  A new simple circuit producing random pulse trains is proposed and
experimentally studied. The circuit is composed of an operational amplifier and
two feedback links, one of which comprises two photodiodes. The photodiodes are
responsible for nonlinearity in the feedback. By varying the illumination it is
possible to control the nonlinearity in the photodiode current-voltage
characteristics and change the degree of randomness in the oscillations. The
circuit's simplicity and optical control make it attractive for coupled map
lattices.
