LM3914N

The LM3914N from Texas Instruments is the industry-standard analog display driver that senses an analog voltage level and drives 10 LEDs to form a bar-graph or moving-dot display. A single pin (pin 9) selects the display mode: tied to V+ for bar mode (all LEDs up to the signal level light), or left open for dot mode (only one LED at the signal level lights). The internal 1.25V reference and precision 10-step resistor divider create 10 equally-spaced comparison thresholds. The LED current is set by a single external resistor connected to the reference output — no individual LED resistors needed. The reference voltage is adjustable from 1.2V to 12V using two external resistors, setting the full-scale input voltage. The input buffer has high impedance (>10MΩ) and accepts signals down to ground, making it compatible with single-supply systems. The LM3914 is incredibly easy to use: a basic 1.2V meter needs only 1 resistor, 10 LEDs, and a power supply. For battery indicators: connect input to battery voltage through a voltage divider; adjust reference for full-scale at the desired voltage. Multiple LM3914s can be cascaded for 20, 30, or more LED displays. The N suffix denotes the PDIP-18 package. Status: Discontinued (TI recommends checking alternatives).

The LM3914N operates as follows: An input buffer (high-impedance voltage follower) receives the analog signal and drives 10 comparators. Each comparator has its inverting input connected to a different tap on an internal 10-step resistor divider. The resistor divider is biased by the internal 1.25V reference (adjustable via external resistors). When the input voltage exceeds a comparator’s threshold, that comparator’s output activates, sinking current through the corresponding LED. In bar mode (pin 9 tied to V+), all comparators below the signal level activate, creating a bar of lit LEDs. In dot mode (pin 9 open), only the highest activated comparator drives its LED, creating a single moving dot. The LED current is proportional to the current flowing out of the reference output pin: I_LED ≈ 10 × I_REF. By choosing the reference resistor, LED brightness is controlled without individual resistors. The internal reference develops 1.25V between REF OUT (pin 7) and REF ADJ (pin 8). Adding resistors from pin 8 to ground and pin 7 to pin 8 sets the reference voltage: V_REF = 1.25 × (1 + R2/R1) + 120µA × R2. This sets the full-scale voltage. The floating resistor divider means both ends (Rhi and Rlo) are accessible, allowing expanded-scale or zero-center meter configurations.

• Simple VU Meter: Audio signal → rectifier → SIG (pin 5); Rhi to REF OUT; Rlo to GND; 10 LEDs on pins 1,18-10; MODE pin 9 to V+ (bar) or open (dot); 1 resistor sets LED current
• Battery Indicator: Battery voltage → voltage divider → SIG; REF set for full-scale at charged voltage; LEDs: green=charged, yellow=mid, red=low
• 20-LED Display: Two LM3914s; SIG of #2 = SIG of #1 (shifted by resistor divider); pin 9 of #1 → pin 1 of #2 for dot carry; bar mode for bar display
• Expanded-Scale Meter: Rhi and Rlo set to a narrow voltage window (e.g., 11V-13V for 12V battery); 10 LEDs span only 2V range for high resolution

LM3914N is a linear-scale dot/bar display driver that drives 10 LEDs proportional to an analog input voltage. LM3915 (logarithmic 3dB/step) and LM3916 (VU scale) are audio-optimized variants. All share the same DIP-18 pinout and support both dot and bar display modes. For digital control, MAX7219 and HT16K33 provide SPI/I2C LED driver alternatives with individual LED addressing.