| Desired Max Voltage | R1 (Brightness) | R2 (Range) | | :--- | :--- | :--- | | | 1.2kΩ | 0Ω (Direct to GND) | | 5V Range | 1.2kΩ | 3.6kΩ | | 10V Range | 1.2kΩ | 8.2kΩ | | 12V Range | 1.2kΩ | 10kΩ |
Old calculators gave you theoretical resistor values like 1,247Ω . A modern "updated" version has a dropdown to snap to (1.2k, 1.5k, 2.2k). It then recalculates the actual dB error (e.g., "Error: +0.2 dB @ step 7").
The LM3915 operates using an internal 1.25V reference between the (Pin 7) and REF ADJ (Pin 8) pins. The relationship between the resistors and the full-scale voltage ( VOUTcap V sub cap O cap U cap T end-sub ) is defined by the following equation:
$$R1 = \frac1.25VI_LED$$