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I have a toy for my son, it's great but it flies round and round in the dark, I was thinking of sticking an LED on it like we used to do at school - back then LED's were rubbish but they had long legs and were designed for electronics and soldering - I am 43 now - but I remember we had to calculate the resistance put in a resistor and then it wouldn't blow the LED .. we made a fuse tester back then..

since then LED's have changed markedly.. but is there still a "brand/family" of LED .. a single stand LED... that will plug straight into a 1.5v AA without blowing that comes with REALLY long wire legs so I can solder it direct to the terminals of the AA battery in series, and fit it into the battery enclosure simply by putting the battery cover on top of it? and do they still come with thin stiff wire long metal legs that can be bent, hold a shape and remain put?

OR. If I need a resister in series what is the resistor I need can someone help me calculate?

and also, they are still a diode, are they forgiving of the polarity or will they blow if placed the wrong way round?

Mr Heelis
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  • 1. LEDs don't blow if installed backwards. As diodes, they will allow no current to flow in that arrangement. 2. They _can_ blow if they don't have a current-limiting resistor in series. 3. Look up T 1-3/4 form-factor LEDs for the type you are thinking of (most likely) – mike65535 Dec 17 '18 at 16:29
  • They still make LED's just like the ones you are talking about. A red LED might turn on with a single AA battery, but it might not. – user57037 Dec 19 '18 at 04:08

6 Answers6

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All non-infrared LEDs I can think of have a forward Voltage (i.e. the voltage across the diode when driven at the recommended operating current for full brightness) significantly enough larger than the 1.5 V you could get out of an alkaline battery.

So, since your voltage is already below what your LED can tolerate at most, you don't actually need a series resistor to drop the voltage difference between supply voltage (1.5 V) and max-current forward voltage (e.g. 1.7V for most red LEDs).

You, however, also don't get full brightness from this.

Problem is that diodes have exponential current/voltage curves, so you go down slightly below the forward voltage, your forward current drastically drops, and thus, the amount of light emitted.

Thus, best case, you need a really low forward current LED. Red is probably your only choice.

But even with a long-wavelength red LED (which can have a nominal forward voltage of 1.6V), 1.5 V (ie. a room-temperature brand new AA battery) is at the very lowest end of visibility.

So: nope, there's no LED that you can directly stick onto a single AA battery.

and do they still come with thin stiff wire long metal legs that can be bent, hold a shape and remain put?

The wired form is typically sold as "for through-hole mounting", and yes, it's still widely available.

Don't really understand where you're going with the "really long wire"; you're soldering, anyway,so soldering wires of exactly the length you need should work.

and also, they are still a diode, are they forgiving of the polarity or will they blow if placed the wrong way round?

The reverse breakdown voltages of LEDs are typically > 4V, so you won't crack that with a single AA battery and are on the safe side.

Marcus Müller
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    Oh really? **no** LED? Not true. I recently moved and have not unpacked my LED inventory yet. I did find an amber LED which has a higher Vf than deep red (660 nm). Powered it with a single AA alkaline battery. Photo in my answer. It should stay lit for a long time. I guesstimate it is drawing way less than 1 mA. I believe it is an OSRAM OSLON SSL GA CSHPM1. – Misunderstood Dec 19 '18 at 05:33
  • That's a really pretty and helpful answer. Upvoted! – Marcus Müller Dec 19 '18 at 16:24
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Indeed you need 1.5-1.6V at least to turn on a red LED. Other colors need more volts.

If the toy has a single AA, as you seem to indicate, this will not work. When the motor of the toy will run, battery voltage will sag.

In this case you will need a DC-DC converter to boost the voltage. A simpler solution would be to get the cheapest single AA LED flashlight you can get... even better if you have one lying somewhere unused... and "recycle" the circuit inside.

Another low-tech solution is to make a LED throwie instead. This is just a CR2032 battery with a LED slapped on top. These batteries are quite weak and have high internal resistance, which will limit the current through the LED to a safe value. Also at 3V it can light more LED colors than red.

