1 Constant Current Charger using LM317 regulator

This project is very similar to universal charger that uses constant current charging method.  But this one is much simpler to construct since you can build it using only 2 parts, the LM317 regulator and resistor. The use of diode D is for short circuit protection. Capacitors C1 and C2 is for good voltage regulation. Resistor R2 acts a dummy load when battery is disconnected. The idea of this charger is the output current is equal to 1.2V divided by the value of R1.
In equation:  Current out = 1.2/R1

Charger schematic

LM317 pins

Part List:
LM317
R1 - see the values in table below
R2 - 2.2 kilo-ohms 1/4W
C1,C2 - 47uF/25V, or any value will do, the higher the better
D - 1N4001 or any similar diode at-least 1A rated

R1 vs  Output current table

You can modify the circuit above and make it simpler by omitting some parts. See diagram below.

Simplified charger schematic

Note: This is not an automatic charger so monitor the battery always to avoid damage. You can charge any battery with voltages from 1.2V to 12V but make sure that the DC input is at-least 3V higher than the battery.

Other useful chargers:
Mobile phone charger - very simple charger intended for cellphones
3V charger - charger powered by 3V battery
1.5V charger - charger powered by 1.5V, for emergency purposes
Universal Charger - charge any battery with different voltages with different charging currents.