Case file — 416B30E7
Launch roast · Show HN
Show HN: I turned my iPhone and a $20 smart plug into an f-stop timer by pentakkusu · HN thread
Unsolicited, from public launch info only. Founder? Reply or ask us to take it down.
The idea
“I turned my iPhone and a $20 smart plug into an f-stop timer https://peterszentkiralyi.eu/darkplug/ Founder's Show HN post: I am a film photographer and do some darkroom printing and always wanted one of the proper, advanced f-stop timers, but never really wanted to spend a couple hundred dollars on one. Eventually I figured my phone was already perfectly capable of doing the timing, it just needed a way to switch the enlarger. So I built Darkplug around a cheap smart plug. The app controls the enlarger over the local network and does regular timing, f-stop timing, test strips, has an intuitive dodge/burn workflow, split grade (2 timing channels) and paper development timing. I’ve been using it for a few months in my improvised bathroom darkroom. Fair warning: the app was blindly vibe coded using Codex and Xcode. Landing page content (Darkplug – Péter Szentkirályi): Darkplug – Péter Szentkirályi # Darkplug A darkroom timer for iPhone that controls your enlarger. Control your enlarger with Darkplug and a compatible smart plug on your local network. Built for film photographers and darkroom printers, Darkplug combines precise timing, guided workflows, and a red-on-black interface designed for darkroom use. ☝️ You only need this beside your enlarger and you are set (EU model pictured). ## Designed for darkroom printing Darkplug times exposures like a classic darkroom timer, and also provides more advanced printing workflows with a very unique feature set. Whether you are making a straight print, testing exposure, planning a complex dodge and burn sequence, or doing split grade printing, Darkplug keeps the process fast and straightforward. Darkplug uses a red-on-black interface designed to stay safe to next to photo paper. The controls are simple, legible in low light, and focused on the task in front of you. The red colour was chosen and tested so that it does not fog paper. ## What you can do with Darkplug - Time straight prints - Add an optional 4-second count-in - Control the timer with F-Stop steps (Darkplug+ feature) - Run guided test strips with second or F-Stops increments (Darkplug+ feature) - Plan and execute complex dodge and burn steps with an easy to understand unique grid system (Darkplug+ feature) - Save and restore dodge and burn plans with reusable plan codes (coming soon as a Darkplug+ feature) - Split grade (two channel) print timer (Darkplug+ feature) - Use a guided paper developing timer (Darkplug+ feature) | | | | | --- | --- | --- | ## Darkplug+ Darkplug+ unlocks advanced tools and workflows for printers who want more control. It is available as a one-time in-app purchase. No subscriptions. ## How it works Set up your smart plug and note its local IP address. Make sure the plug and your iPhone or iPad share the same local network. Connect your enlarger to the smart plug. Enter the plug’s local IP address in Darkplug, and connect. Once set up, you can control the enlarger directly from your iPhone or iPad. ### Compatible smart plugs Darkplug works with the following smart plugs manufactured by Shelly: 1 - Shelly Plus Plug S Gen2 🇪🇺 - Shelly Plus Plug UK Gen2 🇬🇧 - Shelly Plug S Gen3 🇪🇺 - Shelly Outdoor Plug S Gen3 🇪🇺 - Shelly Plug M Gen3 🇪🇺 - Shelly Plug US Gen4 🇺🇸 ## Support For questions, feedback, or support, email me at fill.cans82@icloud.com. ## FAQ See here.”
The bull case
Film printing has a price-sensitive, enthusiastic audience that is still getting its tools from expensive, aging hardware. A $20 plug plus a one-time purchase under about $30 is an easy yes for anyone who wants f-stop timing, split grade, and test strips. Marginal cost is near zero, there are no servers to run, and the Show HN pull (83 points, 35 comments) shows the niche pays attention when someone speaks its language. If the Wi-Fi timing proves accurate, this is a lifestyle product that clears a few hundred sales without ad spend and gets word-of-mouth in a tight community. Because the niche is too small to attract serious competitors, the founder could own it quietly.
The panel
Grounded in live search01 Market
mixedThe live data names no competitors, funding, or market-size figures, so I can't quantify growth or saturation. Structurally, this is a small hobbyist niche of film darkroom printers. The only price anchor is the founder's claim that dedicated f-stop timers cost "a couple hundred dollars." That gap is the strongest advantage: a $20 plug plus a one-time in-app purchase undercuts hardware by roughly 10x. The category is hardware enlarger timers, and I can't assess their activity from the data. The launch history shows only the founder's own Show HN, with 83 points and 35 comments. That's decent hobbyist interest, but it isn't evidence of paying demand. The most serious risk is the ceiling: a tiny niche and a one-time purchase cap revenue. Wi-Fi switching latency also threatens timing precision, which is the core promise.
