Nicad charger uses a transistor as a constant current source. The voltage across 2 diodes 1N4148 diodes biases the base of the BD140 medium power transistor.
The base-emitter voltage of the transistor and the forward voltage drop across the diodes are relatively stable. The charging current is approximately 15mA or 45mA with the switch closed. This suits most 1.5V and 9V rechargeable nicad batteries.