Monitor Light

Wi-Fi control for a monitor light bar with warm and cool white LEDs. An ESP8266 drives the bar and serves a small HTTP API. A Windows tray app gives you one hotkey for on/off and a panel for brightness and colour temperature.

Features

• Ctrl+Alt+L turns the light on and off from anywhere
• Brightness from 1 to 100 % and colour temperature from 2700 K (warm) to 6500 K (cool), mixed from the two LED channels
• Dark Windows 11 style tray panel with a live preview of the light, sliders and Warm / Neutral / Daylight / Cool presets
• Tray icon shows the state: amber = on, grey = off, red = not reachable
• Optionally turns the light off when the PC sleeps or shuts down
• Plain HTTP API, so you can also script it with curl or lamp-control.bat
• No installer and no SDK: the app builds with the C# compiler that ships with Windows

Repository layout

firmware/monitor-light/
  monitor-light.ino      ESP8266 sketch: PWM for the LEDs, HTTP API, mDNS
  secrets.example.h      template for your Wi-Fi name and password
MonitorLight.cs          Windows tray app (single file)
MonitorLight.ico         app icon
build.bat                builds MonitorLight.exe
lamp-control.bat         command-line / menu control
images/                  screenshots

Hardware

Part	Notes
ESP8266 board	Any board that exposes D1 and D2, such as a NodeMCU or Wemos D1 mini
ULN2003A	Darlington driver that switches the LED channels on their ground side
Monitor light bar	Two white LED channels, warm and cool, with a shared positive line (common anode). Most USB light bars are built this way
Power	Usually 5 V USB for the bar; the ESP8266 can run from the same 5 V

Wiring:

ESP8266	ULN2003A	Light bar
D1 (GPIO5)	IN2 (pin 2) → OUT2 (pin 15)	warm channel −
D2 (GPIO4)	IN1 (pin 1) → OUT1 (pin 16)	cool channel −
GND	GND (pin 8)	supply GND
—	—	common + → supply +

Each ULN2003A channel handles about 500 mA. If your bar draws more, tie spare channels in parallel (input to input, output to output). Check your bar's wiring with a multimeter first, because not every bar uses a common anode.

The firmware uses 8-bit PWM at 1 kHz. Brightness sets the total output and the colour temperature splits it linearly between the channels: 2700 K is all warm, 6500 K is all cool, and values in between mix the two.

Firmware

1. Install the Arduino IDE and add the ESP8266 core: in File → Preferences → Additional boards manager URLs add https://arduino.esp8266.com/stable/package_esp8266com_index.json, then install esp8266 in the Boards Manager.
2. Copy firmware/monitor-light/secrets.example.h to secrets.h in the same folder and enter your Wi-Fi name and password. secrets.h is in .gitignore, so it is never committed.
3. Open firmware/monitor-light/monitor-light.ino, select your board (for example NodeMCU 1.0 or LOLIN(WEMOS) D1 mini) and upload.
4. Open the Serial Monitor at 115200 baud to see the IP address. The light also answers at http://desk-lamp.local/ over mDNS (change HOSTNAME in the sketch to rename it).

The light keeps trying to reconnect if Wi-Fi drops. It starts switched off at 100 % and 4000 K after a power cycle, because the state is not saved.

HTTP API

Every response is JSON with the current state: {"on":true,"brightness":60,"temperature":4000}.

Method	Path	Does
GET	/api/lamp	returns the state
POST	/api/lamp?on=true&brightness=60&temperature=3500	sets any mix of on (true/false, 1/0, on/off), brightness (1–100) and temperature (2700–6500)
POST	/api/lamp/on	turns on
POST	/api/lamp/off	turns off
GET	/	service info

Invalid values return HTTP 400 with {"error":"..."}.

curl http://desk-lamp.local/api/lamp
curl -X POST "http://desk-lamp.local/api/lamp?brightness=40&temperature=2700"
curl -X POST http://desk-lamp.local/api/lamp/off

Windows app

Requires Windows 10 or 11. The app uses .NET Framework 4, which is part of Windows, so there is nothing else to install.

Download or build

Download MonitorLight.exe from the latest release and run it; no install is needed. Windows SmartScreen may warn about an unsigned app the first time: choose More info → Run anyway.

Or build it yourself:

build.bat

This produces MonitorLight.exe using the .NET Framework 4 compiler at %WINDIR%\Microsoft.NET\Framework64\v4.0.30319\csc.exe, which ships with Windows. Exit the running app (tray menu → Exit) before rebuilding, because Windows locks the exe while it runs.

Use

• Ctrl+Alt+L: on / off
• Left-click the tray icon: open the panel
• Middle-click the tray icon: on / off
• Right-click the tray icon: Open, Turn on, Turn off, Exit
• Starting MonitorLight.exe again while it runs opens the panel

In the panel, drag the sliders or use the mouse wheel or arrow keys over them, at 1 % or 100 K per step. The chips jump to Warm (2700 K), Neutral (4000 K), Daylight (5000 K) or Cool (6500 K). Moving a slider while the light is off turns it on. Esc or clicking elsewhere closes the panel.

Settings

Setting	Does
Off when the PC sleeps or shuts down	turns the light off just before Windows sleeps or ends the session
Start with Windows	adds the exe to HKCU\Software\Microsoft\Windows\CurrentVersion\Run, starting in the tray

Settings are stored in HKCU\Software\MonitorLight. Only one copy of the app runs at a time.

How it works

The app checks the light every 4 seconds, so changes made elsewhere (the script, curl, another PC) show up. It looks up the .local name once and reuses the IP address, because each mDNS lookup can take a second. The ESP8266 serves one client at a time, so the app sends one request at a time. Slider moves made while a request is in flight are merged into the next one, so dragging never builds up a backlog.

Command line

lamp-control.bat without arguments opens a menu. With arguments:

lamp-control.bat on
lamp-control.bat off
lamp-control.bat status
lamp-control.bat brightness 60
lamp-control.bat temperature 4000

Troubleshooting

• Tray icon is red / "Not reachable": check that the light has power and is on the same network. If desk-lamp.local does not resolve, set the LAMP_URL environment variable to the light's IP address (for example http://192.168.1.50) and restart the app. lamp-control.bat uses the same variable.
• "Ctrl+Alt+L is used by another app": another program has registered the hotkey. The tray icon and panel still work.
• The light flickers on camera: the PWM runs at 1 kHz. Raise analogWriteFreq in the sketch if your camera picks it up.

License

MIT