intelligent NiCad charger

Like all things, NICAD chargers are subject to human error, that is, the batteries can be placed in the holder in "two ways, correctly and incor- rectly. This charger will refuse to operate unless the cells are fitted correctly. The charger consists of a current source (T2) to maintain the output current at about 50 mA. The zener diode D2 and LED hold the base drive of T2 at a constant level and thus also the voltage across R3. The current through R3 is therefore also constant, providing the correct conditions for charging NICADs at the collector of T2. The protection circuit includes T1, D1 and R1. The terminal voltage of +15 V an incorrectly fitted NICAD will turn off T1 preventing the charger from operating. An indication of this will be given by the LED — it will not light. When the battery is fitted correctly T1 will turn on and the charger will function normally. Although the charger is capable of charging up to four penlight cells, it will not detect a single cell being t...