How many volts are minimally required for transistor couplers to operate?

In principle, as long as the voltage at the input LED side is equal to or greater than the maximum value for the LED input forward voltage V_F, transistor couplers can operate. With the above also taken into account, as long as a power supply voltage equal to or greater than a standard DC power supply voltage of 3.3V is assumed, expected operation can be performed.