I want to make this circuit, which is from evilmadscientist.com and includes a Joule Thief:

solar LED light circuit from evilmadscientist.com

It's for a solar dark-detecting LED light; and I'm trying to follow this guide: https://www.instructables.com/id/Solar-LED-Light-Bulb/?ALLSTEPS

I'm having a lot of trouble picking the right material and I'm not sure how to make a safe alteration for my needs (I want to use a brighter LED and a different solar panel than the author of the guide). I've done over a week's worth of research but have gotten vague results.

I've come up with a few possibilities for materials, and I would really appreciate it if someone could tell me if it would be safe to use these specific materials in the circuit. I don't want to burn out the LED or start a fire or something.

I've already purchased solar cells for a previous project, but that project required tiny 52x19mm solar cells. I would like to reuse them; the makeshift solar panel would be made like this: https://www.instructables.com/id/Solar-7-up-Solar-phone-charger-in-a-bottle/step3/Superglue-stack-the-solettes/

Basically, 10 solar cells are glued in series on a playing card (and one more which would be used as a conductor...? Not totally sure how that's supposed to work).

The seller's description of the solar cells:

  • B Grade
  • Average Power (Watts): 0.14 Wp
  • Average Current (Amps): 0.28 Imax
  • Average Voltage (Volts): 0.5 Vmax
  • Efficiency: 17.6%

So with 10 in series, it'd be a 5V, 280mA solar panel. The author of the guide used a 4.5v solar panel, as you can see in the circuit diagram. On sort of a side note, can I use any kind of solar panel? E.g. 3V, 280mA; 10v, 280mA...

Apparently, your diode's amps should greater than your cell's amps to be safe. The only ones I've heard of are the 1N914 diode and the 1N4001 diode. The 1N914 diode seems too small as my solar panel is over 200mA but since 280mA is the MAX current, the solar panel would never reach that current, right? Would it be okay to use the 1N914, or should I get the 1N4001? The 1N4001 seems a bit overkill since it handles 1A. Better suggestions would be much appreciated, as I'm sure there are other kinds of diodes out there that are better suited. Googling stuff like "280mA zener diode" and "300mA zener diode" didn't really get me anywhere.

For the NPN transistor, I was thinking of a 2N4401 transistor because apparently the LED will be brighter because it can handle more amps? And for the PNP transistor I was thinking of getting a 2N4403 transistor, only because it is 40V and 600mA also. I thought I'd make it match... I'm not sure if it matters though? Anyways, please tell me if these are safe decisions.

And for the resistors, the author of the guide used a 5k ohm resistor. Would a 5.1k ohm resistor work okay too? It's hard to find a 5k one. I was thinking of a 5.1k 1/4W carbon film resistor with 5% tolerance. And then a 1k 1/4W carbon film resistor with 5% tolerance that is connected to the Joule Thief.

For the ferrite toroid, I found a FT-50A-75 toroid on eBay. On another site, the product description says:

Excellent attenuation from 0.5 MHz to 20 MHz

  • Inner Diameter: 0.312 inches
  • Outer Diameter: 0.500 inches
  • Height: 0.250 inches
  • AL Value: 3000 µH/100 turns

Is it possible to determine if it's a high permeability toroid from that information? If so, please tell me how. For the Joule Thief to work, apparently you need a high permeability toroid. I'm having trouble getting one at a reasonable price (with shipping included), and this seems to be the best one I have found so far.

For the LED, I want to get a 5mm white straw hat LED. Forward voltage is 3.2-3.4V and forward current is 20mA with 12000-14000mcd (around 63 lumens). LEDs seem to need specific resistors, but I'm not sure. So I would like to make sure that this LED would work perfectly fine on this circuit without a separate, special resistor connected to it. It's just that some other sellers sell LEDs in sets that actually come with separate resistors that are supposed to be used with those LEDs.

Okay, so with a 3.2-3.4V LED and a 5V 280mA solar panel, as well as a Joule Thief, would one AAA NiMH battery with a 1800mAh capacity be all right to use?

