A DC power conversion circuit with constant current output includes a DC voltage source, a driving circuit and a control signal generator. The side walls are joined at a corner. Detection of a change of said logic state at said other of said input 1a, 2a and output 1b, 2b terminals is then indicative of the presence of a plug A, B therein. A photo detector measures the transmitted light through the sample. A combination of high clearance and elastomeric vane and asymmetry of the impeller serve to prevent or reduce clogging caused by filamentary materials.