Second Screen App

Overview

Most people who want a second monitor either buy one or go without. I wanted a third option one that uses the tablet already sitting on the desk. The Second Screen App allows any device with a browser to act as a real-time extended display using WebRTC.

Devices connect in under 5 seconds using a simple 6-digit code, with no proprietary software, no complex setup, and no additional hardware required.

Problem

Studies show that a second monitor can improve productivity by up to 42%. However, external displays cost $150–$400, and wireless screen-sharing tools often require subscriptions.

Meanwhile, tablets and phones are already available but lack a simple, reliable way to function as secondary displays. There was a clear gap for a lightweight, free, and cross-platform solution.

Solution

I built a cross-platform system where a desktop Electron app captures the screen and streams it using WebRTC. A secondary device simply opens a browser, enters a 6-digit code, and instantly receives the display.

Supabase acts as the signaling layer to exchange WebRTC offers, answers, and ICE candidates. Once connected, all data flows peer-to-peer eliminating latency overhead, server costs, and subscription requirements.

Key Features

• Real-time screen streaming using WebRTC (peer-to-peer)
• Under 150ms end-to-end latency on local networks
• 6-digit session code for pairing devices in under 5 seconds
• Works on any device with a browser no install required
• Electron desktop client for screen capture
• Secure signaling using Supabase Realtime
• Session codes expire after 5 minutes or after use
• Built with TypeScript across the full stack

Tech Stack

System Architecture

The Electron app captures the screen using the desktopCapturer API and creates a WebRTC connection. The offer is stored in Supabase along with a session code.

The second device retrieves the offer, sends an answer, and both sides exchange ICE candidates through Supabase until a direct peer-to-peer connection is established typically within 3–5 seconds. After that, all streaming happens directly between devices.

How It Works

1. User opens the desktop app
2. Screen capture starts using Electron's desktopCapturer API
3. A WebRTC session and 6-digit code are generated
4. Secondary device enters the code in browser
5. Supabase exchanges offer, answer, and ICE candidates
6. Peer-to-peer connection is established in under 5 seconds
7. Screen streams live with <150ms latency

Demo

Challenges

• ICE candidate timing issues: Candidates were arriving before remote descriptions were set, causing ~30% connection failures. Solved by queueing candidates until ready.
• Latency & CPU optimization: Screen capture initially used 40–60% CPU. Optimized to 30fps/1080p with controlled bitrate, reducing CPU to <20% while maintaining <150ms latency.
• Session security: Designed a 6-digit code system with expiry and single-use invalidation to balance usability and security.