Revitalising a Sun Jar and the Joule Thief Revisited
All that is needed, in addition to the QX5252F, are an inductor, a solar cell, a rechargeable battery and one or more LEDs. I used a standard AA rechargeable battery, a 3V solar cell for charging the AA, 3 LEDs as I wanted reasonable brightness from the Sun Jar and a 220μH inductor. A restriction on the choice of solar cell was that it had to fit in the opening of the Mason jar – that is within a circle diameter of a little over 74mm. The data sheet uses a 47μH inductance for its ‘typical values’, I chose a rather higher value to keep the current to the LEDs at what seemed like a sensible value (a higher inductance has the effect of reducing the current). The maths surrounding inductance is relatively complex (which is why it doesn’t turn up in GCSE specs), so I tried a few values, settling on something that kept the LEDs reasonably bright. For the LEDs I wanted a reasonably warm colour and chose ‘sand’ coloured – which despite the name is a slightly pink white, reminiscent (to me at least) of an early dawn sky.
First, I built the circuit with the components on a breadboard, to be sure I had everything right. As you can see from Figure 4, the LEDs are off when the solar cell is facing up in a light room (LH image), and on when the cell is turned over (in the same light room, RH image).