Depending on the age of your kid though, make sure the kid doesn't swallow a CR2032.

bobflux
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  • Yeah sadly the CR2032 batteries can kill children and are (in my opinion) something that should have been banned years ago... – Mr Heelis Dec 17 '18 at 23:42
  • I like the idea of pulling apart a torch tho.. good call – Mr Heelis Dec 17 '18 at 23:45
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    I disagree about banning small batteries, you'd also have to ban knives, drain cleaner, small screws, etc... Just had another idea: how about putting a black light (UV) tube in the room, and painting the toy with fluorescent paint which will glow in the black light? That should be simple and safe. – bobflux Dec 18 '18 at 00:11
  • knives don't do this https://www.bbc.co.uk/news/health-37410343 .. plus everyone knows about those dangers.. did you see a sign on the last battery you bought? .. the two most deadly things in the home for Children are blinds and circular batteries... simply because they are not known about.. I can literally imagine no reason why a small battery has to be the shape of a button or a sweet.. even if they coated it in disgusting tasting stuff i'd be slightly sympathetic.. but the world isn't run by those with sense – Mr Heelis Dec 18 '18 at 11:45
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    Lithium coin batteries lodged in the esophagus should be removed immediately. Leakage, chemical burns and potential perforation can occur within hours of ingestion. Seek medical attention immediately Have physician call the NATIONAL BATTERY INGESTION HOTLINE for advice and follow-up (202)-625-3333) collect day or night. – Misunderstood Dec 19 '18 at 03:55
  • @mkeith I'm going to guess you don't deal much with the general public. I do not believe most parents are capable of being educated even if you could reach them with the message. – Misunderstood Dec 19 '18 at 10:07
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An alkaline battery is a poor choice to power an LED without boost step up converter. The voltage starts at 1.5V and discharges down to 0.8v over it's lifespan. A Lithium battery would be a much better choice when using a primary non-rechargeable battery because the voltage does not drop much over its lifespan. For a single LED, a 3V lithium would be a good choice as you can find LEDs of any color with forward voltages below 3V.

A 3.3V Li-ion USB power bank would probably be the ideal way to power a single LED.

For any battery powered LED you should find the most efficient LED with the highest luminous intensity. Not only will the battery last longer but you can run the LED at a lower current which will lower the forward voltage below the typical Vf.

The most efficient luminous color is green. While blue and white are the most efficient LEDs in terms of the number of photons and radiant watts, the Photopic Luminous Efficacy (human sensitivity) of green more than makes up for its radiometric and quantum inefficiency.

If you want white a flashlight is hard to beat.

No matter what color you want the most efficient LED available in that color.

So let's look at green LEDs. You show a preference for through hole LEDs Through hole LEDs are mostly 20 mA LEDs. While there are some 50 mA LEDs that have a higher luminous intensity they are rarely more efficient than the 20 mA LEDs. So let's look at what Digikey has in 20 mA green "indicator LEDs" which is where you find the through hole LEDs. Sort the list by the highest "Millicandela Rating" and while the Broadcomm are brighter the Cree C503B-GAS-CB0F0792 cost $1 less at 25¢ each.

At first glance the Cree LED appears to have a Vf higher than a 3V lithium battery at 3.2V. Except that 3.2V is at 20 mA. With 53,650 mcd we can reduce the current down to 1 mA and still have 2,680 mcd which is probably brighter than most cheap LEDs running at 20 mA.

Now when you look at the datasheet you will see the Vf drops to below 2.9V. Based on other LEDs where their datasheet shows the Vf curve down below 1 mA, this LED will probably have a Vf of about 2.75V @ 1 mA.

A 200 Ω resistor with a 3V lithium battery will likely work very well with the Cree C503B LED.

There are some that do not believe that resistors are an efficient way to regulate LED current but with a 3V battery and a 2.8 Vf, the efficiency is greater than 93% and will improve as the battery discharges.

