normally it takes 3 lemons to get an LED slightly glowing...
with this design you can make a single-lemon-powered flashlight that will run for weeks!!!
it was developed during the exhibition "Cooking and Constructing" at Platform21
joule thief design
Step 1: Prepare the components
- soldering iron
- steel scissors
- sanding paper
- AA battery (for testing)
- 1 kOhm resistor
- NPN transistor
- ferrite bead
- copper wire
- sheets zinc & copper
- white / blue LED
Step 2: Wind the transformer
1. take about 1.5-2 meters of copper wire, bend it in half and wind the coil with it. in this case it took around 50-60 windings arranged in two layers.
2. after the winding is done - cut the double end, so that you have 4 loose single tips.
3. clean the tips from the isolation lacquer with sanding paper
4. with a multimeter set on conductivity test, find two separate windings
5. twist two opposing ends of different windings into one contact
6. pre-solder the tips of the transformer.
your transformer is ready!
Step 3: Prepare the transistor
the middle one - goes straight backwards
the side ones - go forward and a bit more to the sides
afterwards - pre-solder all the legs
Step 4: Solder & test the joule thief
solder the resistor to the middle leg
the short leg of the LED (the minus) goes to the rightmost leg of the transistor, and the long (the plus) leg goes to the LEFTmost leg of the transistor
the transformer gets one of it's single-wire tips soldered to the loose end of the resistor, and the other single-wire tip goes to the long leg of the LED
to test the joule thief - connect the twisted end of the transformer to the plus of an AA battery, and the short leg of the LED - to the minus of the battery
if the LED doesn't light up - check your circuit
Step 5: Make copper electrodes
2. cut pieces that would fit nicely in your lemon
3. pre-solder spots on the electrodes
4. cut a length of copper wire about twice the length of you electrodes combined
5. remove the lacquer from the wire in the same way you did it for the transformer
6. solder the wire to the electrodes
7. check everything is soldered properly with the conductivity test
Step 6: Make zinc electrodes
1. process zinc plates with sanding paper
2. cut pieces approximately the same size as copper ones
3. make small in the top of every piece and bend them
5. prepare the wire in the same way as with copper electrodes
6. clamp the wire with zinc pieces using pliers
7. make the conductivity test
Step 7: Stuff the lemon
1. to reduce your lemon's internal resistance (thus boosting it's current output) you have to smash the lemon several times and roll it on the table until it's all soft an juicy inside
2. mark 8 cuts with as much space between them as the lemon size allows
3. make the cuts as vertical as you can and to the bottom of the lemon
4. insert electrodes in a sequence copper-zinc-copper-zinc... making sure that they don't touch each other
5. measure the voltage between electrodes
copper - "+", zinc - "-"
it should be around 0.8-1.0 Volts
6. measure short-circuit current output
in my case it was around 10 miliAmps - that's a lot for a lemon!