USBee AX PRO input level
R1 and R2 form a voltage divider. If V1 is 18V, OUTPUT will be 5.07V (note that resistor tolerances will be way less precise than the 0.07V exceeding 5V). If you use 10k for R1, OUTPUT will be 5.7V, 15k will give 4.3V, etc..

For other values you can search for voltage divider calculators online or use the following formula:



D1 and D2 are voltage clamp diodes.

D1 makes: Output &lt;= 5V + Vdrop (typically 5.7V)

D2 makes: Output >= 0V - Vdrop (typically -0.7V)

The voltage clamps and voltage divider are somewhat redundant (you could just use a series resistor with clamp diodes).

You might want to add a further series resistor to protect your logic analyzer.







simulate this circuit &ndash; Schematic created using CircuitLab