The Sun Thief
— David Wagner 2007/08/29 12:18
Switched Solar Charging
Most of the same components used to run the LED can also be used to charge a NiCd or NiMH cell by using one or two solar cells in series as a power source and replacing the LED with the battery cell. To use a single solar cell as shown, select a germanium transistor so the circuit will work and be sure to use a Schottky rectifier or larger germanium diode to minimize output power losses. Though tests have been inconclusive, using a transformer with more turns on the feedback than on the primary coil (perhaps 2:1) may improve performance by sharpening the waveform to the base of the transistor so it can will switch faster and stay out of the linear region.
A regular silicon transistor can be used with two solar cells in series, though the efficiency may leave something to be desired.
//
+-------------------------------------->|--+
| +--------------------------------+ |
| | | |
| | +----------------------+-->S--+--||--+
| | | | | |
| | | = = = = |/ C +--Z<--+
| | +--UUU UUU--------+--| |
| | | | | B|\|E |
| | | | | +-------------+
| | | | | |
| | | | +--|<--------------+
| | | +-------+ \ |
| | | 1.4k | 500 \ | |
| | +--/\/\/--+--/\/\/--+-------||--+
| | | | +0.45V| |
| o o | |
|...\.......\. | |
o \o o \o----------------+ |
| | | |
+-+------------------------------||--+
+1.2V|
//
+-------------------------------------->|--+
| +--------------------------------+ |
| | | |
| | +----------------------+-->S--+--||--+
| | | | | |
| | | = = = = 1k |/ C +--Z<--+
| | +-UUU+UUU-+-/\/\/-+--| |
| | | | B|\|E |
| | | | +-------------+
| | | | |
| | | +--|<--------------+
| o o \ |
|...\.......\. \ | |
o \o o \o-------||-------------------+
| | +0.45V| |
| | | |
+-+------------------------------||--+
+1.2V|
//wht
+-------------------------------------->|--+
| +--------------------------------+ |
| | +15u | |
| | +--||---------------+ |
| | | | |
| | +----------------------+-->S--+ |
| | | | | | |
| | | = = = = | 1k |/ C +--Z<--+
| | +-UUU+UUU-+-/\/\/-+--| |
| | | | | B|\|E |
| | | | | +-------------+
| | | // | | |
| | +->|-+ +--|<--------------+
| | | red |
| o o \ |
|...\.......\. \ | |
o \o o \o---------------||-----------+
| | +0.45V| |
| | | |
+-+------------------------------||--+
+1.2V|
This one has a full charge/near discharge indicator?
Note, the lowest practical input impedence seems to be around 3Ω,
so a single 0.45V photovoltaic cell can provide at most 150 mA to this circuit.
Two solar cells in series can supply at most 300 mA so the two PV cells need not be any larger than this, about 
square inches each.
It may be difficult to automate this circuit without an additional light-sensing element. When the battery cell is providing power (switched left) there is no way to detect when the solar cell begins to produce power since this will make no difference in voltage potential anywhere in the circuit.
Using a DPDT switch allows using different base resistors for charging and discharging, and limits reverse current leakage to less than ½μA.
Automatic Solar Charging
$0.10 2 solar cells (0.5in2 ea., 0.9V, 50 mA)
[$10/150in2 as pieces, or use 2/4 of a $1 cell]
0.17 1.2V button cell (70 mAHr) [$1/6]
0.20 1 Ferrite Core [$5/25, $15/100]
0.30 3 Transistors [$1/10]
0.05 1m Magnet wire [$3/200']
0.10 2 Diodes [$1/20]
0.21 6 Resistors [$3.50/100]
0.07 2 Capacitor [$1/30]
0.05 LED [$10/250, $5/100]
0.17 Magnet [$1/ft]
Clear Potting Material
----
$1.42
D1 // +->S-+--------------------------+ +-->|--+ | | | D2 | | | | +-------------|--------------------+->S-+--||--+ | | | | | | C2 | | \|Q1 R1 | = = = = = | R3 R4 Q2|/ | | | _|--+-/\/\/-+--UUUU+UUUU--+-/\/\/-+-/\/\/-+--| +--Z<--+ | /| | T1| | | |\| Z1 | | | | R2 | Q3|/ | | | | +-|<-+-/\/\/-+ | +-----| +-|<-+ | | | | | | |\| | | | | | |R5 | R6 | | | | +------------+-------/\/\/-+-/\/\/-+ | | | | | | | | | | +----------||--+ | | | | \ | C1 | | | | | \ | | | | | | | +-------------||----+----||--------+------------+-----------+ | |P2 |P1 | | | +---------------------------------------+
Two joule thief circuits share one transformer by swapping which winding acts as the the primary and which is used for feedback.
