Mono VU meter with envelope follower output

VU is a highly compact mono VU meter and envelope follower. The VU display consists of 5 yellow, 3 orange, and 2 red LEDs. The module provides an audio input jack on the bottom, and an audio output jack on top, allowing it to be placed right before the master output if so desired. The output jack is buffered, in order to ensure the audio levels don't get affected. Note that there is no AC coupling, so the module can also be used with slow moving CV signals if so desired.

The response curve of the VU display depends on the inserted LED driver chip. Three variants of the chip exist and are interchangable on the PCB. The compatible chips are:
- LM3914 - Linear response curve
- LM3915 - Logarithmic response curve, each LED is 3dB
- LM3916 - VU meter response curve, each LED corresponds to following dB levels: +3, +2, +1, 0, -1, -3, -5, -7, -10, -20

In order to drive the the VU display, the signal is first precision rectified, and then passed through an envelope follower. The falloff of this envelope can be adjusted using the HOLD knob, and in order to expand the functionality of the module, this envelope follower signal can be read from the PEAK output jack. This CV value can be used to control any other module, for instance, to create a sidechain effect. It's voltage ranges from 0V to 9V, depending on the input signal.

In order to make the module compatible for different rack setups, a trimmer on the PCB (T_GAIN) allows you to adjust the internal gain of the envelope follower input. This allows you to adjust the meter so it hits the peak exactly when it would usually clip on the output module, or whatever is after it, ensuring a more accurate display.

Lastly, the brightness of the peak LED can be adjusted with a trimmer on the PCB (T_LED). The brightness can range from around same brightness like all other LEDs, to much brighter.

Features:
- Dual slew limiter
- Independent rise and fall speed
- Linear slew curve
- Status LEDs for output
- Wrong polarity protection
- Power: +12V: 23mA, -12V: 23mA

• 36 mA +12V
• 15 mA -12V
• 0 mA 5V
• 66 mm Depth

Available as an assembled Module and as a DIY project.

This Module is an early prototype, a concept in development, or an AI mockup