In the figure we have an interface using an optical coupler and the comparator LM339 or any equivalent. In this application, operational amplifiers can also be used. The trigger point can be adjusted in the 10 k ohm trimpot, however, nothing prevents us from having a fixed trigger by placing a resistive divider with two 10 k ohm resistors at this point. Remember that the comparators of the LM139/239/339 series have open collector transistors, so a pull-up resistor must be used and current is drained from the output when it goes to low level. The comparator sector power can be done with separate power supply and voltage other than 5 V.