Solar-powered LED Throwie
The simplest and most efficient (80%) way to charge a single NiCd or NiMH battery cell is with five similar solar cells and a blocking diode in series,1) but it is also possible to use only one cell by cascading two joule thief circuits with nearly the same component count. (Here is a similar question, with a silly answer.)
In any case, the limited input current can be offset somewhat by placing a large (super)capacitor across the solar cell. This will store at least some of the excess current produced when the sun shines and release it as clouds go by.
This circuit also has the interesting property that when the solar cell is producing energy and the battery cell is charged enough to require a voltage above the forward voltage of the LED and charge pump diode combined (about 3.5V), the LED will start to glow from the excess solar power, effectively functioning as a full-charge indicator. This will probably never happen. During a normal constant-current charge, the voltage applied to the cell doesn't get much above 1.5V.
2)
3)
4)
Perhaps placing a red LED across the battery cell will do this. Test this.
In this circuit, a photoresistordiode or phototransistor disables the oscillator driving the LED when the sun shines. 5)
However, the capacitance of a photodiode may mess with the oscillator.
(It should also be possible to use a tiny chip of a photocell. Tested: this does not work. )
(: Try using an IR LED.)
$0.10 2 solar cells (0.5in2 ea., 0.9V, 50 mA)
[$10/150in2 as pieces, or use 2/4 of a $1 cell]
0.17 1.2V button cell (70 mAHr) [$1/6]
0.40 2 Ferrite Cores [$1/5]
0.30 3 Transistors [$1/10] (phototransistor: $0.40)
0.10 2m Magnet wire [$3/200']
0.10 2 Diodes [$1/20]
0.07 2 2K? Resistor [$3.50/100]
0.04 1 15K? Resistor
0.10 3 Capacitor [$1/30]
0.05 LED [$10/250, $5/100]
0.17 Magnet [$1/ft]
Clear Potting Material
----
$1.60
Using NPN silicon transistors.
//
+-->|--+
| |
+--------------------------+->S--+--||--+
| | |
| +--------Z<--+
| = = = = 1.4k B|/ C |
+--UUU+UUU--+--/\/\/--+--| |
| | |\|E |
| | +------------+
| | |
| | \\ _ |
| +---\__/----------+
| |
| +1.2V| |
+--------------------------+--||--+
| | |
| |
| |
+--------------------------+->S--+--||--+
| | |
| +--------Z<--+
| = = = = >2K |/ C |
+--UUU+UUU--+--/\/\/--+--| |
| B|\|E |
| \\ +------------+
| +0.9V| | |
+----------||.||------------------+
| | | |
| |
+------||-------------------------+
??
Using a PNP germanium transistor for the solar oscillator.
//
+-->|--+
| |
+--------------------------+->S--+--||--+
| | |
| +--------Z<--+
| = = = = 1.4k |/ C |
+--UUU+UUU--+--/\/\/--+--| |
| | B|\|E |
| | +------------+
| | |
| | \\ _ |
| +--------\__/-----+
| |
| +1.2V| |
+-------+-----------||-------------+------+
| | |
| |
| +--------------------------+-S<--+--||--+
| | | |
| | +-------->Z--+
| | = = = = 1.4k Ge|/ C |
| +--UUU+UUU--+--/\/\/--+--|_ PNP |
| | B|\ E |
| | \ +------------+
| | +0.75V \ |+ |
| +-----------||--------------------+
| | |
+-----------------------------------------+
+1.2 -0.45 = +0.75V
Or this can be done actively.
