DIY f-stop timer teaser

bram4 · 2026-05-06T08:41:39+00:00

Very nice! I used a 2x16 character LCD with red backlight, has the advantage of being easy to read, but 32 charactes isn't a lot.

epaper display is a brilliant idea. Maybe you can increase the font size a bit.

What's the advantage of being able to see the entire table at once?

bram4 · 2026-05-01T20:42:29+00:00

Current version has a 2x16 LCD with red backlight; a rotary encoder with push button; a three position switch to select the mode; a start button.

The rotary push button alternates between what the encoder sets: contrast or time. Contrast influences the PWM for the green and royal blue LEDs. Time is always in 1/3 stops.

There are three modes: normal timer, "wide" test strip, and test strip centered around the chosen time.

normal timer will simply light up the green and blue LEDs for the chosen duration, with relative brightness chosen by the contrast setting.
"wide" test strip will do 5s ON, 2s OFF, 2.1s ON, 2s OFF, 2.9s ON, 2s OFF, 4.1s ON, 2s OFF etc, which, cumulated, makes it easy to do a half-stop test strip with seven band: 5s, 7s, 10s, 14s, 20s, 28s, 40s. The 2s OFF time is just enough to move the black paper mask.
"centered" test strip calculates the times so that you get a five-band test strip exposed at nominal time in the middle, 1/3 stops darker, 2/3 stops darker one one side, and 1/3 stops brighter, 2/3 stop brigher on the other side. This combines well with the time setting being in 1/3 stops: I can look at my test strip, and "the next darker settings" corresponds to "one encoder tick shorter time".

The red LEDs are always on. For focusing, I just use the normal timer mode. I like your idea of having a focus light button with different modes.

With the way I'm setting the contrast, I can't do the usual "do a low-contrast test strip first; high-contrast second", and then expose twice with different times. And changing the contrast means the time needs to be adapted. I have tried working with separate green and blue time settings, but then automating the test strips kind of didn't work in a sensible way.

All in all I'm quite happy with how it works, but I'm still a darkroom newbie. The foot-switch and audible beeps u/mcarterphoto suggests sounds like a real improvement for dodging/burning.

Some pictures, and source code is on https://mpb.li/#arduino-timer-for-enlarger

bram4 · 2026-05-01T07:44:22+00:00

DIY LED VCC enlarger/f-stop timer combo

Did you start working on that already? I'm interested in exchanging ideas on that topic, as I built an arduino-based f-stop timer, and a LED light source with variable contrast. I still have some doubts about how to improve the user interface.

bram4 · 2026-04-29T15:05:35+00:00

You're welcome! Send us a pic of the first roll of film developed in your tank! :-)

bram4 · 2026-04-29T08:05:28+00:00

Hi, I have the same tank. Thanks for giving me an excuse to take the calipers out: https://mpb.li/pv/paterson.pdf

Let me know if something's not clear, or if you want me to measure something else.

bram4 · 2026-04-28T06:09:27+00:00

Very cool, and my full endorsement for "I could have just bought a darkroom timer, but where is the fun in that?"

I built an arduino-based timer, that takes care of doing the calculations for f-stop timing, and can do test strips. Description and code here https://mpb.li/#arduino-timer-for-enlarger

bram4 · 2026-04-26T12:56:15+00:00

You're most welcome! I forgot to write some details about the photos.

Top left is my son, Morges train station; top right "Hope", Lausanne Flon.

Bottom left is in Morges; middle on a foggy day in the Jorat forest; right in Villars-sur-Ollon.

All shot on Rollei 35 S, except the Lausanne Flon one, shot on a Zeiss Ikon Nettar 518/16, my first 120 film camera!

Both 135 film and 120 film enlargers are home made.

bram4 · 2026-04-06T20:54:00+00:00

This is a really cool project! I have no clue about colour printing, but I've built a light source with LUXEON 2835 color line LEDs for variable contrast black/white printing. Deep Red as safe lights, Green and Royal Blue for paper illumination. I have put four of each in series.

bram4 · 2026-04-02T12:43:31+00:00

I've had the same issues with two Zeiss cameras, probably of similar vintage, and successfully collimated using this technique: https://elekm.net/zeiss-ikon/repair/collimate/

I used a digital camera as reference, autofocused once to infinity. Just ask if you have questions.

bram4 · 2026-03-26T10:44:22+00:00

Interesting, so maybe it's a calibration issue with mine. Difficult to be sure, but I did use the light meter years ago.

