One LED, two LED, red LED, blue LED

A reader writes:

Myself and a friend were just reading Big Clive's "Hack your solar garden lights", and we are unsure how he came to those amp readings and the conclusion that two LEDs use less amps than one.

LED brightness comparison

I am assuming we are just missing something, could you please enlighten us?


To oversimplify, two LEDs in series have more resistance, so less current flows. But halving the current passing through an LED doesn't necessarily halve its brightness. Standard high-brightness 5mm LEDs generally have a 20-milliamp current draw on the spec sheet, but will glow from much less, and may be considerably more efficient at small currents.

The reason why this is an oversimplification is that LEDs, unlike incandescent-filament lamps, aren't just a relatively simple resistive device.

(And the "relatively" is in that sentence because not even tungsten-filament bulbs are completely straightforward. They have, for instance, a much lower resistance when cold than when operating. And reducing the power of a filament bulb will generally give you a reduction in apparent brightness that's greater than the reduction in power, because the filament will be cooler and more of its output will be down in the invisible infrared. LEDs, in contrast, only know how to make one colour, even when they're only barely creating a tiny spark of light. This is the case for white LEDs too, because to date all of those are actually blue LEDs with a phosphor coating that turns some of the blue light into other colours.)

Instead of being resistors, Light Emitting Diodes are, yes, diodes, with a constant voltage drop across them at a given temperature. But when they're lit they get warmer, causing them to pass more current and glow brighter and get warmer again, which can rapidly lead to destructive thermal runaway unless the LED is restrained in some way, by for instance limiting the source voltage so the LED will just never be able to get hot enough. Or, more commonly, by limiting the maximum possible current.

You can see how this can get complicated. (Power-supply design in general is a surprisingly tricky field.) Just running LEDs from a simple DC source via current-limiting resistors can be a bit complex; proper efficient LED drivers that deliver a set current no matter what LED you plug into them are more complicated again. (The drivers in garden lights are elegant, but like the "joule thief", not actually very efficient.)

Don't let all this put you off monkeying with garden lights, though; as Clive says, they're both easy to modify and so cheap that it doesn't matter if you wreck something. Just add some of the incredibly cheap high-brightness LEDs you can get nowadays (which I mentioned the other day), and you can make all sorts of decorative, and even useful, solar LED lights for close to no money at all.

Electrochemical Spuds Of Death

A reader writes:

Hello there Mr. Dan. I stumbled across your site whilst googling "can you get hurt making a potato battery". Yep, I googled that.

I (clearly) know little about the electronics/cathode/anode world... but could answer lots of questions about other things non electrical. :)

In planning my son's birthday party, I am considering a potato battery station (sounds odd for a party, but trust me, it fits with the theme).

I have seen several Youtube videos with instructions and examples, some done by children. My main question before I go buy a bag o potatoes and seek out the copper wiring aisle of Walmart is: Can children be hurt doing this? Yes, us grown-up types will be there too, but is there anything I should be concerned about?

Partying Mom

It is theoretically possible to kill yourself with potato batteries, but the chance of a kid managing to achieve this is much, much lower than the chance that one of them will fall over and crack his/her skull in your bathroom, and you probably won't lie awake at night worrying about that.

I could just leave it at that, but of course I won't. This is because I think an understanding of the basics of electrochemistry, which is what potato batteries are all about, is something that all modern humans should have, even if they never put it to use.

You should know why it's warmer in the summer (it's surprising how many people incorrectly say "because then we're closer to the sun", which, even if it were true, would make summer happen at the same time for both the northern and southern hemispheres...), you should know how tax brackets work, and you should also know the basics of the technology that envelops modern humans so completely that we hardly notice it at all.

Sorry, didn't mean to lecture you. This is just something I'm rather passionate about.

Getting back to potato batteries: The power output of an individual potato, or lemon, or what-have-you, "battery" is extremely low, which is why there are few-to-no things you can power from one spud with two pieces of dissimilar metal in it.

