An interesting interface solution that can be provided in some cases is the one shown in the figure in which an adjustment trimpot is employed. This trimpot allows you to bring the signal transition level of the gate where the trigger is to the optimum value. This allows even microprocessors that provide lower output voltages to their ports, as some modern versions that provide 2.7 V or 3.3 V signals can also be used for interfacing external circuits without having to change the circuit. . Note that the reference resistors have been calculated for two comparator supply voltages: 6 V and 12 V. For other voltages the reader can make the calculation, remembering that for a 5 V trip output, the reference voltage level should be 2.5 V. Remember that the outputs of the LM139 / 239/339 comparators are open collector, requiring a pull-up resistor. The maximum current drained from the output is 16 mA (tip) for a 5 V power supply. With this, this step can easily drive transistors and other devices.
