Schematic Volt

The 555 timer can conveniently function as the heart of an automatic Ni-Cd battery charger, the circuit is intended to maintain a full charge on a standby battery supply for an instrument that is always connected to the mains, whether in use or not. lt can also be used for the charger unit for pocket calculators, etc.
The circuit uses the timer’s two on-chip comparators, the flip flop and driver amplifier. A zener provides a reference voltage somewhere near the battery voltage with an allowance for adjustment.

The two potential divider networks supply the comparators with adjustable voltages, one for LOW (switch on) and the other for HlGH (switch off). When on, the output gives a maximum of 10V and when off gives GV, the maximum current is 150mA which is limited by the ·47 ohms and protected by the diode.  The circuit is calibrated by substituting a variable voltage supply for the Ni-Cd batteries., The HIGH adjustment is set first so that the output switches off at the maximum battery  voltage and then the LOW is set for minimum battery voltage. It is simplest to leave the output disconnected from the resistor until after the setting up procedure.