"Battery" is in quotes up there because one tuber and two bits of metal are a single electrochemical "cell"; technically, it's not a "battery" unless it has more than one cell in it. (So, of the things sold in the supermarket as "batteries", AAs and Cs and Ds are cells, but 9V or 6V batteries, composed of six and four 1.5-volt internal cells respectively, really are batteries.)

The open-circuit voltage of any electrochemical cell is determined by the electrode potential of the materials you use for the electrodes. If you build the usual kind of potato battery with copper and zinc electrodes (like, a copper or copper-plated coin, and a zinc-plated galvanised nail), each cell will have an open-circuit voltage of 1.1V, but a current capacity into a short circuit of less than a milliamp.

The larger the surface area of the electrodes, the higher the current capacity will be. But even with really big electrodes you'll probably only get half a milliamp into a short circuit - and the more of the cell's current capacity you use, the lower its output voltage will be.

(For comparison, I just grabbed a rather old but unused off-brand "super heavy duty" - meaning, carbon-zinc, not even alkaline - AA cell out of my Drawer Of Many Batteries, and it still reads more than 1.6 volts open circuit, with a short-circuit current capacity of more than 1.5 amps. Here's a PDF datasheet for an Energizer carbon-zinc AA; they've got a sub-site devoted to these things.)

If you make multiple potato batteries and put them in series and/or parallel, you can increase the voltage and/or current capacity of the whole battery, respectively. Two cells in series (both of which can be stabbed into the same potato; just connect the copper of one cell to the zinc of the next) and you get 2.2 volts open circuit and the same miserably tiny current capacity. Two cells in parallel, and you get 1.1 volts but double the current capacity. Six cells, wired up as series strings of three with the two strings in parallel with each other, and you get 3.3 volts and double current capacity. And so on.

(Many people seem to find the concept of series and parallel circuits tricky to grasp. It's another of those bedrock pieces of information about the world that I urge everyone to learn, though, because it explains a great deal of everyday electrical things. Why does one bulb dying in a string of old Christmas lights kill the whole string? Because they're ten or twenty 12V bulbs {depending on your local mains voltage} wired in series to connect directly to the mains. Why, in contrast, can you have a couple of things turned on and a couple of things turned off all plugged into the same powerboard and have everything work? Because the powerboard's outputs are in parallel!)

Getting back to your actual question, this is how you could, if you tried very hard, kill yourself with a potato battery. 30 milliamps across the heart has a pretty good chance of stopping it, and even lower currents have upon occasion been fatal. Kids might be more susceptible, too; I don't know.

Even sweaty skin is a good enough insulator that sundry low-voltage current sources aren't dangerous - grab the terminals of a 12V car battery with bare wet hands and you probably won't even feel a tingle, though a tiny current really will be flowing through your arms and across your chest. But if you stab probes into yourself, into your hands or preferably into your chest right on either side of the heart, then an array of potato batteries big enough to deliver tens of milliamps really could, if connected to the electrodes, kill you.

(One reason why high voltage can be especially dangerous is that it can spark a hole right through the skin, giving it access to your wet salty conductive innards.)

Given, of course, that this particular means of death starts out with stabbing yourself, you could simplify the process by just stabbing your heart directly.

Hence: Not worth worrying about.

(There's also an outside chance that you could poison yourself by eating a potato or lemon or whatever that's been used as a battery for a while, because it'll now be contaminated with various metallic salts. It probably wouldn't do more than make even a small child slightly ill, though, presuming he or she somehow managed to choke the vile-tasting thing down. This situation is even less likely to happen than chest-stabbing, unless you use some particularly delicious fruit instead of a potato or lemon.)

The great problem with potato-battery demonstrations in the past was not, of course, kids somehow killing themselves, but that it was very difficult to do anything with the extremely feeble output of such a battery. Turning even a tiny motor, or lighting even a grain-of-wheat incandescent bulb, was impossible without a ridiculous number of cells. Getting a feeble glow from a grain-of-wheat bulb rated for 12 volts and 80 milliamps could perhaps be done with as few as 50 potato cells, though I suspect you'd need a hundred or more.

