Can you imagine illuminating four numbers of 1 watt LEDs through a a few 1.5V cells? Looks quite impossible. But it can be done simply using a coil of ordinary speaker wire, a transistor, a resistor and of-course a 1.5V pencil cell.
The idea was suggested to me by one the keen followers of this blog Ms. MayaB, here are the details, let's learn them:
FYI, I tried this simple JT using a 40ft. paired speaker wire (24AWG) purchased at dollar store (of course, for $1).
No torroid, no ferrite rod, just simple air core wound to make it more like a coil (about 3" diameter) and tied the wire with a twistie tie (so that the wire will stay as a coil).
I used 2N2222 transistor, 510 ohm resistor (found out that is the best with a help of potentiometer) and was able to BRIGHTLY lit four (that is all I had) 1-watt high power LED in series (which requires same amount of current as if it was used for only one LED) using two 1.5V AA batteries (that is 3V power supply).
Can be used only one 1.5AA but will be dim (of course). I have also added a diode 1N4148 at the transistor's collecter pin just before the LED but can't tell if it increased any brightness.
Many people have used a capacitor in parallel to the battery claiming it will light the LEDs longer, I have not tested that part yet.
I have read adding a 220uF/50V electrolytic capacitor parallel to the battery would make the lights run longer, adding a 470pF/50V ceramic disc capacitor parallel to resistor will recouple the waste current in the resistor, and adding a 1N4148 diode (it is a switching diode but I don't know how would that effect the brightness) at collector of the transistor before the LEDs in series makes the LEDs brighter.
Using AAA 1.5V Cells
I don't have an oscilloscope to check all that effects. However, I would like to use rechargeable batteries instead of regular AAA 1.5V battery and make it self-regulated (or at least semi-self regulated) circuit by adding a calculator solar cell and a mini Joule Thief on a small toroid to keep charging the battery to last much-much longer.
I indeed need to add a LDR to light the LEDs only at dark and recharge the batteries during the daytime. Your suggestions and ideas are always welcome. Thanks, once again, for your interest.
Feedback from MayaB
Though it is long known Joule Thief circuit, not something new I discovered but thank you for posting a new article on behalf of me, I appreciated it.
How to Improve Brightness of the LEDs
Ps. Over the weekend I hybridized your circuit with the circuit I sent you here and it turned out to be dazzling bright (warning: may blind your eyesight, hehe).
I used the same speaker wire (mentioned above), a 8050SL transistor, 2.2K resistor (paralled with a 470pf capacitor), one 1W high power LED, a 100uH choke (connected from collector of the transistor to the positive rail of power supply), and 1 diode (1N5822 connected at base of the transitor to the positive rail of the power supply).
I used two 1.5V (total of 3V) AA batteries for power supply. And btw, a LDR between 2.2K resistor and the negative rail can be added to turn the LED off during the daylight. Unfortunately, could not light more than one 1W LED with 8050SL transistor in this configuration.