While a CR2032 is a lithium battery it is not intended to supply milliamps of current. Their capacity is rated at 190 µA.

A 3000 mAh(@ 30 mA continuous) lithium CRV3 is more suitable and would last about 150 hours above 2.8V with a continuous 20 mA load.

Bottom Line

Rather than bridge an LED across a battery's terminals and get unpredictable results, use an LED with a forward voltage below the battery voltage and use a resistor to limit the current. Use a battery with a flat discharge curve to minimize the change in brightness as the battery discharges.

When an LED is driven with a voltage source the current will be whatever the voltage is on the IV curve. A aluminum gallium arsenide (AlGaAs) Deep Red and aluminum indium gallium phosphide (AlInGaP) Yellow, Orange, Red will light up with 1.5V but the current will likely be less much less than 1 mA.

A indium gallium nitride (InGaN) White, Blue, and Green will require more than 1.5V to light up.

When an LED's Vf is greater than 3V (e.g. InGaN) you can power it with any type of voltage source including a low impedance voltage source. If the voltage does not exceed the normal operating voltage range of the LED, the LED will likely work okay.

You can connect a low wattage AlGaAs LED to a 3V battery the LED may not burn out but you will very likely reduce the lifespan of the LED.

The LED pictured below is an amber AlInGaP. I connected it to two series AAA alkaline batteries and it survived. But it is a 1 Amp LED with a max Vf of 2.8V. The AAA batteries could not supply an amp of current at 3V. The no load voltage of the batteries was 3.11V which dropped to 2.3v when connected to the LED. When connected to 2 alkaline AA batteries with a no load voltage of 3.23V the battery voltage dropped to 2.7V when connected to the LED and the LED got very hot (over 60° C) very fast (10 seconds). Putting a 2.8 Ω resistor across the two AAAs the voltage dropped to 2.7V. Across the AAs, 2.9V.


Here is an amber LED powered by a single alkaline AA cell.

enter image description here


The same LED powered with two series alkaline AA batteries.

enter image description here

Misunderstood
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  • I am using rechargeables.. they are NiCa .. but 2300mva and very very stable.. they cost a lot and the recharge station I have is digital with a "processor" you can switch each recharge channel and gives very accurate listing of the numbers on it over charge - so I can tell they are stable - because of the v when they are "low" – Mr Heelis Dec 19 '18 at 09:20
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    The problem with Nickel batteries is the 1.2V per cell. You need two of them for red, yellow, or orange LEDs and three of them for white, blue or green. They do have a fairly flat discharge curve and will stay above 1.2V to near the discharge cutoff. The key long battery life is having a high efficacy LED running single digit milliamps. – Misunderstood Dec 19 '18 at 09:59
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The easiest is to use @peufeu's suggestion of a CR2032.

You can also get a small boost converter module, available from China for about 50 cents each including shipping if you're willing to wait some weeks. About 11mm x 11mm and they will work down to 0.9V which will allow you to get most of the juice out of the battery.

That will take your 1.5V (nominal, it will drop as the battery discharges under load) and convert it to 5.0V (regulated), and you can use one of the numerous online calculators for the resistor value. For example, to run a 3V white LED at 10mA you would need a resistor of about 200 ohms. A small 100uF cap across the battery wouldn't hurt.

You can't hurt the LED if you connect it backwards with a 5V or less supply (assuming a reasonable series resistor, of course). White and blue LEDs can be static sensitive however, but that's unlikely to burn you unless you're quite careless with static sparks.

Spehro Pefhany
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  • https://www.amazon.co.uk/Xintiandi-Outdoor-Portable-Flashlight-Keychain/dp/B07G363XWN/ref=sr_1_12?ie=UTF8&qid=1545133827&sr=8-12&keywords=cheap+1+battery+aa+torch can cannibalise this – Mr Heelis Dec 18 '18 at 11:53
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Why do you talk about 1.5V? A single disposable battery cell is 1.5V to 1.6V only when it is brand new and has not been used. It drops to 0.9V during its life. You need light when the battery is 0.9V to 1.6V.

