Here is a totally different take on the Joule Thief (JT) circuit commonly found in garden lights. Instead of charging a 1.2v battery directly from the solar cell and converting the power to run a 3-volt LED, we'll be using the JT to convert the output from the solar cell and charging a Lithium battery first. Then when night falls, the battery is used to drive the LED directly.
This method has some advantages: (1) the Lithium cell that was chosen here (and avialable for $2 here) has an output of 3-volts, which can drive a White LED directly; it also has a huge capacity (800mAH) and very low leakage. (2) The solar cell normally can only charge the NiCd battery in full, direct sunlight, but, with the JT circuit, it is able to deliver power to the Lithium cell even on overcast days.
Step 1: The 'Reversed' Layout.
A look at the circuit will tell you this is not a run of the mill JT configuration. Most obvious will be the fact that there is only a single coil involved (the 220uH) - we are using a second transistor (Q2) and C1 to take over the timing requirements. This allows us to use a wider range of coil values, as well as operate over a larger voltage range.
Besides reversing the charge / discharge order, this circuit also reverses the location of the driver transistor and the coil, but wait, that's not all! The transistors all have reversed polarities, and even the output voltage is reversed!
No, it isn't an error! Diode D1, the LEDs and the charged battery all have their polarities reversed! That's because this Joule Thief is configured as a voltage inverter. This arrangement was chosen due to its advantages for this kind of application.
To improve efficiency, the traditional JT relies on a fairly constant battery supply (over a millisecond or so) to give it a boost when it is delivering power. With the limited output from a Solar Cell, we have to store all its power in C2 and feed it into the Lithium in one big pulse, meaning the capacitor will be "empty" for the few critical millisecond, cancelling the 'kick' the normal JT requires to work well.
Our 'Reversed' JT circuit will work as a regular JT - without the 3v Lithium load, an input of 1.2v will light up the LEDs quite nicely. Not strictly necessary, the LEDs are there so you can SEE the system working, and also to prevent the battery overcharging.
Step 2: The Light
Because the complex stuff is done already, the light itself is quite simple. A BC327 transistor is used as the switch to turn on the LED. The 'inverter' circuit allows us to combine a 'dark' detector, as well as a over-charge limiter in one. Output from the Solar Cell and the charge on the Lithium is monitored through R3 and R4. If the Solar Cell sees no light, or if the Lithium goes over 3.6-volt (the safe charge limit for this cell), LED3, a 100mA unit turns on.
Switch S2 is optional and allows you to run the LED at full power, otherwise R2 will only allow 20mA to the LED.
The second picture shows the two halves of the circuit together.
Step 3: Parts List
Solar Cell. 2-volts with 100-ohm load
Q1,Q3 BC327 PNP. Can be any low-signal amp of sufficient current rating (>100mA)
Q2 BC337 NPN. Most will work but if you change Q1, Q2 or L1, you may need to adjust R1 for best performance (Try 3.3k to 15k)
D1 1N4148 or 1N914 or similar
LED1 Blue or White LED
LED2 Red LED
LED3 100mA (1/2W) White LED
C1 220pF. Can be 150-500pF
R2 330-ohm (use 470-ohm for longer run time)
R4 6.8k-ohm (use these values instead of the one on the schematic)
R5 100-ohm. Go as high as 220-ohm for lower brightness.
L1 100-500uH. Many home-made ones will work.
The second image shows the waveform measured at the top of the coil. The portion above the tag (2) is the charge stored in C2 fed into the coil. The sharp negative going pulseis the battery being charged.
Step 4: Testing the Solar Cell
If you, like me, have collected Solar Cells from busted Garden Lights, it is important to choose one which has good performance.
A simple test is to take them out on a sunny day and measure their output with a Voltmeter that has 100-ohm resitor across the + and - leads. This puts a load on the Solar Cell and will give you some idea of how 'powerful' it is.
In this picture, the meter shows almost 2 volts in a slight shade. Using Ohm's law, we know that 20mA (2-volt / 100-ohms) is also flowing from the cell, and this is the values we will use to design our circuit.
Step 5: Some Calculations
To get an idea of how well our system can perform, we need to figure out how much power we can get, which dictates how much power we can use up.
Using the numbers of 2-volt and 20mA from before, we now know that we can get 40milli-watts by multiplying Volt with Amp. Over the course of an 8-hour sunny day, we should be able to get 320mWH of power from our system.
With a 100mA LED running full steam, it will draw (3.3-volt x 100mA), or 330 milli-Watts! Divide this number into the 320mWH we get from sun-power, it means we'll use it up in less than an hour! Of course the battery will continue to supply current from its reserve, but that is power that would require extra days to replenish.
Step 6: Variations
This circuit performs the same function, but switches the input's polarity so that the output is more 'normal' looking -- positive at the top and negative as ground. It also shows the use of 2N390x transistors and 3.7v Lithium-ion cells, which can also be 3 NiMH batteries in series.
The values of R4 and R5 are important: not only is Q3 tasked with switching on the light after dark, it is also responsible for switching the LED off when the battery drops below 2.9v, as well as to prevent over-charging. In fact, it will delay switching on the light if the battery has not received a full charge during the day.