10-LED bar voltmeter

LM3914

The LM3914 is a linear 10-LED display driver. It integrates a precision divider network tapped by ten comparators spaced evenly along it, a programmable 1.25 V voltage reference and a high-impedance signal buffer. Each output is a constant-current sink that sets the current through its LED on its own — no series limiting resistor needed.

A mode pin picks between dot (only the LED that matches the level) and bar (all LEDs lit up to the level) display, and the scale is programmed with two external resistors. The IC runs from a single supply between 3 V and 25 V, which lets it sit alongside most analog circuits.

How it works

The signal to measure enters at pin 5 (SIG) and reaches the internal buffer, which compares it in parallel against the ten taps of the internal divider chain. That chain spans from pin 6 (RHI, high end) to pin 4 (RLO, low end). In this circuit RLO is tied to ground and RHI is tied to pin 7 (REF OUT), so the full-scale voltage equals the programmed reference.

Pins 7 and 8 form the programmable reference. R1 (1.21 kΩ) between REF OUT and REF ADJ, together with R2 (3.83 kΩ) between REF ADJ and ground, set the voltage at REF OUT to about 5 V — the full-scale value for a 0 V to 5 V input range.

Pin 9 (MODE) picks the presentation: here it goes straight to V+, so the display works in bar mode: the higher the signal, the more LEDs on at the same time. C1 (2.2 µF) decouples the +12 V rail that feeds the LED anodes and absorbs the current step as they turn on. D1–D6 are green, D7–D8 yellow and D9–D10 red, reproducing the classic “green-yellow-red” visual code as the level climbs toward full scale.

Since each output is a built-in constant-current source, no series resistor is needed on any LED: the LM3914 itself sets the same current for all ten.

Application design

Full-scale voltage

With R1 and R2 between REF OUT, REF ADJ and ground, the internal 1.25 V reference is amplified as:

$V_{REF} = 1.25\ \text{V} \cdot \left(1 + \frac{R_2}{R_1}\right) + I_{ADJ} \cdot R_2$

where $I_{ADJ} \approx 80\ \mu\text{A}$ is the typical REF ADJ pin current. With $R_1 = 1.21\ \text{k}\Omega$ and $R_2 = 3.83\ \text{k}\Omega$:

$V_{REF} = 1.25 \cdot \left(1 + \frac{3.83}{1.21}\right) + 80\ \mu\text{A} \cdot 3.83\ \text{k}\Omega \approx 5.5\ \text{V}$

Enough to cover a 0 V to 5 V input with a small margin above. Each LED lights roughly every 500 mV ($V_{REF}/10$).

LED current

The LED current is ten times the current flowing through the internal divider chain, which R1 pins down:

$I_{LED} \approx \frac{12.5\ \text{V}}{R_1} = \frac{12.5}{1\,210} \approx 10.3\ \text{mA}$

A comfortable value for standard 3 mm or 5 mm LEDs. To boost brightness, lower R1 (and recompute R2 to keep the full-scale value); to dim the display, raise it.

Supply and bar mode

In bar mode with all ten LEDs on, the IC draws about 100 mA from the +12 V rail, so the supply must handle that comfortably. The 2.2 µF C1 capacitor close to the IC is a must: it prevents oscillation and keeps the anode rail stable when the LEDs switch on together.

Other applications

The LM3914 fits any analog indication where a 10-step bargraph reads better than a dial or a numeric display:

• Analog panel voltmeter. The direct use of this schematic, with the scale programmed by R1/R2 to any range.
• Battery charge indicator. With a divider on the input the full scale is calibrated to the typical maximum (e.g. 14.4 V for lead-acid).
• Audio VU meter. Preceded by a rectifier and a low-pass filter it shows the level of an audio signal on a bargraph.
• Thermometer with the LM35 or similar. The 10 mV/°C output of the LM35 drives SIG directly and the scale covers the range of interest.
• Fluid level or position indicator. A potentiometer or distance sensor provides the input signal for a 10-step visual strip.