Background:
I should explain what I’ve been considering. My door has southern exposure so I would like to put a solar cell on it along with a long-running joule thief. The LED will be pointed at the keyhole in the lock. During the day the solar cell will charge a supercapacitor and run the Joule Thief after dark. When it’s dark, the keyhole will be easily seen and opened. I have another option and that is to use a battery. This could be a AA cell or AAA cell. I have been experimenting with low power Joule Thiefs and their battery life can be a month or more. If I use a rechargeable cell, the solar cell could recharge it during the daytime.
I found a Joule Thief on Youtube from xee2vids that he calls Self Running. This JT has two things that are different than the conventional JT: the cathode of the LED is connected to the positive, not the negative. Also the resistor is much higher; he uses 51k. Also a small value capacitor is across the resistor. In the version I built I used 150 k resistor and a 470 Picofarad capacitor.
The reason for connecting the LED cathode to the positive is so that if the voltage on the input rises above the LED’s forward voltage, nothing happens. If it was connected to the negative then the higher supply voltage would light the LED without the Joule Thief. In this case Xee2vids and I are using 6 volts for the supply voltage and the LED must be connected to the positive to prevent it from lighting.
The reason for using such a high supply voltage is because xee2vids’ circuit uses a 10000 microfarad capacitor for the supply. In order to make the Joule Thief run longer he charges it up to 6 volts. In my case, I used a bank of 1 Farad super capacitors charged up to 6 volts. I connected nine 1 farad supercapacitors in series parallel to give 1 farad at 7.5 volts. The JT has a Fair-Rite 2673002402 coil core with turns of wire for each winding. The transistor is a PN2222A.
I charged up my JT to 6 volts and timed the lit LED. At the start it was moderately bright. After 30 minutes the voltage had dropped to 2.4 volts and the LED was still moderately bright, just a little bit dimmer. After 45 minutes the voltage had dropped to 1.66 volts and the LED had dimmed noticeably and the light was not good enough for illumination.
My conclusion is that this circuit can run for about a half an hour with a one farad capacitor initially charged to 6 volts. If I increased the capacitance to 10 Farads, the led should stay lit for about 5 hours. A capacitance of 15 Farads should give about 7.5 hours, which is enough to keep the circuit lit from sunset until midnight on a winter day.
A typical Solar garden light runs about this long using cheap nickel-cadmium rechargeable AAA cell. It’s a lot cheaper then 15 farad capacitor but the capacitor doesn’t lose its capacity after a year or two. Obviously this price difference is the reason why solar garden lights don’t use capacitors, they use cheap rechargeable batteries.
To get 15 farads at 7 volts, I would need to put 3 capacitors in series, each capacitor would be 45 farads. The closest value would be 50 farads. This capacitor would be about the size of a single AA cell, and three of them of course would be much larger.
I have to design a small enclosure that can be fastened to the door and hold the LED properly pointed in the right direction. Most of the room inside the enclosure will be taken up by the super capacitors.