Dirt cheap NiCad charger
Your circuit is a step in the right direction.

V per cell of 1.4V is OK - note that this will change somewhat with temperature and also with manufacturer and model. Slightly down is better but will not give 100% capacity BUT prolongs cycle life.

Diodes or zeners do not have  sharp enough cutoff "knee" - They will allow charge when they ought not and will discharge the battery when they should be off. The cheap &amp; widely available TL431 shunt regulator and one transistor provides a very sharo edge and can have hysteresis added.
 TL431 - 31 cents each in 10's, 8 cents each in 100's in TO92 at Digikey. 

Basic circuit - add a resistor between T:431 and transistor base - works as shown but "naughty" Can use a P Channel MOSFET. . 



If desired these can be provided across as few as each PAIR of cells in a series string so that cell balance is maintained. Each cell-pair will stop charging at the "correct" point and excess current will be sent 'down the chain' to either other batteries or ground. A PAIR must be used as even the low voltage TLV431 at 1.25V Vref cannot provide a low enough clamp voltage for a single cell :-(. TheTL431 or TLV431 cathode drops to ABOUT a diode drop below Vref at turn on, but my personal experience shows that the circuit is marginal and device dependant at 1.4V. There are lower voltage devices but cost is much higher. eg The ZXRE060 from Zetex is superb but 80 cents in 10's and 41 cents in 100's.