So potato batteries usually ended up doing something lame like powering a pocket transistor radio with a piezoelectric earpiece, which is a feat that you can more impressively achieve with no battery at all.

Today, you could similarly fail to impress the youngsters by potato-powering one of those little LCD clocks and kitchen timers that're meant to run from a couple of button cells. Two or three potato cells in series might, at a stretch, be able to run one of those. A far better target, though, is lighting a light-emitting diode (LED).

A modern high-intensity red or amber LED will only want about two volts and a couple of milliamps to light dimly, and will be quite impressively bright at only 10mA. Ten parallel strings each containing two potato cells ought to be enough to give a pretty bright light, and each two-cell "string" could be only one potato.

Here's a red LED...

LED and lemon battery
(image source Flickr user trvance)

...just barely glowing from only three copper/zinc lemon cells in series...

Multi-cell lemon battery
(image source Flickr user s8)

...and here's an excellent example of multiple cells in one lemon...

Joule Thief lemon battery lighting LED
(image source Flickr user s8)

...which works extremely well because it's cheating, and using a simple four-component circuit (counting the LED) called a "Joule Thief", which I learned about years ago on the excellent Web site of the inimitable Big Clive.

I recommend you provide sufficient spuds and/or lemons, electrodes and alligator-clip leads to make lots of cells, and also provide a grab-bag of water-clear high-intensity LEDs so the kids don't know what colour they've got until they get it to light up.

A lot of LEDs will not cost you a lot of money. I find it mind-blowing that the going price on eBay for a pack of a hundred mixed waterclear high-intensity LEDs has, for some time now, been under five US bucks, delivered. I suggest you get 5mm LEDs, not the 3mm ones that're the absolute cheapest, because the smaller ones are a bit fiddly even for kids' hands.

(I don't actually need any more LEDs, but I just felt morally obliged to buy this hundred-5mm-LED pack, from this seller, for $US2.99 delivered. At this price you could use these things, which were a miracle of the age in the 1970s and have for years now been revolutionising a significant portion of the lighting industry, as notice-board pins. They are literally cheaper than thumbtacks. Even the ones with three different-coloured dies and an invisibly minuscule controller chip built in cost damn close to nothing.)

You should play with this stuff yourself before the party, so you can introduce the kids to the series/parallel idea, and help them if they don't know to chain the cells nose-to-tail (copper to zinc or zinc to copper, not copper to copper or zinc to zinc), and also see which way round you have to connect the LEDs to make them work. (They're light-emitting diodes; they only work one way around. Long leg positive.)

It would also be a really good idea to get the finest, cheapest digital multimeter eBay has to offer, so you don't have to rely on licking the ends of wires to estimate how many volts your potatoes have managed to make. Every home should have a crappy ten-buck yellow plastic multimeter; you may not use it often, but it can be very handy at times. (Put it in the kitchen drawer with the screwdriver, the hammer, the random screws and washers and the polycaprolactone.)

Depending on age and disposition, the kids may figure this all out for themselves, of course. LEDs only work one way round, a battery setup that'll light a 1.8V red LED probably won't light a 3.6V blue or white one, a setup that'll light a blue LED may very satisfyingly turn a red one into...

Dead LED

...a friode, you can series- and parallel-wire LEDs as well as batteries...

While you're shopping for quantum-physics miracles on eBay for three cents each, you could add a couple more things that used to be super-tech and are now super-cheap: Lithium coin cells, and rare-earth magnets.

2016 (20mm diameter, 1.6mm thickness) and 2032 (3.2mm thick) coin cells aren't as cheap as LEDs; if you buy them in a supermarket or pharmacy you can pay dollars for one. Again, though, just hit eBay and you can find fifty for less than 15 US cents each.

