Using this circuit diagram can be made a very simple charger for NiCd batteries, using a timer 555. Internal comparison window is fixed at 4.7 V by Zener diode D1. If potential of pin 6 exceeds this level, the output voltage at pin 3 is "0". If potential pin 2 drops below half the reference voltage (2.35 V), the output voltage becomes "1".
When voltage of the battery is low, the output voltage of IC1 is high, the battery will be charged by R6 and D2 until it reaches "full charge voltage", fixed with P2. When the battery is charged timer changes state and stops charging process.
When, due to auto discharging, battery voltage falls below value established by P2, timer result in resumption of charging. D3 bicolor LED glows red while charging and green during the rest period.
With values given in the scheme, the charger is suitable for 6, 9 V volt batteries.