I made this JT with a transformer I pulled out of an old switch mode power supply from a PC (see the first photo). The upper coil had a single turn as it came from the PS, so I removed that and wound 15 turns of 26 AWG enameled wire on the bobbin. Its inductance was somewhat more than 100 uH, and the turns ratio of it and the other (bottom) winding was about 1 to 5. The upper winding became the primary (connected to the collector). The transistor was a BC337-40, and the resistor was a 220 ohm 1/4 watt. The LED was a yellow superbright.
The LED was connected anode to the collector and cathode (flat spot) connected to the base. This takes advantage of the greatest difference of potential when the transistor shuts off.
The second photo shows the LED glowing with 100 millivolts or 1/10 volt from the supply. The LED is still glowing, though weakly. The circuit starts at 0.408 volt, and will still keep running below 100 mV, but the LED will be very dim. The frequency was 3 kHz.
The supply current at 1/4 volt was 46 mA, and at 1/2 volt was 96 mA. I think this circuit would make a good converter for a single solar cell that puts out 1/2 volt. But the 220 ohm resistor will have to be adjusted to accommodate the current output of the particular cell that is being used. The idea is to have the circuit put the optimum load on the cell when it’s putting out at its best.
Back to experimenting…