Rare-earth magnets can be even cheaper. If you restrict this search to Buy It Now items more suited to the impatient, you can get twenty 8mm-diameter 1mm-thickness neodymium-iron-boron disks for less than ten cents each; hundred-packs drop it to about seven cents apiece.

Why am I suggesting you buy these items?

Because you can light an LED by just pressing its legs to either side of a coin cell...

LEDs on a coin cell
(image source Flickr user spike55151)

...and if you put LEDs (preferably diffused 10mm ones, but any with legs will work), coin cells and magnets together, you get...

LED throwie production line
(image source Flickr user c3o)

..."LED throwies".

LED throwies
(image source Flickr user chopsueyphoto)

Which are easy to make, and awesome.

Psycho Science is a regular feature here. Ask me your science questions, and I'll answer them. Probably.

And then commenters will, I hope, correct at least the most obvious flaws in my answer.

Catches fire, would buy again, AAA+++!

I bought a couple of sets of red LED lights on eBay; two ten-metre 100-LED strings for $US15.96 delivered.

You know the ones. Little lightweight controller box that always starts in cycle-through-all-modes mode, with a button that has to be pressed seven, or is it eight, times to get the darn things to just stay on constantly (or as close to it as the flickery PWM controller can manage).

Generally these cheap lights seem great. I've been very pleased with the others I've bought in the past, most recently the 220V-rated multicoloured ones from this seller, which seem to work very nicely from Australian power.

So I bought some red ones, alleged to work from 110 to 220 volts, from this other seller.

I plugged these new ones in while holding the wound-up lights in my hand, just to see if they worked at all, and they seemed OK.

And then, there was a pain.

In my hand.

A... burning pain, restricted to a few very small spots.

This puzzled me.

I adjusted my grip to avoid the ouchy spots, and observed a few thin trails of smoke rising from the wound-up lights.

I unplugged them.

I tried the other set.

Same deal.

These sorts of LED lights are configured as several long series strings, with a single inline current-limiting resistor (which, being one resistor at the start of a long series string of LEDs, probably doesn't actually limit current very well at all) in series with the first LED in each string.

[UPDATE: Now that I'm peeling one of the lights apart, it's apparent that they've actually got resistors on several of the LEDs early in each string. Here's a great analysis of these things and how to stop them flashing and flickering, forever.]

These resistors were getting very hot, very fast, and raising smoke from the clear PVC insulation over them.

Seizing the opportunity to use my variac and its delightfully mad-scientist-ish giant knob, I tried feeding the lights 110V instead of Australia's nominal-230V mains.

Now, they worked fine. The resistors got a bit warm, but not unduly so.

Fault located, then.

Next, like a damn fool, I told the seller that they were selling devices that were a fire hazard in 200V+ countries, and they should probably stop doing that, and could I have my money back, please?

Anybody who's ever filed an eBay/PayPal dispute over a defective item of low dollar value sold by some dude in China knows what happened next.

I opened a Dispute, I asked for a refund, they told me to get lost. I escalated the Dispute to a Claim, and eBay/PayPal in their wisdom told me to send the items back to the seller via registered mail to get my refund, which would of course be five bucks less than it'd cost to send the goods back.

(And if the seller decided to tell eBay that what I'd sent them was a box of newspaper, I probably wouldn't even get that.)

Perhaps if I'd lied and said the goods never showed up at all, I might have had a chance. Since I tried to warn the seller about maybe setting their customers' houses on fire, though, I got to pay the price.

Which is not in itself a big deal, of course, besides THE PRINCIPLE OF THE THING GRRR. It's not a dead loss, either; I can always chop the LED strings off the controller box and run them from some appropriate non-flickery DC power supply. This is not very difficult to do, and involves a lot less soldering than building an LED array used to.

I feel such a tit, though. Every time, I go through this idiotic routine, like Charlie Brown with Lucy's damn football.

