Overview

The LM3915 is a monolithic integrated circuit manufactured by Texas Instruments that senses analog audio signal levels and drives a 10-LED dot or bar graph display logarithmically. The internal divider is scaled to 3dB per step across a total 30dB dynamic range, matching human auditory perception of volume.

Widely used in audio equipment, mixing consoles, stereo amplifiers, and DIY VU meters, it accommodates signals from 10 mV10\text{ mV} up to 12V12\text{V} without external attenuators. Like the LM3914, it includes a 1.25V internal reference, programmable LED currents (2 mA…30 mA2\text{ mA} \dots 30\text{ mA}), and a MODE pin to switch between Moving Dot and Bar Graph display modes.

Quick reference

Supply Voltage (V+)3.0 V to 25.0 V DC
Decoded LED Outputs10 Regulated Current Sink Outputs (LED1 to LED10)
Logarithmic Step Scale3 dB3\text{ dB} per step (30 dB30\text{ dB} total dynamic range)
Display ModesBar Graph (Mode Pin tied to V+) / Moving Dot (Mode Pin floating)
Internal Reference Voltage1.25 V nominal
Programmable LED Current2 mA to 30 mA (set by resistor between Pin 7 & 8/GND)
Package18-pin DIP / PLCC-20

Pinout (DIP-18 Package)

text
             ┌───┴───┐
        LED1 1│ 1   18│ LED2
          V- 2│       │17 LED3
          V+ 3│       │16 LED4
         RLO 4│ LM3915│15 LED5
         SIG 5│       │14 LED6
         RHI 6│       │13 LED7
     REF OUT 7│       │12 LED8
     REF ADJ 8│       │11 LED9
        MODE 9│       │10 LED10
             └───────┘
PinNameTypeDescription
1LED1OutputOpen-collector LED 1 driver output (−27dB-27\text{dB} threshold)
2V-PowerSignal and power ground reference (0 V)
3V+PowerPositive supply power pin (+3.0V to +25.0V DC)
4RLOInputLow-end of internal reference divider (Connect to GND)
5SIGInputAnalog audio signal input pin
6RHIInputHigh-end of internal reference divider (Connect to REF OUT)
7REF OUTOutputInternal 1.25V reference output / LED current programming pin
8REF ADJOutputVoltage reference adjust pin (Ground for 1.25V output)
9MODEInputDisplay mode select pin (V+ = Bar Graph; Floating = Moving Dot)
10–18LED10–LED2OutputOpen-collector LED driver outputs 10 down to 2

Logarithmic Step Threshold Table (3dB / Step)

LED StepRelative Level (dB)Threshold Voltage (VREF=1.25VV_{REF}=1.25\text{V})
LED1−27 dB-27\text{ dB}0.056 V0.056\text{ V}
LED2−24 dB-24\text{ dB}0.079 V0.079\text{ V}
LED3−21 dB-21\text{ dB}0.112 V0.112\text{ V}
LED4−18 dB-18\text{ dB}0.158 V0.158\text{ V}
LED5−15 dB-15\text{ dB}0.223 V0.223\text{ V}
LED6−12 dB-12\text{ dB}0.315 V0.315\text{ V}
LED7−9 dB-9\text{ dB}0.446 V0.446\text{ V}
LED8−6 dB-6\text{ dB}0.630 V0.630\text{ V}
LED9−3 dB-3\text{ dB}0.891 V0.891\text{ V}
LED100 dB0\text{ dB} (Full Scale)1.250 V1.250\text{ V}

Basic Audio VU Meter Application Circuit

text
                      +9V to +12V Power
                             │
                     [Pin 3: V+] ────────── [Pin 9: MODE] (Tie to V+ for Bar Mode)
                      LM3915
                     [Pin 2: V-] ────────── GND
                             │
  AC Audio In ─[10µF]─►|─┬─► [Pin 5: SIG]
                    1N4148│
                         [100k]
                          │
                         GND
                             │
                     [Pin 6: RHI] ──┬────── [Pin 7: REF OUT]
                     [Pin 4: RLO] ──┴──┬─── [Pin 8: REF ADJ] ───[1.2kΩ Resistor]─── GND
                                       │
                                      GND
   LED Anodes (+VCC) ──► (10x LEDs) ──► [Pins 1, 18..10: LED1..LED10 Outputs]

Common mistakes

  • Connecting AC audio directly to Pin 5 without rectification: Negative swings of an un-rectified AC audio signal going below −0.5V-0.5\text{V} on Pin 5 will cause false triggering or damage internal comparators. Use a simple half-wave diode rectifier (1N4148 + 100k parallel resistor/capacitor smoothing filter) before Pin 5.
  • Excessive supply voltage in Bar Graph mode: Operating at 12V…15V12\text{V} \dots 15\text{V} in Bar Mode driving 10 LEDs at high current causes heavy IC heating. Power the LED anodes from a separate lower voltage source (3.3V…5V3.3\text{V} \dots 5\text{V}).
  • Ground loops between audio signal ground and heavy power ground: Connect the signal ground (Pin 2, RLO) directly to the audio input ground point to prevent ground hum from riding on the display.

Notes

  • LM3915 vs LM3914 vs LM3916: LM3915 has 3dB3\text{dB} steps over 30dB30\text{dB}; LM3914 has linear steps; LM3916 has a standard VU scale (−20dB-20\text{dB} to +3dB+3\text{dB}). Two LM3915s can be cascaded for a 60dB60\text{dB} display range.

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