Fig 2 shows an implementation of the ‘joule thief’ circuit using a transformer wound with 0.5mm diameter enamelled copper wire on an inexpensive ferrite core available from RS Components (stock no: 467-4239). (The coil ends have been terminated with 0R resistors to make them easier to plug into the breadboard). My year 10 students, who have just made a few of these, have dubbed it the ‘Flat’ battery torch.
When connected to a single solar cell in bright light, the joule thief works perfectly!
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