Circuit is very easy. The input, not stabilized Voltage is from greatz bridge and filtered by caps (this voltage is simulated by dc plus sawtooth wave) the opamp output current is flowing to the darlington base. This current is proportional to the output voltage and load. Voltage from output voltage divider is compared with voltage from the potentiometer and opamp is doing all what he can to make, the output voltage (but divided by the divider) the sam as the voltage from the pot. . The 5V source can be 7805.
