DX Patrol PA Monitor

PA monitor with PWR, SWR, and temperature readout for Arduino Nano (ATmega328P), designed for the DX Patrol 20W PA (IU0PXK).

1. Project Overview

This repository contains firmware for a power amplifier monitor that measures:

• Output power (PWR)
• Standing wave ratio (SWR)
• Heatsink temperature

Readings are displayed on a 20x4 I2C LCD.

Available firmware variants:

• v2.0.0: Base monitor (PWR, SWR, Temperature)
• v3.0.0-4wire: Base monitor + rotary encoder menu + 4-wire PWM fan control
• v3.0.0-3wire: Base monitor + rotary encoder menu + 3-wire fan control

2. Repository Contents

File / Folder	Description
src/main.cpp	Firmware v2.0.0 (base monitor)
src/main_4wire.cpp	Firmware v3.0.0-4wire
src/main_3wire.cpp	Firmware v3.0.0-3wire
calibration_fit.py	Polynomial calibration helper script
platformio.ini	PlatformIO project configuration
RELEASE.md	Release notes

3. Base Version (v2.0.0)

Key Features

• ADC averaging over 16 samples (noise reduction)
• Polynomial PWR calibration (Horner evaluation)
• Physical SWR computation (gamma = Vref / Vfwd)
• Exponential moving average on temperature (alpha = 0.15)
• Non-blocking loop with millis()
• Differential LCD update to reduce flicker
• Optional serial debug via #define DEBUG

Calibration Constants

Constant	Value	Description
TEMP_OFFSET_MV	464.0 mV	Temperature sensor offset at 0 C
TEMP_MV_PER_DEG	6.25 mV/C	Sensor sensitivity
PWR_ADC_OFFSET_MV	298.0 mV	FWD channel offset
SWR_ADC_OFFSET_MV	1288.0 mV	REF channel offset
RAW_PWR_NOISE_FLOOR	8	ADC threshold for power channel
RAW_SWR_NOISE_FLOOR	4	ADC threshold for SWR channel
VREF_MV	5000.0 mV	ADC reference
ADC_SAMPLES	16	Samples per average
LOOP_INTERVAL_MS	100 ms	Monitor update period

Hardware Pins

Arduino Pin	Signal
A0	Temperature sensor
A1	SWR channel (REF)
A2	Power channel (FWD)
SDA/SCL	I2C LCD (0x27)

4. v3.0.0-4wire (4-Wire Fan + Encoder)

Added Features

• Rotary encoder on D2 (CLK), D3 (DT), D4 (SW)
• LCD settings menu (3 editable parameters)
• Hardware PWM fan control on D9 (OC1A, Timer1)
• EEPROM settings persistence (magic byte 0xA5)
• Hysteresis-based fan control
• Menu timeout after 15 s

Extra Pins

Arduino Pin	Signal
D2 (INT0)	Encoder CLK
D3 (INT1)	Encoder DT
D4	Encoder switch
D9 (OC1A)	4-wire fan PWM

4-Wire Fan Menu Parameters

LCD Label	Meaning	Range	Step	Default
Fan ON thresh.	Fan turn-on threshold	20-85 C	0.5 C	45.0 C
Fan OFF thresh.	Fan turn-off threshold	15-(ON-1) C	0.5 C	40.0 C
PWM minimum	Minimum PWM output	0-254	5	80
Save & Exit	Save settings and exit	-	-	-

4-Wire Wiring

Direct PWM connection (no MOSFET required):

Arduino D9 (OC1A)  -> Fan pin 4 (BLUE, PWM)
Arduino GND        -> Fan pin 1 (BLACK, GND)
+12V supply        -> Fan pin 3 (RED, +12V)
Fan pin 2 (YELLOW, TACH) optional / unused

5. v3.0.0-3wire (3-Wire Fan + Encoder)

Differences vs 4-Wire

• Timer2 PWM on D11 (OC2A) at about 61 Hz
• N-channel MOSFET low-side switching required
• Kick-start support (high duty at startup)
• Optional TACH readout on D5
• EEPROM layout for this variant (magic byte 0xB6)

3-Wire Fan Menu Parameters

LCD Label	Meaning	Range	Step	Default
Fan ON thresh.	Fan turn-on threshold	20-85 C	0.5 C	45.0 C
Fan OFF thresh.	Fan turn-off threshold	15-(ON-1) C	0.5 C	40.0 C
Min duty	Minimum running duty	20-95 %	1 %	40 %
Kick-st	Startup kick duty	50-100 %	5 %	100 %
Save & Exit	Save settings and exit	-	-	-

3-Wire Wiring (MOSFET low-side)

Recommended components: N-channel MOSFET (for example IRLZ44N), 100 ohm gate resistor, 10 kohm gate pull-down.

+12V supply         -> Fan RED wire
Fan BLACK wire      -> MOSFET Drain
MOSFET Source       -> Common GND (Arduino + 12V supply)
Arduino D11 (OC2A)  -> 100 ohm -> MOSFET Gate
MOSFET Gate         -> 10 kohm -> GND (pull-down)
Fan YELLOW (TACH)   -> D5 (optional with TACH enabled)

6. Version Comparison

Feature	v2.0.0	v3.0.0-4wire	v3.0.0-3wire
PWR/SWR/Temperature monitor	Yes	Yes	Yes
Encoder + menu	No	Yes	Yes
Fan control	No	Yes	Yes
Fan type	-	4-wire PWM	3-wire MOSFET
PWM source	-	Timer1 (D9)	Timer2 (D11)
Typical PWM frequency	-	about 50 kHz	about 61 Hz
Kick-start	-	No	Yes
TACH RPM	-	No	Optional (D5)
EEPROM settings	-	Yes (0xA5)	Yes (0xB6)

7. Calibration Procedure

1. Collect (mV_ADC, real_power_W) pairs with a trusted wattmeter.
2. Update the data list in calibration_fit.py.
3. Run:
   • python3 calibration_fit.py
4. Copy printed coefficients into PWR_COEFFS[] in the target firmware file.
5. Rebuild and upload.

8. Build and Upload

Requirements

• PlatformIO CLI
• Board: nanoatmega328new
• Library: LiquidCrystal_I2C (resolved by PlatformIO)

Commands

# Build
pio run

# Upload
pio run --target upload

# Serial monitor
pio device monitor --baud 115200

9. Notes

• Display format uses comma-style decimal output (for example 12,34).
• Small values are forced to zero to suppress ADC noise jitter.
• Tune RAW_PWR_NOISE_FLOOR and RAW_SWR_NOISE_FLOOR if needed.
• In the 3-wire variant, KS is shown during kick-start time.
• v3.0.0-4wire and v3.0.0-3wire EEPROM layouts are intentionally incompatible.

License

MIT License

Copyright (c) 2026 Alessandro Orlando (IU0PXK)