A simple device for the production of varying potentials which represent four functions of the formf(t) =at, f(t)+at+bsin&ohgr;t, f(t) =at2+b, andf(t) =afor (50 msec≤t≤100 msec) and a rate of operation of 5 cps is described. It makes use of a rotating disk of variable radius to control the light incident upon the cathode of a photomultiplier tube. The accuracy of the functions is about 4%, with the exception that near points of discontinuity, maximum errors of 4 to 10% were observed. The method of preparing the disk, use of the functions in the solution of linear, ordinary differential equations, and an analysis of the error are presented.