Example: For 20 mA (typical bright LED), ( R_\textset = 12.5 / 0.02 = 625 \ \Omega ). Use 620 Ω standard. Design goal: Audio level meter for -30 dBV to +6 dBV (36 dB range, but LM3915 only does 30 dB, so compress or shift). Desired: LED1 = -30 dBV, LED10 = 0 dBV (30 dB span). Reference voltage = 5.0 V (from 12V supply). LED current = 15 mA.
( V_\textRHI = 1.5 ) V. Check: 1.5 V peak corresponds to ~1.06 Vrms → ~0.5 dBV (close to 0 dBV). LM3915 Calculator
But for simplicity, designers often set ( V_\textRLO = V_\textLO ) and ( V_\textRHI = V_\textref ) (if ( V_\textref ) is scaled to match highest LED threshold). More practically: The LM3915’s internal divider has a ratio of ~1.25 dB per step in voltage terms, so the voltage at step n is: Example: For 20 mA (typical bright LED), ( R_\textset = 12
[ \textAttenuation factor = \fracV_\textref,desiredV_\textmax ] Desired: LED1 = -30 dBV, LED10 = 0 dBV (30 dB span)