Help - I want to power a 5 mm LED from 1.5V alkaline cell

I want to use my partially depleted D cells to power a single 5mm white LED. I want to use only one cell at a time. How do I go about it?

I don't have any particular purpose for this light, it is just a fun project. Well, I will probably use it as a camp marker and in tent illumination.

Is there a small cheap driver that could boost the voltage to 3.2-3.5 Volt. If it had multiple modes, like 10, 20 and 40mA it would be great. Or could I use the driver and a switch + a resistor in parallel with the LED to drop the intensity, but that wouldn't be very efficient, would it? Else should I try a joule thief (and forget about multi modes)?

What would be the most efficient solution?

If you have any other suggestion, I'm listening.

Thanks in advance for your help.

Calina said:

Else should I try a joule thief

Click to expand...

I'd recommend this circuit, more efficient than the Joule thief.
I used it to upgrade a bike light, see this thread.
You can choose the value of the LED current by changing R1.

I would prefer a cheap driver from DX or such but it is a nice circuit and if R1 is set in parallel with a small variable resistor it would be quite flexible, albeit the circuit would be slightly larger. Maybe a switch to choose between two resistor values would be a sturdier solution.
My problem if I decide to do it, will be the transformer build. I'll check in my scrap box if I can find a suitable core and some wire.

Merci, (je suppose que vous êtes francophone). Pouvoir lire le tchèque serait bien utile ici quoique le circuit soit assez clair.

You can also wire an old ArcAAA circuit. The start-up voltage on it is around 0.8V, IIRC. Not as low as the Joule thief, but once the circuit is running, you'll run a D-cell for a looooooooooooong time.

Thanks greenLED,

That is pretty much what I'm looking for, but I don't have an old Arc to start with . Any idea were I could find a small driver? I searched to no avail.

Could wire up an E01 head. 0.4v startup. You're talking WEEKS of runtime...

Calina said:

Thanks greenLED,

That is pretty much what I'm looking for, but I don't have an old Arc to start with . Any idea were I could find a small driver? I searched to no avail.

Click to expand...

You might be able to buy them from Arc. Some of the modders may have a couple of spares; might check the Homemade and modified BST?

I made a circuit that works for the opposite reason: Testing LED's. Wow, you made me go back a few years!

It pretty much runs any color LED at a low current, just enought to make sure it works and know the color.

It was a fun project. I got the circuit from a library I believe. Pre-internet! If I can find it I'll come back and post it.

The DIP switch is just a single pole, all wired in parallel. Only need to turn on one...

The big inductor results in a flicker-free light.

I recommend doing this project the fun way... :twothumbs

Calina said:

My problem if I decide to do it, will be the transformer build.

Click to expand...

I used an off-the-shelf 100 uH inductance like this one (one euro here in France).
Because the inductance was higher than suggested on czech page, I removed 1/4 of the turns, and used the wire to wind the secondary winding. It worked perfectly and I am very satisfied with the result.
I contacted the author about how important the inductance was and he answered:
If inductance will be higher, the LED current vill be smaller. If inductance will be small, the efficiency will be decreased. The ratio between primary and secondary wind doesn't important, 1:3 to 1:10 will be right.
Please note that this circuit is designed to power two LEDs in series, but it should probably work for one LED only by choosing the correct value for R1 (start with a high setting on a potentiometer, and decrease gradually while controlling the current).

Another option for your project would be to use one of the chips from Prema Semiconductor, especially the PR4401 or PR4402. Take a look at their detailed datasheet, you will see this is quite a versatile device since you can modify the output current by changing the value of the external inductor. Note that this is a SMD device so need to be able to handle such tiny components (which I am not).
You may have a hard finding a retailer for these in Canada, so if you PM me your address I'll be happy to send you one. In return, simply let us all CPFers know how it went .

Merci, (je suppose que vous êtes francophone). Pouvoir lire le tchèque serait bien utile ici quoique le circuit soit assez clair.

Click to expand...

Cher cousin d'outre-Atlantique, je t'en envoie une traduction (faite par un collègue tchèque et corrigée par moi) par MP. Tes corrections éventuelles de l'anglais seront les bienvenues.