Now, what baffles me most is the wires. Magnetic wires, solid wires, insulated wires... 22 gauge, 30 gauge, which one should I use? Some say magnet wire is best to use for the Joule Thief (maybe better because they're thinner) and others say the wire has to be insulated, and still others say it doesn't matter... I don't even have any possible material in mind. I hope someone can shed some light onto the wires for me. I thought this would be the simplest part. Wires also seem to cost way more than I thought they would. I might be looking in the wrong places and typing in the wrong things, so I would really appreciate some help. Also, is it called "hook-up wire"? I mean, would something like this work?: http://www.ebay.ca/itm/Aapex-17-Feet-18-Gauge-OFC-COPPER-RED-Positive-Primary-Turn-on-hookup-WIRE-CABLE-/250922109505?pt=US_Car_Audio_Power_Speaker_Wire&hash=item3a6c1f8a41

Should I use a different kind of wire for the Joule Thief and a different kind of wire for the rest of the circuit? If possible, I would like to use the same kind as I really do not have money to spend.

I would extremely appreciate any answers or even partial answers (even if you don't tell me about ALL components and just tell me if, for example, the LED and resistors will work). I've honestly been agonizing over this for a while, and have put in a lot of effort before trying to get help. I know this is very long, so thank you for even spending time on reading through it. I would appreciate some explanations, but even just a simple "yes, x, x, and x are safe" and "no, x is NOT safe" would help.

  • \$\begingroup\$ Deep discharging of NiMH cells, like with a joule thief will shorten their lifetime substantially. It is better design a circuit that detaches the load when the batteries are empty (approx. 1V, check the datasheet) \$\endgroup\$
    – jippie
    Commented Mar 30, 2013 at 21:41
  • \$\begingroup\$ Thanks, but I don't have the required knowledge for designing my own circuit and I have no idea how to change the circuit according to your suggestion. So just to let everyone else know too, I have really limited knowledge on electronics. You can probably tell from my question though. \$\endgroup\$
    – Seven
    Commented Mar 30, 2013 at 21:47

3 Answers 3


Carbon film resistors aren't used much these days. Metal film are much better. That 5k resistor isn't critical, a 5k1 will be fine.

  • \$\begingroup\$ But the carbon film would still be okay? They're the cheapest (including shipping) on eBay, and I'm trying to save every dollar I can. \$\endgroup\$
    – Seven
    Commented Mar 30, 2013 at 20:53
  • \$\begingroup\$ Yes. A carbon film resistor is fine. Any resistor should be okay. \$\endgroup\$ Commented Jan 29, 2022 at 17:37

It is a long question, but better than a short one, as you've shown your own research.

1) Solar cells. If you're stacking your own ones, stack 9 of them and get the 4.5V of the original circuit.

2) Battery charging. Batteries are the only thing you've left out of your spec. This is an area where the circuit design relies on cutting a lot of corners. In theory it might be out of spec, if you were to put 4.5V at 280ma through AA NiMH cells indefinitely. In practice, you don't get full sun all day, you'll be using it indoors, and you're not going to get optimal power transfer from the cells, so this isn't going to cause problems.

3) Diode. It's just a regular diode, not a zener. Current through it is actually determined by the battery and right hand side circuit, not the solar panel - the transistor is off when the panel is generating electricity. The original 1N914 will be fine. 1N4004 will also be fine.

4) Resistors: not a precision component here, use whatever meets your cost constraint. 5.1k for 5k is fine.

5) Wire: not critical. Your ebay link looks suitable. Thinner is better for the toroid.

6) Transistors: stick with the exact part numbers. Design may rely on specific parameters.

7) LED: again, this circuit relies on cheating. Normally a white LED won't run from two NiMH cells. The joule thief part provides a boost converter that gives small pulses of higher voltage. It doesn't have the capacity to provide a lot of current at that voltage. In combination with the pulsing this means there should be no risk of damaging it.

(A proper analysis of this circuit would be good, if nobody else supplies one I'll do it in a few days).