//
+-->|--+
| |
+--------------------------+->S--+--||--+
| | |
| +--------Z<--+
| = = = = 1.4k B|/ C |
+--UUU+UUU--+--/\/\/--+--| |
A2 | B1 | |\|E |
| |/ +------------+
| +----| |
| | Ge|\| |
| | +-----------------+
| | | |
+----------------+--||--+ |
| | +1.2V | |
| +--------------------------+--||--+
| | | |
| | |
| +--------------------------+->S--+--||--+
| | | |
| | +--------Z<--+
| | = = = = 1.4k Ge|/ C |
| +--UUU+UUU--+--/\/\/--+--| |
| A2 | B1 B|\|E |
| | +------------+
| | \ \ |
| | +0.45V +----+ |
| +----------/--- / |
| | +----+------------------+
| | | |
+-/\/\/-+------||------+ |
22k? ?? ---
-
Now, the other germanium transistor could be replaced with silicon if I can find a way to hit it with battery cell voltage as soon as the solar cell gets its capacitor charged to start the circuit oscillating. It should continue to run with the 0.45V from the cell, it just won't bootstrap a silicon transistor with the voltage that low.
Try this to replace one germanium diode with silicon.
//
+-->|--+
| |
+--------------------------+->S--+--||--+
| | |
| +--------Z<--+
| = = = = 1.4k B|/ C |
+--UUU+UUU--+--/\/\/--+--| |
A2 | B1 | |\|E |
| |/ +------------+
| +----| |
| | |\| |
| | +-----------------+
| | | |
| +--||--+ |
| | +1.2V | |
+--------------------------+--||--+
| | | |
22k? | | |
+--/\/\/--->|--+ | |
| | |
+--------------------------+->S--+--||--+
| | |
| +--------Z<--+
| = = = = 1.4k Ge|/ C |
+--UUU+UUU--+--/\/\/--+--| |
A2 | B1 B|\|E |
| +------------+
| \ \ |
| +0.45V +----+ |
+----------/--- / |
| +----+------------------+
| | |
+------||------+ |
?? ---
-
Solar-charged Supercapacitor
// +------------------------------------+-->|--+ | | | | +--------------------------+-->S--+--||--+ | | | | | | | = = = = |/ C +--Z<--+ | +--UUU+UUU--+--/\/\/--+--| | | | | B|\|E | | | | \ +------+ red | | | \ | | // | o\ o--+----------||--------+ +-->|--+ \ +0.45V| | 0.1uF| o +--||--+ | | | | | 50F | +---------------------------------+--||--+
At first glance a solar-charged supercapcitor seems an ideal application, especially if using it with a germanium -based discharge circuit to drain nearly all the energy out of the cap. But you must be very careful not to overvoltage the capacitor. Perhaps the easiest way to do this is to put a red or orange LED and a small, fast mica or ceramic regular capacitor across the supercap.
The usable energy into or out of a capacitor is E = ½ C ΔV², where E is in joules (watt-seconds), C is the capacitor's capacitance in Farads, and ΔV is the change in voltage from charging or discharging the capacitor.
If a germanium-based circuit gives a usable glow down to 0.3V, a 50F, 2.3V capacitor will charge and discharge up to E = ½ * 50 * (2.3-0.3)² = 100 W-sec = 27.7 mW-hr. This is enough to run a single 20 mA LED for about twenty minutes6), but it will be difficult to get both decent efficiency and brightness over the full discharge voltage range. (Or perhaps not so difficult. See the lemon vampire circuit.)
- Typical 2.3V carbon-aerogel capacitors: $3/10F, $6/50F.7)
Compare this to one of the tiniest rechargeable cells I can find, a 70 mA-hr NiMH button for about 17¢ (surplus). It can supply 84 mW-hr, enough for over an hour8) of fairly constant illumination.
0.369 hr = 22.16 min
1.12 hr