02 Tech
mixedThe hardest problem is exposure accuracy over Wi-Fi. A burn of a fraction of a second, or a 1/6-stop step on a 4-second base, is a difference of tens of milliseconds, and HTTP round-trip jitter to a relay can exceed that. The fix is to send the plug a single "on for N ms" command so the plug's own timer ends the exposure. The landing page never says whether the app does this, and "blindly vibe coded" suggests it may not. The Shelly-only, local-API choice is the right buy over custom hardware. The weak spot is manual IP entry, since there's no discovery. There is no moat: the local API is open, and the dodge/burn grid can be cloned in a weekend. The well-chosen part is the cloud-free local control, which works in a bathroom darkroom with no account.
03 Finance
mixedDarkplug+ is a one-time in-app purchase, price unstated, so LTV is a single payment with no recurring revenue. CAC only works if it's near zero: darkroom printers are a tiny niche, paid acquisition can't recover a one-time fee (my rough estimate: $10-30), and the Show HN spike is a one-off. Pricing fits the value. Against the "couple hundred dollars" dedicated timers, plus a ~$20 plug, anything under about $30 is an easy yes, but the ceiling is low. Apple's 15-30% cut trims it further. Shelly-only hardware limits the addressable market, and you can't verify revenue because traction shows only upvotes. What breaks first is support time, since manual IP setup and a vibe-coded app will generate email. A working point: near-zero marginal cost and no servers mean it can be profitable at a few hundred sales.
04 Timing
mixedWell-timed, though for a small market. The film revival has pushed more people into home darkroom printing, and many are price-sensitive hobbyists who balk at a couple-hundred-dollar f-stop timer. That is the gap Darkplug fills. The macro trend that matters most is the durability of that revival, which depends on film and darkroom paper staying available and affordable. If supply tightens or prices climb, new darkroom entrants will dry up. The window is open and not closing fast, because the niche is too small to attract serious competitors. The real risk is cheap clones, since AI-assisted coding makes a copy easy. The favorable factor is that cheap smart plugs with local network control now exist, so no custom hardware is needed.
Cause of death
The core promise, timing precision, is unproven
The Tech Agent flagged that a short burn or a 1/6-stop step on a short base time comes down to tens of milliseconds, and Wi-Fi round-trip jitter can exceed that. The landing page never says whether the app sends one "on for N ms" command so the plug ends the exposure itself, or whether the phone toggles on and off over HTTP. For a product whose pitch is "proper f-stop timer," inconsistent prints will kill trust fast in a community that obsesses over repeatability.
The revenue ceiling is low and the data to prove otherwise is missing
The Finance Agent estimates a one-time price somewhere in the $10-30 range (an estimate; the price isn't stated), minus Apple's 15-30% cut. Darkroom printers are a tiny niche, the Shelly-only requirement narrows it further, and upvotes aren't revenue. It can be profitable at a few hundred sales, but there is no recurring revenue and no paid channel that pays back a one-time fee.
There is no moat, and AI-assisted coding makes cloning cheap
The Tech Agent notes the local API is open and the dodge/burn grid can be rebuilt in a weekend. The Timing Agent points out the niche is too small for serious competitors, but a hobbyist with Codex can do the same thing the founder did. The product also leans on a supply-dependent film revival. If paper or film gets scarce or expensive, new entrants dry up.
Blind spot
The failure mode isn't inaccuracy, it's a stuck "on." If the phone sleeps, the app backgrounds, or the Wi-Fi drops mid-exposure, the enlarger lamp can stay lit and fog an expensive sheet of paper. Printers will forgive a few milliseconds of drift in a cheap tool, but not ruined fiber paper and a lost session. The founder uses this daily in his own setup and may not feel how often a stranger's flaky bathroom Wi-Fi breaks the chain. Manual IP entry also invites support email, and every support email is time the one-time revenue doesn't cover.
What would need to be true
Wi-Fi-controlled exposures must repeat within a tolerance that serious printers accept, with plug-side timing and a safe failure mode.
Enough film printers must be willing to buy a Shelly plug plus a Darkplug+ upgrade to reach a few hundred paid users without paid acquisition.
The film and darkroom paper revival must keep drawing new, price-sensitive printers, with supply and prices staying stable enough that the niche isn't shrinking.
Actions to take this week
Measure your own exposures this week: run 50+ exposures each at about 0.5s, 2s, and 8s, and record the real lamp-on time with a light sensor or high-frame-rate video. A positive signal is a spread of under about 20ms. Publish the results on the landing page.
Check whether the app sends one timed "on for N ms" command so the plug ends the exposure. If it toggles on and off from the phone, change it, and add a plug-side failsafe that shuts the lamp off if the connection drops.
Pull your App Store Connect numbers for the Show HN spike to see how many downloads turned into Darkplug+ purchases. If conversion is under about 2-3% of downloads, test a different price point or move one more feature into the free tier.
Email or reply to the Show HN commenters who asked questions and ask three things: which timer they use now, what they paid, and what price they'd pay for Darkplug+. A positive signal is five or more naming a number at or above your price.
Cut setup friction to reduce support load: add local-network discovery or a QR-based setup in place of manual IP entry, and state the Darkplug+ price on the landing page.
Work through these, then roast the idea again with what you learned.
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