A universal-voltage discrete input circuit uses a high voltage depletion-mode field effect transistor in combination with a low-voltage, adjustable precision shunt regulator and an isolation circuit for interfacing a low voltage digital logic circuit to a switched external voltage ranging from about 7 volts to about 1000 volts AC or +/−DC, at a low fixed current. In addition to the wide input voltage range accepted at a uniform low current value, very high voltage isolation is provided between the external voltage and the low voltage digital logic circuit, and elimination of ground loops and common mode noise.