Non-linear Voltmeter

Arduino-driven 0-16 V panel meter with a Gaussian-weighted non-linear scale that expands resolution around 12 V — designed for monitoring a 12 V DC bus (e.g. Synology NAS power rail).

Circuit

The bus voltage is fed through a resistor divider (R1/R2/R3) with diode clamping (D1/D2) for overvoltage protection. An L7805CV regulator (U1) powers the Arduino from the same bus. The PWM output is smoothed by C4 before driving the panel meter.

How it works

A Gaussian sensitivity function concentrates ~60% of the meter's arc in the 10-14 V range, making small deviations from 12 V easy to read at a glance. A Python script integrates this sensitivity curve to produce a 1024-entry ADC-to-PWM lookup table. The Arduino reads voltage via the resistor divider on A1 (scaled 0-16 V to 0-5 V), applies an EMA filter, and drives an 85C1-style panel meter with PWM on pin 5 (D5).

Color bands on the scale face indicate operating zones:

Band	Range	Meaning
Gray	0-5 V	Device off
Maroon	5-10.5 V	Brownout / corruption risk
Amber	10.5-11.4 V	Brownout warning
Green	11.4-12.6 V	Safe range (±5%)
Amber	12.6-13.2 V	Overvoltage warning
Red	13.2-16 V	Overvoltage damage

Simulator

Interactive Tkinter simulator with needle, slider, and PWM readout:

python3 simulator/meter_gui.py

Files

File	Description
firmware/meter.ino	Arduino sketch — reads ADC, applies LUT, outputs PWM
firmware/generate_lut.py	Generates the ADC→PWM lookup table (prints Arduino C array)
firmware/meter_lut.csv	LUT as CSV (voltage, duty_cycle_pct)
firmware/voltage_mapping.csv	Full mapping (bus_voltage, meter_voltage)
scale/generate_cricut_svg.py	Generates a 1:1 SVG for Cricut "Print Then Cut"
scale/generate_concept.py	Renders the non-linear scale face as a PNG (matplotlib)
scale/meter_scale_cricut.svg	Cricut-ready SVG output
scale/meter_scale_cricut_6x3.57cm.png	Scale face PNG (transparent background)
scale/concept_scale.png	Concept visualization of the scale
scale/alignment/	Alignment validation tools and reference photos
circuit/meter.fzz	Fritzing project file (schematic + breadboard)
circuit/schematic.png	Circuit schematic
circuit/breadboard.png	Breadboard layout
simulator/meter_gui.py	Interactive Tkinter simulator with needle, slider, and PWM readout
simulator/screenshot.png	Simulator screenshot

Tuning

Edit the parameters at the top of firmware/generate_lut.py:

• CENTER — voltage the scale expands around (default 12.0)
• SIGMA — width of the expanded region (default 1.4)
• BASE_GAIN / PEAK_GAIN — sensitivity floor vs. peak at center

Custom Scale Face (Cricut)

scale/generate_cricut_svg.py generates a 1:1 physical-scale SVG for replacing the factory 0-5 V scale on the Baomain 85C1 with the custom non-linear 0-16 Vdc scale using a Cricut "Print Then Cut" workflow.

python3 scale/generate_cricut_svg.py           # produces meter_scale_cricut.svg + .png
python3 scale/generate_cricut_svg.py --ruler   # adds mm ruler ticks for verifying dimensions

Cricut workflow

1. Import meter_scale_cricut.svg into Cricut Design Space
2. Select all layers except the magenta cut outline → click Flatten (merges into a single "Print Then Cut" layer)
3. The magenta cut outline remains as a Cut operation
4. Select both the flattened print layer and cut outline → Attach (locks relative positioning)
5. Click Make It — the Cricut prints the design with registration marks, then cuts the outline
6. Trim and mount the printed scale face onto the meter

Tuning physical dimensions

All measurements are configurable constants at the top of the script (mm). Print at 100% scale (no fit-to-page) and compare against the original plate. Key parameters:

Parameter	Default	What it controls
PLATE_W / PLATE_H	60 × 35.7 mm	Overall plate size
ARC_RADIUS	31.5 mm	Scale arc radius
PIVOT_X / PIVOT_Y	30.0 / 39.2 mm	Needle pivot point
NOTCH_W / NOTCH_H	22.7 / 5.85 mm	Bottom cutout size
TOWER_W	1.2 mm	Tower column width flanking the dome
VALLEY_DEPTH	2.0 mm	Valley floor height between towers and dome
DOME_N	2.283	Superellipse exponent for dome profile
FILLET_R	0.3 mm	Fillet radius at 90° corners
MOUNT_HOLE_*	2.5 mm dia, 14/3 mm inset	Screw hole placement

License

Public domain — see UNLICENSE.