PURPOSE: To increase or decrease receivers in number without changing circuits on a transmission side by driving the light emitting diode of a reception-side photocoupler by a constant current circuit.

CONSTITUTION: The output signal of a transmitter A is at a level H or L, in a binary code; when a transistor TR1 is on, the voltage of a signal line D is almost zero volt, the light emitting diode LED is off, and the signal of a receiver B1 is at L. When the TR1 is off, on the other hand, a current flows from a resistance R1 to the signal line D and resistances R3 R4 of the receiver B1 to turn on the LED. The voltage obtained by subtracting the base-emitter voltage drop of the TR from the Zener voltage VZ is applied to the emitter Q of the TR2 as far as the voltage of the signal line D is higher than the voltage VZ. Therefore, the voltage at the emitter Q is constant regardless of the number of receivers B1, B2∼ and the output signal level of the photocoupler 1 is constant, so that the number of receivers are increased or decreased without changing transmitters.