Would one of the circuit boards from one of the cheap Dorcy AAA lights work in a situation like this?

THIS LIGHT


:edited to ad link

thezman said:

Would one of the circuit boards from one of the cheap Dorcy AAA lights work in a situation like this?

THIS LIGHT


:edited to ad link

Click to expand...

I forgot completely about the Dorcy AAA but I must admit that taking apart a perfectly good light just to get a driver sort of makes no sense to me.

greenLED said:

You might be able to buy them from Arc. Some of the modders may have a couple of spares; might check the Homemade and modified BST?

Click to expand...

I still have to check with Arc if I can buy only the driver. Would it be multi mode?

EngrPaul said:

I made a circuit that works for the opposite reason: Testing LED's. Wow, you made me go back a few years!

It pretty much runs any color LED at a low current, just enought to make sure it works and know the color.

It was a fun project. I got the circuit from a library I believe. Pre-internet! If I can find it I'll come back and post it.

Click to expand...

Did you find the circuit and would you care to explain how it works?

Thanks everybody for the suggestions up till now. I find it hard to believe that a small ready made low current multi mode driver doesn't seem to exist anywhere.

Calina said:

Did you find the circuit and would you care to explain how it works?

Click to expand...

Still looking for it...

I'll have to say, paul, that thing looks awesome!

Craig

Cemoi sent me a PM with a co-worker's translation of the Czech site for which he provided a link : http://www.belza.cz/ledlight/m2d.htm

The circuit might be more powerful than what I've been looking for but since it could still be of interest to others, here it is.
_____________________________________

Single cell – two white LED voltage converter

I tried to design a simple converter to the light, able to supply more LEDs from one Ni Cd (NiMH) battery. Most converters I have ever found, were able to supply only one LED from one converter. Even if I didn't achieve the requested goal (4 LEDs), the circuit has some interesting characteristics.
The inspiration for this circuit was lights described at http://elm-chan.org/works/led1/report_e.html and http://elm-chan.org/works/led2/report.html . I was quite successful in achieving better efficiency and performance using voltage of 1.25 V. The converter was based on the self-oscillating converter principle. The experiments showed that the choice of the transistor is very important. The transistor must have a very small saturation voltage and , if possible, high gain. I tried several transistors, and BD243C was the best one, but other transistors BD24X worked much worse, so it probably depends on the manufacturer. BD433, originally used in nf amplifiers with small voltage (cars), was the second best. The special "low saturation" transistor BD433 was not very good, just a bit better than BC639. I finally used BD433, because box TO220 of transistor BD243 was to big.
The efficiency of the converter strongly depends on the capacitance of C2. The optimum was about 20nf, also with other transistors. I tried also a capacitor with higher capacitance in series with a small resistor, but the results were worse. Another component with effect on efficiency, was the capacitor C1. The capacitor should have a very small series resistor, otherwise the efficiency decreases. I achieved the best efficiency (about 5% better) with condensers 1000 micro Farad on C1 and C3, obtained from the circuit voltage converters for processor from a faulty PC motherboard.I didn't try more diodes, I used the popular Schottky diode 1N5819.
The transistor base is supplied through R1 from the converter output. This arrangement reduces the dependency of output current to input voltage and allows the converter to work also with a very small voltage. The transistor should have a high gain, to have a R1 resistor as high as possible. The amperage going through R1 affects the converter output and makes the efficiency worse.
The variation of the output amperage and efficiency for the two settings of converter are shown on fig. 2 and 3. Using a variable resistor (or potentiometer) as R1 the LED amperage was set up to 20mA for voltage 1.25, resp. 2.4 V. Using 1.25 V, the maximum of converter output amperage was about 25 mA. When I decreased the resistance of R1, the converter failed. To be exact, I notice that the supply amperage of the converter was 168mA in first case (supply voltage 1.25V), in the second case 73.5 mA(2.4V). The voltage on two LED resistances in series and resistor 10 ohm (I measured amperage as decrease of voltage on this resistor) with amperage 20mA was 6.85V.
The important part of the converter is the transformer. The core was taken from a radial inductance 09P-331K (GM), on which I wound 47.5 and 9.5 turns of core wire, size 0.3mm. The primary inductance is about 75 µH.
________________________________

I hope the translation is clear enough to be understandable, and that I didn't mess it up. If anybody here speaks Czech and wants to comment, correct or improve, feel free to do so.

