The charger described in this article can charge packs of 4 to 7 cells with capacities ranging from 600mAh to 2Ah. The charger automatically begins charging when a pack is connected. It charges at a (nearly) constant current (adjustable), and terminates the charge when the pack begins to get warm (a NiCd pack begins to warm up when it has reached full charge). An LED indicates that charging is in progress.
The circuit for the charger (Figure 1) is simple. Z1A is a comparator which compares the voltage on its two inputs and produces a high output when the "+" input (pin 5) is higher than the "-" input (pin 6), and a low output otherwise.