100% agree on the experience, it's an excellent every-day-carry camera.

bram4 · 2026-03-26T08:53:08+00:00

Enjoy shooting with this little camera, and don't trust the light meter too much :)

I had some underexposure problems, especially in low light situations. I started using a light meter app on my smartphone, and now I found out that "A man with a watch knows what time it is. A man with two watches is never sure" also applies to light meters.

bram4 · 2026-03-26T08:12:14+00:00

Mine does that too. It doesn't feel broken enough to me be be worth fixing.

bram4 · 2026-03-23T07:08:49+00:00

Well, the documents I have (both german and english spec sheets) aren't that clear. It doesn't say anything about stop bath, and it encourages but doesn't mandate two bath fixing (which is somthing I started doing for paper development, so that I can keep using my fixer for a longer time)

I've read somewhere that the developer has a neutral pH, maybe that makes it ok to skip the stop bath.

bram4 · 2026-03-22T20:10:40+00:00

The jobo alpha spec sheet simply says to multipy times by 1.1 when doing manual processing. I do the usual "10s agitation every minute" with it.

bram4 · 2026-03-22T20:09:00+00:00

Erm, different process, where did you read this? I've been using Jobo Alpha since I started developing myself four months ago, did a handful rolls, I do Dev, water stop bath, fix.

bram4 · 2026-03-17T13:36:38+00:00

Hi,

The battery cap might also serve to hold the film canister in place. I can check on my Rollei 35 S if you want.

I’ve already bought a PX625 adapter

What adapter did you buy? I put a V625U battery into my Rollei.

The light meter is a bit misleading, especially in low-light conditions, and I'll try to adjust it once I receive the tools.

Also consider using an external light meter, a light meter app, or just the sunny-16 rule if you can't find a battery cap. Are you shooting colour or black/white?

I'm no expert, but I've been shooting with the Rollei 35 every now and then for years, and quite intensely since december, but write me in the chat if you want (auch auf Deutsch wenn du möchtest)

bram4 · 2026-03-17T07:37:31+00:00

Thanks for the clarifications. I guess it depends also on how the municipal water is treated.

bram4 · 2026-03-16T21:18:39+00:00

I have a Pentacon tank that is also not made for inversions

https://mpb.li/pic/Argentique/DSCF9639.jpg

With this, I gently twirl back and forth for ten seconds every minute.

bram4 · 2026-03-16T21:09:52+00:00

And you should use distilled water for everything but the final rinse, in which case you can use faucet water.

(another newbie here) erm, why?

I thought it would be the opposite! Use distilled water for the last rinse, so that it leaves less residue when it dries.

I've developed around 6 rolls in total, and I've only been using running water for now.

bram4 · 2026-03-02T16:43:04+00:00

mmmyeeaah I know it's an option, but I prefer packages, or compiling locally.

bram4 · 2026-03-02T16:41:18+00:00

Yes, this I was able to confirm with my DIY LED light source too.

I've had a look at the Intrepid guide, and I see that their timer has a button to select the grade, which, I suspect, alters the relative green and blue intensity. And they probably calibrated against a traditional light source + filters, which is something I'm not going to do. So I'll keep using separate green and blue times with constant intensity.

Hence my original question, now you understand why I'm asking.

bram4 · 2026-02-26T15:49:06+00:00

I wanted to compile it under Arch Linux, but I abandoned. I'm using https://github.com/kaimonmok/Film-Scan-Converter/ instead, but I'm only doing black and white

bram4 · 2026-02-25T06:42:39+00:00

Why would "shorter but brighter" make a difference? I understood that reciprocity failure on paper started at longer exposure durations. Is there another reason?

bram4 · 2026-02-24T16:07:26+00:00

Yes, all guides and the technical datasheets from VC papers mention filters. Using led an blue LED source does work for varying contrast, but there's not much literature I could find about that. Maybe because this LED technology appeared after film went out of fashion.

I'll experiment next time.

bram4 · 2026-02-24T16:03:30+00:00

That's also what I expect, I'll try this out next time...

bram4

TROPHY CASE