I bought some LEDs for a school science lab, and they say they are for 1.5V throught 3V. I can’t get them to light with 2 AAA batteries. I’ve switched the input direction to account for the fact that they’re diodes, trying both directions. What am I missing? Here is the eBay listing. Pardon Our Interruption...
Link doesn’t work.
Did you just connect the LED leads to the battery? If so, they lit for a millisecond or 2 before burning out.
Loose LEDs need a current-limiting resistor in the curcuit somewhete.
URRRGGHH. The link works only for me, it seems.
OK, a resistor like what?
The strength of the resistor depends on the rest of the circuit you’re designing and what it’s for. This is very basic electronics, but if all that is new to you, you probably want to read or watch some electronics hobbyist stuff to get the beginnings of knowledge and understanding about all this stuff.
Here is an article which explains how to choose a current-limiting resistor for an LED:
https://eepower.com/resistor-guide/resistor-applications/resistor-for-led/#
Without any data on the actual LED (i.e., operating current) it will likely take some trial and error.
I see 30 mA. Thanks for the link. I can calculate the resistor needed. Then the challenge is finding one.
LEDs are “happiest” when they are driven by a constant current (CC) source, and unhappiest when they are driven by a constant voltage (CV) source. Connecting an LED directly to a battery is an example of the latter. The reason is due to the LED’s negative temperature coefficient. Powering it with a CV could cause it to go into thermal runaway.
A resistor in series with a battery is not a CC or CV. It is sort of “in between” a CC and CV. A low voltage in series with a low resistance is closer to a CV, and a high voltage in series with a high resistance is closer to a CC. So be careful if you’re using a low voltage in series with a low resistance, as it could sort of look like a CV and thus cause the LED to go into thermal runaway.
The best solution, IMO, is to use build a CC using an LM317 and one resistor. It’s just one more component (vs. a simple resistor), but offers much better performance and protection.
Ok, thanks. The thing is that voltage range is 1.5-3V and 30 mA. To put in 2V and keep it in the middle of the range, we use: 3V - 1V / .03 = 66.7 Ohms. Right?
Now I see that the setup linked by @Crafter_Man gives a max of 10 mA. I’m not experienced enough to go and devise something that is different. I’m a HS chem guy, and I can plan a buffer to get a 9.2 pH, but I haven’t done this kind of thing. Is asking for help on this too much? At some point there is a thing as asking for too much free work.
Yeah, no need to overcomplicate this for your presumably simple application. If these are loose LEDs with assorted colors, then the voltage range indicated might just be an approximation of the range of “forward voltages” of the LEDs in the pack. Different LEDs (particularly different colors) have different forward voltages. Assuming you grab an old-school red LED, 1.8 V would be a safe guess.
Anyway, take your applied voltage, subtract the LED’s forward voltage, divide by 0.02 A (a fine target current), and whatever resistance you get out, that’s your resistor. As noted upthread, you might opt for a higher voltage and higher resistance to ensure a more stable current (since variations in the LEDs forward voltage would matter less), but honestly these things are pretty robust as long as there’s some current limiting resistor in there.
So, a red LED with a 9 V battery would suggest a 360 ohm resistor, via (9 V - 1.8 V)/(0.02 A) = 360 ohm.
(As a side note, two AAA batteries won’t be enough to overcome the forward voltage of some LED types, and for those where it can, it leaves only a small excess voltage to judge the resistance (and thus regulate the current) by. You might be less frustrated with at least three AAA batteries, if not a different voltage source entirely.)
No, using an LM317 + “programming resistor” as a CC source can source well over 100 mA.
You just have to make sure the source can provide the necessary voltage.
When powering a bunch of unknown LEDs with a current regulator as described above, I would recommend a source voltage of at least 6 V.
The eBay seller said he used a 3V button cell, and that does work. I’m stumped why bigger AAA batteries don’t work.
Button cells have much higher internal resistance than a AAA, like a hundred times higher. His 3 V button cell probably pushed only 50 mA through the LED due to the cell’s internal resistance. Your AAA batteries would have peaked briefly at maybe two amps or so and then stopped, since the LED was instantly fried.
I have indeed seen people light LEDs by pinching them onto a button cell, but it only works by the luck of the internal resistance.
OK, I’ll go to my main point. I can start another thread if that’s better.
I had the idea to get a light that would brighten as the electricity passed through better and better media: water < dirty water < slightly salty water < quite salty water. LEDs will dim, so I thought of this. The conductivity meters often only read out in digital form, and I can tell you that HS kids will respond to a brighter light much better than just a bigger number. You have to think of many of them as emotionally in about 6th grade. So maybe I’m asking too much from this idea, but I thought that with changing the solution and the distance between the wires, we could make the LED brighten and dim. I may have invented a too complicated idea.
You have. 
Well, not too complicated to make it work, but more complicated than you thought it was.
With a DC source, your electrodes (which I’m guessing are just the tips of wires) will get an electroplated coating of whatever minerals are present (especially in the “dirty water” case), and they will also instantly start forming a layer of bubbles: gaseous hydrogen, oxygen, and chlorine from the electrolysis of the water and from the dissociation of the NaCl that you’re presumably using. So, the resistance of the system will spike very quickly from these insulating effects on the electrodes. Using AC instead of DC would solve these issues. If you do that, and maybe go to a slightly higher voltage source for better current control, I think you will get where you want to get. I’d also recommend attaching the ends to some chunky pieces of metal.
The switch to an AC source, though, is key to a smooth experiment here. Keep a current-limiting resistor in the circuit tuned to the upper end of your brightness range, so that when you touch the test leads together (no resistance in the “water”) you are still good (i.e., not cooking the LED). You can try for, say, 60 mA and see if the LED survives (assuming you have spares), but the max steady current the LED can handle will depend very much on the specific LED.
Note that for AC sources, the stated voltage will be the “RMS” voltage, and the actual peak voltage during the cycle will be sqrt(2) higher than that (whether the “stated” voltage is from a label or from an AC voltmeter). You’ll want to tune your target max current accordingly.
So the light would light up 60x /s?
Yeah, they would be turning on and off with the cycling. You’d have to test it out to see how noticeable or annoying the flickering was with your specific LEDs. The underlying brightness demonstration would still work.
What I’m not understanding is why you’re fiddling with LEDs.
A plain old incandescent bulb like from an old-fashioned flashlight has exactly the behavior you want. Put the bulb & your resistance water tank in series, add a battery to complete the circuit, and away you go.
That bulb will also effectively filter any AC ripple if you choose to go with an AC power supply.
You can power an LED with AC. Just stick a regular diode (e.g. 1N400X) across it to safely clamp the reverse voltage.
But as mentioned by @Pasta, measuring the resistivity of water is kinda hard. Not only must AC be used, by the dynamic range is huge.
Here’s what I would do: forget the LEDs. Put some distilled water into a clean cup. Grab an ohmmeter. Clean the probes with distilled water, then stick them in the water. Note the reading. It will be high, and discuss the reason for it. Add a little bit of salt. Note how the resistance instantly decreased, and then slowly increased. Have a discussion on why this happened.
In other words, instead of using AC - which is the proper way to measure the resistivity of water - use the improper way (DC) and explain the results.