The most common type of ratchet is an on/off ratchet which generates a sawtooth potential in its “on”-state and a flat potential in its “off”-state. Here, the idea is that particles are localized in the potential wells when the potential is on. When the potential is off, the particles will diffuse and spread over a wider area. Consequently, when the potential is turned on again, re-forming the sawtooth potential, the particles that had diffused over an edge will drift to the bottom of the re-formed well. Because of the asymmetric shape of these wells, a net motion is achieved by periodically turning on and off the potentials. 
Rectification of electromagnetic radiation
An example of a non-directional perturbation is an alternating electric field; rectification of this might find use in e.g. novel RF antennas. When we subject charges (electrons or holes) that sit in a periodic asymmetric channel, i.e. a ratchet, the asymmetry will result in a net DC current that can power electric circuitry. The effect can be investigated for materials with different mobility and the charge carrier densities can be controlled via doping and/or a gate field in a so-called field effect transistor.
 Appl. Phys. Lett. 102, 073104 (2013) doi: 10.1063/1.4793198