Stabilizer LP2951 is manufactured by National Semiconductors (datasheet). It provides the implementation of the built-in current limiting function and allows generating a stable charge voltage level of a lithium-ion battery at the circuit output.
The magnitude of the charge voltage is 4.08 – 4.26 volts and is set by resistor R3 when the battery is disconnected. Voltage is held very accurately.
The charge current is 150-300mA, this value is limited by the internal circuits of the LP2951 chip (depends on the manufacturer).
Diode to apply with a small reverse current. For example, it can be any of the 1N400X series that can be purchased. The diode is used as a blocking one to prevent the reverse current from the battery to the LP2951 chip when the input voltage is turned off.
This charge produces a fairly low charging current, so that any 18650 battery can charge all night.