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VoltMeter (Arduino Uno) — Command & Setup
Manual
Firmware Version: VoltMeter v1.2 (Memory Optimized)
1. Introduction
This manual explains all available serial commands for the VoltMeter firmware and provides a clear stepby-step guide to set up, calibrate, and operate the system. The commands are entered through the Serial
Monitor (or any serial terminal) at 115200 baud with newline line endings.
2. Basic Setup Procedure
Hardware Connection
Voltage Divider Wiring:
Connect the input voltage (Vin) → Top resistor (RTOP)
Connect midpoint between RTOP and RBOT → A0
Connect bottom resistor (RBOT) → GND
Make sure A0 never exceeds 5 V (for DEFAULT reference) or 1.1 V (for INTERNAL reference).
Power the Arduino from a stable USB or regulated 5 V supply.
Software Setup
Upload the VoltMeter firmware to your Arduino Uno.
Open Serial Monitor at 115200 baud.
Set line ending to Newline.
You should see the startup report and the line:
Type HELP for commands.
3. Command Reference Table
Command Function Example Description
HELP Show help list HELP
Lists all available commands
with basic usage examples.
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Command Function Example Description
SETREF DEFAULT
Set ADC
reference to Vcc
(5V)
SETREF DEFAULT
Use the Arduino’s operating
voltage as reference. Suitable
for mid-to-high voltage dividers.
SETREF INTERNAL
Set ADC
reference to
1.1V
SETREF INTERNAL
Uses the internal bandgap
reference for higher accuracy
when A0 voltage ≤ 1.1V.
SETREF AUTO
Auto-select
reference SETREF AUTO
The firmware tests the signal
level and automatically picks
DEFAULT or INTERNAL.
CAL <volts> Calibrate using a
known voltage
CAL 12.000
Runs calibration assuming the
applied input is 12.000V.
Calculates scale factor (K) and
saves to EEPROM.
CAL <volts> RBOT
<ohms>
Calibrate with
known bottom
resistor
CAL 12.000 RBOT
10000
Uses known RBOT to estimate
RTOP from measured K.
CAL <volts> RTOP
<ohms>
Calibrate with
known top
resistor
CAL 12.000 RTOP
22000
Uses known RTOP to estimate
RBOT from measured K.
CAL <volts> [RBOT/
RTOP] SAMPLES <n>
Calibrate with
custom sample
size
CAL 12.000 RBOT
10000 SAMPLES
512
Performs calibration using <n>
samples for more precision.
PRINTCAL
Print current
calibration data
PRINTCAL Displays stored calibration info
(K, vref, resistors, stats, CRC).
CLEARCAL /
RESETCAL
Erase calibration CLEARCAL
Clears calibration block from
EEPROM. The voltmeter will
require recalibration.
MEASURE
Take one
reading
MEASURE
Captures a single ADC
measurement and prints full
stats and computed Vin.
LOG ON
Enable
continuous
logging
LOG ON
Starts automatic measurement
every second (default interval:
1000 ms).
LOG OFF
Disable
continuous
logging
LOG OFF Stops periodic measurement
output.
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Command Function Example Description
FACTORYRESET
(optional)
Reset
calibration,
settings, and
counters
FACTORYRESET
Clears EEPROM calibration and
resets counters to default
values. (Only available if added to
sketch)
FULLRESET (optional,
dangerous)
Wipe entire
EEPROM
FULLRESET
Erases all EEPROM memory
(including boot counter). Use
only for recovery. (Optional, must
be added manually)
4. Step-by-Step Setup & Calibration
Step 1 — Power & Verify Communication
Open Serial Monitor at 115200 baud.
You should see a startup banner like:
================= Voltmeter Startup =================
Firmware: VoltMeter v1.2 (mem-opt)
Boot Cnt: 5
EEPROM: No valid calibration (CRC fail or empty).
Run CAL <volts> [RBOT|RTOP] to calibrate.
=====================================================
Step 2 — Choose Reference Mode
Depending on your divider ratio and input voltage range: - For 0–5 V ADC range, type:
SETREF DEFAULT
- For 0–1.1 V ADC range, type:
SETREF INTERNAL
- Or let the firmware decide automatically:
SETREF AUTO
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Step 3 — Run Calibration
Apply a stable, known voltage (measured with a DMM) to the divider input (Vin).
Enter a calibration command such as:
CAL 12.000 RBOT 10000
The system will sample and calculate ratio K = Vin / Vadc , then save it to EEPROM.
Wait for the confirmation message:
[CAL] Calibration saved.
To verify calibration:
PRINTCAL
Step 4 — Start Measuring
To measure once:
MEASURE
To enable continuous measurement (logging):
LOG ON
To stop logging:
LOG OFF
5. Understanding Measurement Output
Example output:
t=10233 ms, ADC_raw_mean=682.35 (min=676, max=689), vref=5.000000,
Vadc=3.333100, Vin=11.936821 V ±0.012, Vin_IIR=11.940832, ref=DEFAULT, cal=YES
Explanation: - t — time since boot (ms) - ADC_raw_mean — average ADC value (trimmed) - vref —
active ADC reference voltage - Vadc — measured voltage at A0 - Vin — actual computed input voltage
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(calibrated) - ± — uncertainty derived from ADC noise - Vin_IIR — filtered Vin (IIR smoothing) - ref —
reference source (DEFAULT/INTERNAL) - cal — calibration status (YES = valid calibration loaded)
6. EEPROM & Persistence
Calibration and counters are stored in EEPROM and loaded automatically at startup.
If CRC fails or calibration was cleared, the system requests recalibration.
Boot counter increments each power-up.
7. Factory Reset & Recovery
If calibration or EEPROM data becomes invalid: - Run:
CLEARCAL
to remove calibration only. - Or (if added):
FACTORYRESET
to restore all settings to default.
8. Tips for Reliable Operation
Ensure A0 voltage never exceeds Vref.
Use metal film resistors (1% or better) for long-term stability.
Keep divider impedance < 100 kΩ total.
Recalibrate after hardware changes or if voltage readings drift.
Use a stable power supply when operating in DEFAULT mode.
Minimize EEPROM writes — avoid frequent unnecessary calibrations.
9. Example Workflow Summary
Upload firmware and open Serial Monitor.
Enter SETREF DEFAULT .
Apply a known voltage (e.g., 12.000 V).
Type CAL 12.000 RBOT 10000 .
Wait for [CAL] Calibration saved.
Type PRINTCAL to confirm.
Type LOG ON to begin continuous voltage readings.
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10. Command Quick Reference
HELP — Show all commands.
SETREF DEFAULT|INTERNAL|AUTO — Set ADC reference mode.
CAL <volts> [RBOT|RTOP] [SAMPLES <n>] — Calibrate.
PRINTCAL — Show stored calibration data.
CLEARCAL / RESETCAL — Clear calibration.
MEASURE — Take one measurement.
LOG ON / OFF — Toggle periodic logging.
FACTORYRESET (optional) — Reset calibration, counter, and settings.
FULLRESET (optional) — Erase all EEPROM data (dangerous).