automatic NiCad charger

This little circuit will produce a DC output that is almost twice the supply voltage. A square-wave input is required of sufficient level to turn TI fully on and off. When TI is conducting, C2 is charged to just under the supply voltage. When TI is cut off, T2 starts to conduct and raises the voltage at the negative end of C2 to just under the positive supply level. This implies that the voltage at the positive end of C2 is raised to almost twice the supply voltage, so that C3 will ultimately charge to this level.