Solar Powered Joule Thief

After building the Wearable Joule Thief project I became quite intruiged by the circuit and wanted to play around with it a bit more. My particular desire was to make it solar powered with the objective being that it should be able to run through the night even during dark winter months. A Joule Thief will easily run for a week on a fully charged battery so I figured that it should be able to run for one night even if not fully charged. To achieve a good runtime the circuit must consume the least amount of current possible and have the highest rated charging components possible (solar cell and battery) within the size restraints of the enclosure used. As battery life was a major factor in the building of this project I knew that I needed to spend some time getting the transformer right. Although the type of core, size of wire and the number of turns used are not too critical for the circuit to work, they do affect the current consumption. The classic way of making a Joule Thief transformer is to use bifiliar windings where two wires are wound around a toroidal core together and then the end of one is connected to the beginning of the other, effectively forming the centre tap that goes to battery positive. I found though that simply winding the coil linearly around the core with a tapping point along the way is much easier and works just as well, with the shorter coil going to the base, the longer coil going to the collector and the tapping point going to battery positive


Although the size of core isn't critical, smaller ones will fit more easily onto the veroboard which incidentally is my favourite size of 9 x 25 holes. Suitable cores can often be salvaged from scrap power supplies and for some reason I find that Green ones always seem to work! When winding the coil there is a 'sweet spot' where you get minimum current drain, so if using a random size core it's worth experimenting to find the best number of turns. Current consumption can be further reduced by changing the value of the base resistor. The original Joule Thief circuit used a value of 1K but here it has been increased to 10K which greatly reduces the current drawn (to approx 14mA at 1.4V) albeit at the expense of slightly reduced light output. I think it's a good trade off though. AA batteries are traditionally used in this type of application but I decided to use a single AAA instead as they are now available with fairly large capacities and being smaller will take up less space on the veroboard allowing room for a mounting hole to be drilled at each end if desired. A PCB mounted battery holder was obtained with long wires that could be folded horizontally under the board and soldered to the appropriate points. Although not shown on the diagram, remember to cut the tracks underneath the holder or else the battery will short out! An extra little addition to the board was added in the form of a Molex KK plug used in reverse to make a PCB socket that the LED can be directly plugged into. This is handy if you want to quickly try different types and colours. Incidentally, if you want to make the light from a clear LED less directional, it can be diffused by 'roughing up' its surface with sandpaper (first photo shows KK socket and 'frosted' LED)

Veroboard layout

I won't go into the operation of the Joule Thief circuit here as there is a ton of information on the internet already, so I'll just explain the three extra components required (4 including the panel) to make it solar powered. Originally I considered using a constant current charging circuit but then reminded myself that the beauty of the Joule Thief is its simplicity and so kept it basic. Charging is carried out by a standard 1N4001 silicon diode placed between the solar panel + and battery +. This allows charging current to flow from the panel into the battery during the day but will stop the battery discharging back into the panel at night. A silicon diode will drop 0.6 volts across it and so a single NiMH battery can be charged using a 2 volt solar panel (2V - 0.6V = 1.4V). I've found that Schottky diodes are unsuitable in this application as they allow some current to pass in the reverse direction. Voltage from the solar panel is monitored by a 4K7 resistor and when there is enough light to produce 0.6V the first transistor will turn on effectively grounding the base of the Joule Thief transistor which stops it oscillating. The charging diode also prevents the first transistor being turned on by the battery. The solar panel should ideally match the type of battery, so for example if you use a 750mAh AAA then the panel chosen should be able to supply approx 75mA to fully charge it in the usual 'rule of thumb' 16 hours but... as we never get 16 hours of sunshine here, a panel with a slightly higher current can be used. I chose a 2V 130mA panel coupled with a 1100mAh AAA battery which is fine because in practice the panel never achieves its rated current anyway

Details of the transformer

Toroidal Core - Green toroids always seem to work (salvaged so type unknown)
Number of turns - 24 turns in total tapped at 9 turns (9t+15t) for a toroid of approx 11mm OD x 5mm ID x 6mm HT

Number of turns - 25 turns in total tapped at 9 turns (9t+16t) for a toroid of approx 9.5mm OD x 3.5mm ID x 3mm HT
Type of Wire - 0.45mm approx OD enamelled copper wire (measured with vernier calipers)
Note: 0.45mm includes the enamel. If purchasing from a supplier, choose a size of approx 0.4mm (27SWG or 26AWG)


The Joule Thief was assembled inside a Kilner Jar with the solar panel sealed onto the lid with epoxy potting compound
(yes a hole does need to be drilled through the glass!)

Internal view showing the veroboard mounted on a 70mm x 3mm laser cut acrylic disc (eBay) attached to the lid with epoxy