August 12, 2008

You could try a brand new alkaline battery. A 1.5V battery is more like 1.65V (or more) when new, and the voltage drops from there, so 1.5 is just a ballpark average. A fresh battery might supply enough voltage to light the LED.

But you're probably better off doing it properly - use two batteries in series. Connect them to the LED for just a split second (it will over-volt and be very bright, but if it's just a split second, you won't burn it out) to confirm that it works, then set about figuring out the resistor side of things, to make it into a proper device.
posted by -harlequin- at 1:26 AM on August 12, 2008

LEDs aren't that picky. The brightness from one LED to another can vary quite a bit if you only use a resistor to regulate current. Other than burning them out, that is really the only issue.

It happens that driving LEDs with pulses is an easy way to control them for certain purposes, but that is all about practical control circuitry, and nothing to do with fundamental properties.
posted by Chuckles at 1:28 AM on August 12, 2008

Another vote for load resistor.
posted by whoda at 6:35 AM on August 12, 2008

You need a resistor and another battery.

LED's (diodes in general) are like a switch. Below a certain voltage, they are off and don't conduct at all (as you discovered). Above that voltage, they are on, and conduct freely. Thats not good, because too much current will cause damage or at least drain your battery really quickly.

Ohms law is: V=IR
Consider the wire: on one end, you are holding the voltage with your battery at 3V (if you use two batteries to increase the voltage. The other end you are holding at 1.7V, because of how the LED works (This is an approximation, but good enough for our discussion). If the wire has 0 resistance, then I = V/R = (3-1.7)/0 = Infinity! Obviously in reality, the current will not be infinite - the diode voltage will increase, and the battery voltage will decrease, and your wire is not exactly 0 ohms. But you get the point. If, however you include a resistor, you can control the current to a reasonable value (depending on the size of the resistor, see Ohms Law. The smaller resistor you use the brighter your LED, and vice versa.
posted by jpdoane at 7:39 AM on August 12, 2008

Are you sure you have a 1.7 V LED? 1.7 V sounds like a red LED which sounds like the wrong color for a reading light. If you bought / scavenged a white LED it's probably 3.5 V (or slightly higher if it's a superbright). If that's the case, 2 AA batteries will not cut it.

I find that 9v batteries are a good choice for very simple projects like this. It will NOT be the efficient choice, but:
-it's only one battery and gves you more than enough voltage for most small projects
-the connectors are small and cheap
-they are easy to solder to if you're really lazy (don't want to buy a connector)

you can also test out your LED with a 9v battery and no soldering. Put the longer leg of the LED against the small (+) battery terminal and *tap* the shorter leg against the larger (-) terminal. Don't hold it there or you'll burn out the LED but it should be easily able to accomodate a short tap. You should see the LED flash.
posted by aquafiend at 8:06 AM on August 12, 2008

Wow, these are great responses!

It looks like at the very least I'm going to need a different set of tools. As much fun as it is to sit on my living room floor and solder direclty to batteries, I think a battery holder and breadboard are definitely in order - especially since I hope to ultimately wire more than one of these in series! This is the thing I eventually want to make, sans microcontroller for now (another project for another day)

The solar light instructable looks great, headless. And thanks to everyone for the crash course in resistance and voltage -- it helps to talk through it step by step, even for the stuff I sort of know. I also had no idea that LEDs had a voltage threshold - that changes a lot!

Once again, metafilter to the rescue!
posted by puckish at 4:45 PM on August 12, 2008

Late to the party, but what the heck. The alternative to finding/sizing the correct resistor is to use a battery that's bad at delivering current, and which will limit itself. The ideal here is a 3V lithium watch battery like the CR2032 that's used everywhere.

Having a 3V source gets you over the diode hump, and watch batteries are horrible current sources, so you won't pop the LED. You can assemble the two pieces together with a piece of tape and call it a day.

This is the basic idea between the Throwies that were made by the zillions at a Maker Faire a few years back. I had a bunch of student make them and track the lifespan; typically they stay bright for a couple of days of continuous use.

I'll also point out briefly that reading from a single normal LED is going to be kind of annoying; they don't give off a ton of light, so you'll end up holding the book pretty close the LED and illuminating a small circle. What you really want is something like a Luxeon LED, but that's another Instructable.
posted by range at 4:49 PM on August 12, 2008 [1 favorite]

Very late to the party but: Yeah you need a 3V supply and a resistor. Don't solder anything, just use alligator clips. Twist the stripped wire ends together then clamp an alligator on the wires. Once you know things work, then you can start soldering things and use shrink tubing to make it pretty.

FWIW, soldering onto batteries is a bad idea. They contain nasty chemicals and can leak or explode if heated. Buy a little 3V AA battery holder.
posted by chairface at 10:06 PM on August 12, 2008

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