"Not bad for a forty-year-old piece of silicon," he whispered, as the Oracle dimmed into a soft, steady heartbeat. Should we focus on a story about the technical build process of this calculator, or would you prefer a tale about the mysterious calculations it performs?
4.1 LM3915 Basics The LM3915 senses an analog voltage and lights LEDs according to a 3 dB-per-segment log scale by default. To achieve 2 dB/step over 10 segments (20 dB total), modify the reference network so the full-scale input range maps to the LM3915’s reference span. The LM3915’s internal reference sets the LED currents via a single resistor (Rset) between pin 7 and ground.
Built-in regulator creates a stable reference voltage from 1.2V to 12V.
4−1=R21200 Ω4 minus 1 equals the fraction with numerator cap R sub 2 and denominator 1200 space cap omega end-fraction
VREF=1.25×(1+R2R1)cap V sub cap R cap E cap F end-sub equals 1.25 cross open paren 1 plus the fraction with numerator cap R sub 2 and denominator cap R sub 1 end-fraction close paren : Determines the brightness of each LED.
For decades, the LM3915 has been the undisputed champion of analog bar graph display drivers. Whether you are building a vintage VU meter for a hi-fi amplifier, a DIY audio spectrum analyzer, or a simple battery level indicator, this IC from Texas Instruments (originally National Semiconductor) has been a go-to component. However, one significant hurdle has always plagued engineers and hobbyists:
The LM3915 establishes a nominal 1.25V reference voltage between the REF OUT pin (Pin 7) and the REF ADJ pin (Pin 8). The total output reference voltage relative to ground is determined by the ratio of R1cap R sub 1 R2cap R sub 2