Turn a circuit on and off when a capacitor reaches a certain voltage

Question

I have a circuit that takes an input voltage of 0.5-1.5 volts and uses it to charge a supercapacitor up to around 4 volts. However, I only need the capacitor to be charged up to 3 volts. What is the simplest way with commonly available discrete components (including resistors, capacitors, NPN and PNP transistors, power transistors/power MOSFETs, 2N6027 PUTs, and various diodes) to turn off power to the charging circuit when the capacitor's voltage goes over 3 volts and turn it on when the voltage drops below around 2.8 volts?

^{simulate this circuit – Schematic created using CircuitLab}

Answer 1

I would perhaps use a relay, with discrete components you won't necessarily get good accuracy. If you don't want a huge relay hanging around, I would use a comparator with voltage reference. I'd say using discretes in this case won't really cut it, unless you aren't looking for accuracy. Another option is to build the comparators out of transistors, but that sounds pretty painful.