Sometimes there's a bit of variety, like when I was trying to get a refund for an item described as new which turned out to be used, and the Hong Kong seller seemed to sincerely believe that "but if I give you a refund, I will lose money!" was an ironclad reason why he need not do so.

(Eventually he tried "OK, we'll give you a few bucks back, provided you lie in your feedback and say there wasn't a problem.")

I love the PayPal replies, too. You've proved that sending the item back will cost more than the refund? Well, now apparently it's a "judgement call" whether you should do so!

And then, "We know situations like this can be difficult and appreciate your patience and cooperation as we work toward resolution."

I really wish eBay/PayPal would be realistic in these exchanges and just say "hey, it's a flea market, almost always it works OK, but you got ripped off this time, it happens". Instead, just to twist the knife, when you give up and Cancel a PayPal claim, " agree that this complaint has been resolved to your satisfaction"!

(The only alternative is to wait until the clock runs out, whereupon PayPal tell you that the lack of resolution of your complaint is entirely due to your tardiness.)

So, in summation: EBay/PayPal aren't getting any better about this stuff.

And, if you're in Australia and want cheap twinkly LED lights in many colours, try these.

And don't buy stuff from this dickhead.

UPDATE: Lo, a message has arrived from the dickhead him or her self!

I'm sorry for that that our product make you no happy,
anyway, can you help to revise the feedback to positive and we'll refund

Yeaahhh... no. Product still fire hazard. Bad seller! Bad!

Tiny amps and huge speakers

A reader writes:

I have a quick question regarding speaker sensitivity as I'm somewhat confused. I've always liked nice sound, but it's not a big passion of mine, recently a read a review on a small tube amp and thought that might provide me with a good starting point to sit and listen to some albums at home rather than just listen to things on the train/bus.

Anyway, this small tube amp is cheap and cheerful, putting out a beastly sum of 3.5 watts/channel @ 8 ohms.

So I started to look for speakers to match the amp. If you want something with a high sensitivity, as they recommend, you start running into some big dollars, which to me kinda negates the cheap amp! Reading around people recommend a 90db+ @ 1m set of bookshelf or similar speakers (Even as far as 97db @ 1m!). I happened to come across these large 3 way tower Jamos for a mere $500 delivered, offering a sensitivity of 89db @ 1m, which is better than say bookshelf speakers such as the AudioEngine P4 which are 88db @ 1m!

I guess ultimately I want to know can this tiny amp power such large tower speakers and still be listenable in my small lounge room (in my small apartment)? Also am I right in saying if you got both sets of speakers (Jamos and the AudioEngines) the Jamos would be noticably louder given the same wattage inputted?

Thanks for your assistance and hope you can help!


First up, I implore you to spend less money and get a proper transistor amplifier. Not because the little valve amp you're looking at has feeble output power - which it does, but that's really not very important - but because tube amps do not actually sound better.

Tubes in guitar amps, that are meant to be driven into distortion, sound very different from transistors, and you may well find yourself turning up a tiny tube amp far enough that it goes into clearly audible distortion, but this is a bad thing. Hi-fi amps are not meant to have any audible distortion, and at normal volumes for normal amps this is the case for all of them. In this situation, tubes and transistors sound exactly the same. Various golden-eared audiophiles insist that this is not the case; none of them score better than chance in blinded tests.

Speaker design can make a huge difference to the sound of a hi-fi system; amplifier design does not, unless there's something terribly wrong with the amp (which doesn't mean audiophiles won't still insist it's awesome).

OK, lecture concludes.

As regards getting decent listening volume from a weedy amp, your room and how close you are to the speakers matters more than the raw loudspeaker efficiency numbers (which are measured with a 1kHz test tone, and so don't take bass or treble response into account; you may prefer quieter speakers if they have a flatter response curve). In your little room in your little flat, you'll probably be able to get away with just about anything.

