I am also stuck on Level 31, controlled memory cell (D flip flop). I believe it has all of the functionality but it's still saying i'm wrong. I have the single bit memory cells all hooked to the reset with diodes to stop them interfering with each other. I also have a single bit memory that is connected to the reset, control, and memory used output. I also have and gates that go from the control to each input, then to the respective bit memory cell(there is a diode preventing the control memory from interfering with the and gates). So the functionality is no matter what input i ad to the system nothing will change until I activate the control, then the input will be recorded and the memory used will be activated. Nothing will change until the reset is activated.