  • \$\begingroup\$ It's actually a AAA NiMH battery (1800mAh) that I was planning on using; I don't know if it was a typo, but just thought I'd point that out in case a AA battery is okay but a AAA battery is not. Thanks so much for answering by the way. The wire part was bugging me a lot, so thanks for clearing that up. \$\endgroup\$
    – Seven
    Commented Mar 30, 2013 at 21:43
  • \$\begingroup\$ Would this wire be suitable for the entire circuit as well? ebay.ca/itm/… It's thinner so I guess that's better for the joule thief. It just looks kind of fishy. \$\endgroup\$
    – Seven
    Commented Apr 1, 2013 at 22:30
  • \$\begingroup\$ That looks like it's being sold by the foot, so is more expensive if you want more than a tiny quantity. (Have you considered salvaging wire from dead appliances, if it's that cost-critical? You might find a 1n4001 diode as well. Just checking - you already have a soldering iron and solder to hand?) \$\endgroup\$
    – pjc50
    Commented Apr 2, 2013 at 0:15
  • \$\begingroup\$ I probably won't be able to identify a 1n4001 diode, and I have considered salvaging from dead appliances, but - I know it's hard to believe - I don't have anything to salvage from. I actually was worried about the soldering iron (the cost) but I found out yesterday that my school has soldering equipment that I'll be allowed to use. They also said I could take some wire, but I think I should still buy some (not too sure if they'll be very happy about me taking more than "some" wire). Thank you so much for all the help! \$\endgroup\$
    – Seven
    Commented Apr 6, 2013 at 21:47
  • \$\begingroup\$ @Seven the 1N4000 series diodes are usually marked with their part number. Higher numbered ones, e.g. 1N4004, would work just as well as 1N4001. \$\endgroup\$
    – Simon B
    Commented Dec 21, 2021 at 10:10

Regarding your inductor - I had a quick look on the internet to find out about joule thieves and one of the things that was said on wikipedia was: -

The predominant mode of operation relies on the non-linearity of the inductor (this does not apply to air core coils). As the current ramps up it reaches a point, dependent upon the material and geometry of the core, where the ferrite saturates (the core may be made of material other than ferrite). The resulting magnetic field stops increasing and the current in the secondary winding is lost, depriving the transistor of base drive and the transistor starts to turn off. The magnetic field starts to collapse, driving current in the coil into the light emitting diode (raising the voltage until conduction occurs) and the reducing magnetic field induces a reverse current in the secondary, turning the transistor hard off.

What this implies is that you have to be a little careful when choosing the toroid core. You want one that will saturate and from the your coil spec (3000uH/100 turns) it looks like a fairly low permeability type and this may not saturate as expected.

I'm not saying it won't work but i am saying in my experience that when I calculate the inductance per single turn (3000uH / turns-squared) I get 300nH per single turn and it strikes me that the inductor core is a possibly a low saturation type given the size it appears to be on the videos i saw plus number of turns and likely current flow.

  • \$\begingroup\$ What would be considered "high" saturation? \$\endgroup\$
    – Seven
    Commented Mar 30, 2013 at 21:49
  • \$\begingroup\$ @Seven it's really difficult to say - there are several factors that I'm just guessing at really; number of turns increasing can make a core saturate, higher currents, smaller toroid dimensions. The core I saw looked big-ish and given the currents involved it might need a specific type of core material that saturates more easily. Try it and if it works then no problem - I don't think you'll break anything - try a 100R resistor in series with the led just in case \$\endgroup\$
    – Andy aka
    Commented Mar 30, 2013 at 21:59
  • \$\begingroup\$ So smaller means more saturate? Whenever I look online people are really vague and just say "ferrite toroid", and the best I've found is "high permeability ferrite toroid". Well, I'd love to be able to just try it, but I can't afford to waste money on this stuff. I know it's like 6 bucks, and if I'm lucky, it'll work, but if I'm not (and I'm usually not), I'll have to buy another, and in the end it might turn out that I waste $15. I really don't mean to be so picky but it all adds up and my current situation doesn't really allow for me to spend money like that. \$\endgroup\$
    – Seven
    Commented Mar 30, 2013 at 22:08
  • \$\begingroup\$ @Seven You might try to a get toroid detail from the guys doing the video. Smaller does mean it saturates at a lower current for the same number of turns - this means it doesn't act like a 1:1 transformer any more and wiki seem to be saying it nneds to go non-linear in this department. Try looking up Fair-rite - I've always found them to be very helpful and I would half-guess they may know what toroid to suggest. \$\endgroup\$
    – Andy aka
    Commented Mar 30, 2013 at 22:41
  • \$\begingroup\$ I contacted the author of the guide and was told to take one out of an old DVD player; will get one from a friend in a week or so, and I'm hoping there will actually be one in there and that it'll work. Thanks for all the help. \$\endgroup\$
    – Seven
    Commented Apr 1, 2013 at 22:28

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.