Calina said:

I still have to check with Arc if I can buy only the driver. Would it be multi mode?

Click to expand...

single mode only

hmmm...
the simplest form of a boost circuit away from building transformer driven joule thiefs is using NPN transistors like this


EDIT: Better yet, I buy most of my passive components from AllElectronics
L1, Given the current you'll be working with, I think ceramic would be sufficient, but in the future if you want to expand on it I'd consider something bigger
C1, While I would rather work with monolithic caps as thay are not as prone to chipping as ceramic disc caps...but for this particular instance, disc caps are cheaper

*I am not an electrical expert, so YMMV

Design:
Thru-hole mounting is much easier on the hands if your hands shake or your not good with the iron. For Homemade projects I'd consider buying some small perf board to work with, if you are unsure with the circuit layout, get yourself a breadboard and test the circuit before soldering them in place. [both boards mentioned above is sold at radioshack]:thumbsup:

Illum:

• What is the LED current with this circuit?
• What is the efficiency?
• Will it work with two LEDs in series?
• How about two NiMH (input voltage around 2.5V) and two LEDs?

• Its used to support LEDs up to 3.6V 20ma requirement
• Starts up at 0.7V
• Loop-back circuit with either transistor open at any given time and timed by the resistors
• With a 1.0-V battery voltage, the average battery current is 41 mA, and the average LED current is 14 mA.
• the nominal input/output power efficiency is 23%, 34%, and 72% with a battery voltage of 0.8 V, 1.0 V, and 1.5 V, respectively.

I have no idea what site I got this off of, but based on the concept notes I've copied onto a .txt file, here goes

The cycle starts with the battery voltage slightly above Q2's VBE. This creates a positive Q2 base current:
iB = (battery voltage − VBE)/R2

and Q2 turns on, thus switching inductor L1 to ground. Q1 is off. Energy stored within L1's magnetic field builds as L1's current rises with a positive di/dt. As this current rises, it also flows through Q2's RSAT (D1 is off). Q2's collector voltage becomes sufficiently large to turn on Q1. Q1's base voltage is connected to Q2's collector by the feed-forward network of R1 and C1. R1 also serves as Q1's base current limit.

As Q1 turns on, the previous base drive to Q2 is then shunted to ground and Q2 turns off. The switching off of Q2 discharges L1's energy into the LED (D1) as the magnetic field collapses. This flyback action of L1 forward-biases D1, which gives up photon illumination in the form of white light. With L1 discharged, Q1 turns back off. The self-oscillating action repeats until the battery voltage falls below Q2's VBE.

L1, Q2's RSAT, and the switching characteristics of Q1 and Q2 dominate the period and duty cycle of this oscillation. LED brightness depends directly on the average current flow through D1. D1 is on while Q2 is off and off while Q2 is on.


There's a limitation on the largest battery voltage that can drive this boost circuit. Because the base of Q2 connects directly to the battery by R2, a battery voltage greater than 1.5 V will cause excessive Q2 base current. A single alkaline-cell battery (<1.5 V) is best.

Click to expand...

Concerning efficiency, i have this graph to go with it but I have no idea whats happening because I can't read it


I think its a safe assumption to say this is a single 5mm LED driver. two LEDs in series would require a minimum of 7.2V @ 20ma and two LEDs in parallel would be 3.6V 40ma [better yet 50ma] and neither seems capabable by this particular circuit

For reference, theres another method to this design but its significantly more complicated in terms of passive components required, but its still a 2x 2N3904 Signal transistor circuit

Here is your solution for $0.62. I also included a link to the data sheet for the driver.
http://www.mouser.com/Search/ProductDetail.aspx?qs=7EMYER6HMn0PEIRolSjI1g==

Data sheet: http://www.diodes.com/datasheets/ZXSC380.pdf