Digital input circuit

A digital input circuit comprising:

• a pair of input terminals through which an input signal of a digital voltage signal is inputted;
• a power supply;
• a photocoupler serving as a signal transmitting element for transmitting the input signal from the input terminals to an input port of a microcomputer and having one light emitting diode and one phototransistor, wherein the light emitting diode of the photocoupler is connected between the input terminals, a collector of the phototransistor is connected to a positive side of the power supply, and an emitter of the phototransistor is connected to ground via a first resistor;
• a second resistor for pulling up the input port of the microcomputer;
• and
• a bipolar transistor provided between the ground and the second resistor, wherein a collector of the bipolar transistor is connected to the second resistor, an emitter of the bipolar transistor is connected to the ground, and a base of the bipolar transistor is connected to a connection node between the emitter of the phototransistor and the first resistor.