An invisible infrared LED uses 1.2V, a red LED uses about 1.8V and other colors use about 3.2V. My cheap (one dollar) solar garden lights use a 0.8V to 1.6V battery cell with a little voltage booster circuit to light any color of LED.

Audioguru
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The current limiting resistor values for the LED are needed for the specific power rating of the LED. Sure, you could find an industrial LED that can handle the raw current without a resistor, but it wouldn't be very suitable. You should find the power rating of an LED you DO have, or can get cheaply, and then calculate a suitable current through it. As it is a diode, it has a forward voltage - this is the voltage drop across is that occurs to turn it on. The power rating (max power dissipated) of the LED will be in watts - which means we can find the max current that can go through it. We know P = V * I, so divide the power rating by the forward voltage (this should also be on the datasheet - and can be around 1.5v or even larger which means the LED won't be as bright depending on its forward voltage) to get our current. Now we have the max current - using Kirchoffs Voltage Law we know the rest of the voltage will be dissipated through our resistor (Use the max power remaining [nominal battery voltage 1.5v ] - forward voltage so we can be safe).

Use this current and voltage value to find a suitable resistor. You won't find one exact (unless you are very lucky) but a close one will suffice. For ex, lets say the forward voltage is 1v and the LED has a power rating of 1W. Max current is 1A, and we know that at most, 0.5v will be disspated through the resistor. So our resistance is 0.5V / 1A = 500 Ohms.

On another note, I recommend wiring the LED and resistor with the switch rather than the battery - so when the toy is off, it won't still power the LED unless the battery is physically removed (compared to your design of wiring to the terminals).

QuickishFM
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  • The forward voltage of an LED is dictated by the color it needs to emit, not by the "ruggedness" or anything. – Marcus Müller Dec 17 '18 at 16:13
  • @MarcusMüller I have made a mistake; I see it may have come across as the voltage, though I meant that certain LEDs may be able to tolerate higher currents instead. I will edit my response, thank you for alerting it to me. – QuickishFM Dec 17 '18 at 16:15
  • But which LEDs exist with a forward voltage <= 1.5V? – Marcus Müller Dec 17 '18 at 16:18
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    @MarcusMüller a Lumex SML-LX2530SRELC-TR deep red has a Vf of 1.2V @ 20 mA. Likely lower at 1 mA. – Misunderstood Dec 19 '18 at 04:07
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    @Misunderstood read that [spec sheet](https://www.lumex.com/spec/SML-LX2530SRELC-TR.pdf) carefully: It says "FORWARD VOLTAGE(IR/SR) 1.2/1.8"; the infrared version of that LED has a lower forward voltage (see my answer excluding IR), the visible red one has 1.6V. As said, the type of junction defines the band gap, which defines both the forward voltage as well as the wave length. – Marcus Müller Dec 19 '18 at 10:44
  • @MarcusMüller Good catch, but no IV curve which leaves possibilities. I'm still going with a 660 nm deep red can be powered with 1.5v. If I can get an amber AlInGaP LED to light up with 1.5V a 660 nm AlGaAs should be a piece of cake. I'm sure I have a few different 660 nm deep red to test with. I have 1.5V far red too but that is pushing it a bit too far for luminous intensity. Far red is considered a visible and is listed on the Relative Sensitivity Curve for the C.I.E. Standard Observer with a 105 on a scale of 100,000. – Misunderstood Dec 19 '18 at 12:24
  • @MarcusMüller Not sure what you mean by "type of junction" I assume materials used which would determine forward voltage. I agree band gap influences wavelength. I disagree Vf is a function of band gap or wavelength. With AlGaAs and AlInGaP Vf increases as wavelength decreases. With InGaN Vf increases as wavelength increases. Therefore Vf cannot be a function of band gap. Vf is mostly a function of the resistivity of the materials used. Photon wavelength energy has an insignificant contribution compared to material resistivity IMHO. – Misunderstood Dec 19 '18 at 12:41