It's perfectly possible to get room-filling audio from a few watts per channel. That's all that most listening actually ever needs, because of the logarithmic response of the human ear - you need something like ten times as much power to make the music sound twice as loud. Extra wattage is nice to have for parties, or extraordinarily low-efficiency speakers, and an amp with high burst current capacity can make sudden crescendos, the cannons in the 1812 Overture, et cetera, sound better. But low output power is not a big deal for most purposes.

There are, by the way, many perfectly-fine low-wattage amps in the the Class D or "Class T" categories, which even many audiophiles appear to like, possibly because of some warped sense that these amplifiers have as little output power as a frou-frou tube amp, and so they must sound good. There's no pressing reason to get a Class D amp instead of an ordinary one if you're not short of space or want an amp that'll run from 12 volts, but there certainly are a lot of super-cheap 12V Class Ds on eBay these days.

As a general rule, sealed-box loudspeakers have terrible efficiency of about 1% at best, but even they can be OK from a few watts per channel if the room's small and/or the speakers are close to the listener. Ported (or "bass reflex") loudspeakers still only have an audio efficiency of a few per cent, but they beat the heck out of sealed boxes for loudness and can be tuned to have a response hump down around the bass-drum area, which is why practically every low-end speaker these days is ported. There are other designs - transmissions lines, horns, electrostatics and several more - but they're all in the pretty-darn-expensive-unless-you-build-it-yourself category.

You definitely can connect a tiny amp to big speakers, by the way, and that's actually a good way to get plenty of sound from a small amplifier. As a general rule, the bigger the speaker, the higher its efficiency. This is why the best way to upgrade a cheap-'n'-crappy plastic department-store midi system is to replace the standard speakers with much, much bigger ones. You can actually blow up big speakers with a small amp, if you turn it up way past the audible-distortion line and it starts sending some really nasty waveforms to the speakers; generally, this happens at parties when everyone's too drunk to notice how awful the music suddenly sounds (and how it gets progressively worse, as the tweeters die first, then the midranges...). As long as you know that, though, there's no down side to pairing big speakers with a small amp.

If you know which end of a screwdriver is which, I recommend you check out nearby loudspeaker-kit companies; you can get great speakers very cheaply if you do just a little bit of screwing and gluing. Also, remember thrift shops and garage sales; most of the speakers you'll find there will be pretty awful or need significant repairs, but you could just as easily find some nice full-sized three-ways from 1985 whose only problem is that some kid poked the woofer and put a dent in the dust cap. (Which, by the way, won't hurt the sound. If the speaker's been poked so hard that the voice coil scrapes on the magnet, or if it's a fragile little dome tweeter that's been crushed, that's bad. Damaged or missing dust caps on bigger drivers don't matter, though. It's also possible to replace rotted or ripped roll surrounds around big drivers, either with a repair kit that comes with surrounds the exact right size, or more annoyingly with a reel of straight roll surround.)

Try eBay, too, avoiding surprise expenses by searching for speakers within whatever distance you find acceptable of where you live. I'm up in the mountains without a whole lot of nearby options, but searching for used speakers within 25km of my mum's house in the Sydney suburbs shows me some awesome Seventies Technics monsters in good condition, various little brand-name surround-speaker sets that can be perfectly fine even without a subwoofer, some Tannoys that'd be great except some loony will probably bid them up to a zillion dollars, some odd-looking little Mission bookshelf speakers, some nice little Gale bookshelfs too (Gale are sort of the very top of the off-brand mountain; good designs, low prices), and the list goes on.

Above all, don't think that you have to get something with a Major Hi-Fi Brand on it to get decent sound. The Jamos you mention look very nice for the money (not so nice for their much higher alleged list price), but they might not be great for a small room, because they have a rear-firing bass driver and port, so you can't push them hard up against a wall at the back without losing a lot of bass. But there really are a lot of other options.

I dunno - maybe I just enjoy shopping for used speakers in the same way many blokes enjoy shopping for used cars!

And now, let the argument in the comments about valves and transistors... commence!