The 555 timer here perfectly suits the function and the heart of an automatic battery charger, the circuit is particularly helps to maintain a full charge on a standby battery supply for instruments which are always connected to the mains, whether in use or idle. It can also be used for charging pocket calculators, cell phones etc.
The circuit incorporates the timer's two ON chip comparators, the flip-flop and driver amplifier.
A zener diode provides a reference voltage somewhere close to the battery voltage with a certain space for the adjustments.
The twin potential divider networks provide the comparators with
When switched ON, the output provides a maximum of
This nicad battery charger circuit is set up by substituting a variable voltage supply for the connected batteries.
The HIGHER limit setting is done first so that the output switches OFF at the maximum battery voltage.
Next, the LOWER limit is set for minimum battery voltage level. It is recommended to leave the output disconnected from the resistor until after the setting up procedure has been completed.
COMPLETE CIRCUIT DESCRIPTION AND DIAGRAM IS GIVEN HERE.