Lemon! 🍋

Stereo 20-LED VU Meter

When life gives you a lemon, plug your music into it.

Lemon is a compact stereo VU meter built around two LM3915N-1 LED bar-graph driver ICs.

The design provides 10 LED levels for each stereo channel, giving a total of 20 LEDs. It is intended to be used as a standalone audio visualizer or integrated into an amplifier.

Features

• Stereo audio level display
• 2× LM3915N-1
• 20× rectangular LEDs
• 10 levels per channel
• Independent sensitivity adjustment for left and right channels
• 3.5 mm stereo audio input/output
• Adjustable DOT / BAR display mode
• 15–25 V DC supply
• Designed for integration into an audio amplifier
• Two-layer PCB with large GND copper zones

Design

• Lemon is based on the LED VU meter circuit published by Elliott Sound Products.

• The original circuit uses an LM3915 to drive ten LEDs according to the level of an incoming audio signal. Lemon uses two identical sections for the left and right channels.

• Each channel has its own sensitivity adjustment, allowing the two VU meters to be calibrated independently.

Signal Flow

• The audio signal is passed through the board while each stereo channel is sampled by its respective LM3915 section for visual level indication.

Power

• Lemon is designed for a 15–25 V DC supply.

• The PCB uses a single positive supply and ground connection. The LED supply path and LM3915 supply path are separated on the board according to the original circuit.

• The EARTH connection is kept separate from the main GND plane through the original 10 Ω connection, allowing the board to be integrated into a larger amplifier system without unnecessarily tying the two together.

PCB

The PCB was designed in KiCad with a focus on:

• Clean stereo symmetry
• Short and organized signal paths
• Large GND copper zones
• Easy component identification
• Through-hole construction
• Simple integration into an amplifier enclosure

The two rows of LEDs form the main visual element of the board, with the LM3915 circuits and adjustment controls positioned behind them.

• Components

Quantity Component

• 2 x LM3915N-1
• 20 x Rectangular LEDs
• 2 x 50 kΩ potentiometers
• 2 x 3.5 mm stereo TRS jacks
• 2 x 82 Ω / 0.5 W resistors
• 2 x 1.2 kΩ resistors
• 2 x 2.2 kΩ resistors
• 2 x 10 kΩ resistors
• 2 x 10 Ω resistors
• 4 x 100 nF capacitors

Component values and quantities should be verified against the final schematic and BOM before assembly.

Project Status

• The VU meter circuit is based on the design published by Elliott Sound Products.

• Lemon is an independent PCB implementation and stereo adaptation of the circuit, with a custom PCB layout, mechanical arrangement, and visual design.

License

GNU General Public License v3.0