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Project ideas from Hacker News discussions.

Show HN: I turned my iPhone and a $20 smart plug into an f-stop timer

Original Article

Hacker News Discussion

📝 Discussion Summary (Click to expand)

1. Latency and timing accuracy of smart switches
Commenters repeatedly questioned how quickly a command from the app translates to the switch actually turning on/off, and what jitter this introduces for precise photographic exposures.

  • “I always assumed … there would be too much jitter in the actual timing … how stable is the delay from the app sending a command until the switch operates?” — arnarbi
  • “I'd probably put the typical smart switch hardware/software stack at around 0.25 seconds. Maybe you can … but I don't think I'd try to rely on anything much faster than that.” — alnwlsn

2. Darkroom‑specific display requirements
The discussion emphasized that the UI must use a red‑on‑black (or similarly safe) palette to avoid exposing photographic paper, while also noting the need for readability outside the darkroom.

  • “Isn't it more than a nod, and actually a hard requirement to be considered for use in an active darkroom?” — baoooooooooooo
  • “I get that red‑on‑black color scheme is a nod to the red lighting of a dark room, but the contrast is so low I can hardly read the example screenshots.” — pimlottc

3. Hardware implementation choices
Several users weighed using stock smart‑switch hardware against flashing custom firmware (e.g., ESP8266/ESP32) or employing OLEDs to achieve low latency and proper spectral output.

  • “If you did, I would go in the custom firmware for the smart switch direction. A lot of them are just an ESP32 inside …” — alnwlsn
  • “It’s not a nod. It’s required to not ruin the paper during use. A specific yellow would also work but phone screens often output more than just the desired color. Red is the safest.” — guax

🚀 Project Ideas

Precise Darkroom Exposure Firmware for ESP32 Smart Plugs

Summary

  • Replace stock smart‑plug firmware with a low‑latency ESP32 binary that accepts millisecond‑precision on/off commands over Wi‑Fi or MQTT.
  • Provides sub‑millisecond switching (limited only by AC zero‑crossing) for reliable darkroom exposure timing.
  • Core value proposition: deterministic control of enlarger light without relying on cloud‑induced jitter.

Details

Key Value
Target Audience Darkroom photographers, DIY electronics hobbyists, makers using ESP32‑based smart plugs
Core Feature Precise timing control (≤0.01 s latency) with configurable exposure sequences via simple HTTP/MQTT API
Tech Stack ESP32 IDF, FreeRTOS, MQTT or HTTP server, optional OTA updates
Difficulty Medium
Monetization Revenue-ready: One‑time hardware sale $29 (pre‑flashed module)

Notes

  • HN users complained about timing jitter (“0.25 seconds”, “need 0.001 seconds”) and expressed interest in custom firmware (“go in the custom firmware for the smart switch direction”).
  • A deterministic local controller eliminates dependence on app/cloud latency and directly addresses the need for repeatable exposure times.

Darkroom‑Safe Timer App with Local Network Control

Summary

  • A mobile app (iOS/Android) that displays a red/orange UI safe for darkroom use, lets users set exposure durations, and sends instant on/off commands to locally‑connected smart plugs (via Wi‑Fi or Bluetooth LE) while measuring and compensating for device‑specific latency.
  • Core value proposition: accurate, repeatable exposure timing without exposing the user to unsafe light or relying on uncertain cloud delays.

Details

Key Value
Target Audience Film photographers who use enlargers and smart plugs in a home darkroom
Core Feature Red‑safe UI + timer + local‑network plug control with automatic latency compensation
Tech Stack React Native (or Flutter), Redux, ESP‑Now/BLE or Wi‑Fi socket, optional calibration routine
Difficulty Low
Monetization Hobby

Notes

  • Commenters noted the UI contrast problem (“contrast is so low I can hardly read”) and the necessity of red lighting (“hard requirement to be considered for use in an active darkroom”).
  • The app solves both the visibility and timing accuracy concerns raised by arnarbi, jrflo, and alnwlsn.

Smart Plug Latency Calibration Service

Summary

  • A lightweight service (runs on a Raspberry Pi, laptop, or in the cloud) that measures the end‑to‑end delay from command issuance to actual relay actuation for a given smart plug model/store, then applies a programmable offset to achieve target exposure times.
  • Core value proposition: turn any off‑the‑shelf smart plug into a reliable darkroom timer without firmware flashing.

Details

Key Value
Target Audience Users who prefer to keep their existing smart plugs (e.g., TP‑Link, Sonoff) and want precise timing
Core Feature Automatic latency measurement + offset application via local API or Home Assistant integration
Tech Stack Python, pigpio/gpiozero for GPIO probing, MQTT broker, optional Docker container
Difficulty Medium
Monetization Revenue-ready: SaaS subscription $5/month (hosted) or free self‑hosted

Notes

  • alnwlsn estimated “0.25 seconds” latency and suggested custom firmware; this service offers a non‑invasive alternative that HN tinkerers could run locally.
  • Provides a practical utility for discussion: users can share calibration curves for different plug models, fostering community‑driven improvements.

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