  In this paper we summarize the existing principles for building
unconventional computing devices that involve delayed signals for encoding
solutions to NP-complete problems. We are interested in the following aspects:
the properties of the signal, the operations performed within the devices, some
components required for the physical implementation, precision required for
correctly reading the solution and the decrease in the signal's strength. Six
problems have been solved so far by using the above enumerated principles:
Hamiltonian path, travelling salesman, bounded and unbounded subset sum,
Diophantine equations and exact cover. For the hardware implementation several
types of signals can be used: light, electric power, sound, electro-magnetic
etc.
