Electronic – How to use YX8122 led driver

It is a long question, but better than a short one, as you've shown your own research.

1) Solar cells. If you're stacking your own ones, stack 9 of them and get the 4.5V of the original circuit.

2) Battery charging. Batteries are the only thing you've left out of your spec. This is an area where the circuit design relies on cutting a lot of corners. In theory it might be out of spec, if you were to put 4.5V at 280ma through AA NiMH cells indefinitely. In practice, you don't get full sun all day, you'll be using it indoors, and you're not going to get optimal power transfer from the cells, so this isn't going to cause problems.

3) Diode. It's just a regular diode, not a zener. Current through it is actually determined by the battery and right hand side circuit, not the solar panel - the transistor is off when the panel is generating electricity. The original 1N914 will be fine. 1N4004 will also be fine.

4) Resistors: not a precision component here, use whatever meets your cost constraint. 5.1k for 5k is fine.

5) Wire: not critical. Your ebay link looks suitable. Thinner is better for the toroid.

6) Transistors: stick with the exact part numbers. Design may rely on specific parameters.

7) LED: again, this circuit relies on cheating. Normally a white LED won't run from two NiMH cells. The joule thief part provides a boost converter that gives small pulses of higher voltage. It doesn't have the capacity to provide a lot of current at that voltage. In combination with the pulsing this means there should be no risk of damaging it.

(A proper analysis of this circuit would be good, if nobody else supplies one I'll do it in a few days).

1. Yes, you should care. The BCR421 will try to maintain a (roughly) constant current at the collector terminal, however if Vout drops too low then it will not be able to maintain the constant current, as you can see from the graphs you posted. What will happen is that the output will saturate at a lower current than you want. To find out if this will happen, assume that the maximum desired current is flowing through the LED. Subtract the maximum possible LED voltage (which will occur at the maximum current) from the minimum possible supply voltage and compare with the dropout voltage of the BCR421 at that current.

   So the LED drops 1.35V typical at 100mA (1.6V maximum). Your supply voltage is 3.3V nominal, let's dock 5% for tolerance to give 3.13V. That gives the chip 1.54V to work with. The parameter VSmin indicates that typically 1.4V is enough at 25°C, and the graphs you posted indicate that above about 1.1V it's fairly constant current. So it's likely okay unless you have some extreme operating conditions.

2. The current through the external resistor when it is regulating correctly will be approximately Iout - 10mA. The internal resistor conducts the 10mA. It will be slightly higher because of base current (maybe 0.5%), but that's immaterial in this situation, given the +/-10% tolerance on the 10mA current. As @jippie points out in a comment, 100mA through a pot wiper is probably not a good idea, depending on the pot construction (okay for a wire-wound, but they are expensive). Minimum resistance may also be a problem. It's generally recommended not to put much current through pot wipers- they work better as voltage dividers than rheostats.

3. This circuit maintains a fairly constant current regardless of changes in the LED forward voltage (or differences from unit-to-unit when first assembled) and regardless of changes in supply voltage.