The simplest flip-flop can be built with two NAND gates as shown here. This is a Reset-Set [S-R] type or R-S flip-flop [also called Set-Reset [S-R] in some publications]. If the inputs are kept unconnected when the circuit is not triggered, the inputs must be connected to +V using pull-up resistors. Typical values for these resistors are 100 k ohm. This circuit is set and reset with negative pulses applied to the input. The same configuration can be built using two Schmitt NAND gates, such as the ones found in a 4093. Notice that set and reset are done with negative pulses applied to the inputs.
Flig-Flop Using NAND Gates
