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 .