Power White LED Light Properly

The cheap LED light simply drives a parallel string of white LEDs with two or three batteries.  There is no other circuit elements, not even a current limiting resistor.  The most annoying problem is that the light gets dim fast as the battery voltage drops when there is still plenty of battery capacity left.  The proper way of doing it is a constant current boost converter,  LT1932 is such as an IC that comes in a small package, works down to 1V and is simple to hook up.  Here is the circuit diagram,

Here is the actual circuit board,

LT1932 can supply up to 40mA.  For higher current, we can take a boost regulator like LTC3429 (600mA 500KHz synchronous boost) and close the loop with current sensing.  The following circuit sources 120mA and works down to 1.8V.  It has about 90% efficiency.

The actual circuit board,

Another incarnation with higher current (250mA) and in the size of a flashlight bulb,