I found a DIY TLR camera kit that lets you build the camera from parts, then use it to take photos. What I don't understand, is that the specs list:
1/150s shutter speed; f/11 aperture
From that description, it makes it sound like it has a fixed shutter speed and aperture. Assuming I use 24 or 36 exposure 35mm film, am I limited to one set of exposure values for the entire 24 or 36 frames? I am thinking I might be misunderstanding how TLR cameras work.
This is very similar to a Holga medium-format toy camera, where the shutter speed is approximately ¹/₁₀₀th of a second (give or take the particular camera you have and how it is feeling today) and the aperture is about f/13 (regardless of whether you have the alleged aperture lever set to one of its two non-functional options).
So how do you get the exposure right? You shoot in lighting that's right for the film speed you've chosen. You depend on the greater exposure latitude of film when it's off by a bit, and don't worry so much about getting it perfect. Or close to perfect. If you wanted it perfect, you wouldn't have a plastic DIY camera, right? Rather than control and execution of vision, it's about happenstance and creation through serendipity.
PS: this isn't normal for TLR cameras in general. It's normal for toy cameras, though.
This is meant as an amplification of mattdm's answer, not as a competitor. Editing part or all of this into what is already a good answer would be every bit as welcome as leaving this standing as an addendum. It's just some old-fogey background.
Believe it or not, that's how most "consumer" cameras worked for a very long time, and even the idea that you have to focus would have been foreign to the average holiday snapshooter of a couple of generations ago. Brownies and Instamatics (and similar cameras from other makers) were what most people used. Automatic exposure for civilians came along when I was a kid, and the only knobs and buttons most people ever had on their cameras were the film advance knob and the shutter release button. Let's just say that you'd be amazed at the latitude of film, especially B&W.
f/11 at 1/150s is just barely more than a two and a third stop overexposure for the canonical "sunny sixteen" scenario for ISO (ASA) 400 film. That may sound like a lot, but keep in mind that most Zone System photographers found their N (normal) exposure for Tri-X (and other 400-speed films of a similar generation) at 200 or 250, so conservative development means that this is only about a stop and a half overexposed. You might get some blocking in the extreme highlights under the brightest conditions, but most people wouldn't be worried about holding details in the clouds. By the same token, a two-stop underexposure (relative to N) isn't a complete tragedy -- you can easily get an acceptable print, even if it isn't quite up to Ansel Adams' standards. Plus-X (ASA 125 film) was a better "outdoors" film, and a good match for the non-TTL-regulated, can't-see-for-a-week output of flashbulbs. (If you're too young to remember them, be thankful—the Christmas morning snaps were crippling, and if you were the photographer you got a few burnt fingertips before you remembered that "hot enough to soften glass" is very hot indeed.)
Colour print film isn't quite as forgiving as B&W, but between a stop and a half overexposure and a stop or so underexposure of the rated speed you can still get very good results, and an extra stop in either direction is snapshot salvageable. (A third to a half over is a "normal" exposure for better-saturated colours for most print films.) An arsenal of films at 100-, 200- and 400-speed films will see you through most situations, especially if you can pair them with an auto flash (if you have a mind to, there's no reason why you shouldn't be able to build an X-sync, or even an M-sync if you want to try flashbulbs, into this beast). If graininess is what you want, then higher speed films (400 and up) with an easily-fabricated and cheap slip-on ND filter (or even an external aperture stop) will bring you safely into the bright sunlight.
Shooting chromes (does anyone do that anymore?) will take a lot more work. They don't have a lot of latitude since the initial exposure is the final product and you can't soup individual frames. You'd have to work with an external light meter, the actual T-stop (the transmissivity of your lens, which may be significantly different from the f-stop) and external ND filters or aperture stops. It's certainly possible, but it seems, somehow, to be at odds with the philosophical aim of the camera.
Judging by the parts, I'd say the shutter speed is fixed and there is nothing there to make a variable aperture, unless they have swappable discs that can stop down the lens. I think that's how the lensbaby works. But otherwise yes, for this $20 DIY camera you're going to have to take everything at a fixed EV :)
Luckily, modern film has great latitude in exposure. You could always apply the "Sunny 16" rule. That is, for any film, at a shutter speed of 1/ASA (for example 1/400 for 400 ASA, 1/250 for 250 ASA) , the following apertures apply :
f/16 Sunny
f/11 Slight overcast
f/8 Overcast Barely visible
f/5.6 Heavy overcast
f/4 Open shade/sunset
Since your speed is fixed at 1/150, and your aperture is fixed at f/11, using a (close to ASA 150) film like Fomapan Classic ASA 100 35mm Black and White film in Sunny, Slight Overcast, or Overcast conditions should work out just fine.
