View Full Version : DIY light meter and bulb exposure timer

c.a. church
03-26-2009, 04:40 PM
Hi all - it's been a while since I've posted on here, but I haven't forgotten about f295 *grin*!

This past fall I was working on a project for dSLR time-lapse, but had pinhole photography support as well.

The goal was to create an off-camera light meter that would control the exposure of a camera directly (in bulb mode, using a standard wired remote cable from the device) during a shot interval sequence, controlling exposure in 1/100EV steps. Regarding pinhole, I intentionally let the f/stop setting go on until a full floating point integer had been filled (that's pretty darned high). So, it can easily be used to meter strange combinations like f/375 and ISO 25. The only change that would have to be made to the existing code would be to remove the intervalometer settings, and make one of the buttons trigger the camera exposure. If anyone's interested I can provide a patch in no time. It controls exposure time in millisecond increments, up to about 48 days exposure.

It can be built for less than $100 in parts, using an arduino board, a cheap lcd, a simple opto-coupler, some buttons, whatever connector styles you want, and a chip from taos. If you wanted to trigger an old-style mechanical cable release, that can be done using a simple linear actuator, I'm sure, and I could write that up for you as well. I've also done some write-ups on how to do photographic measurements and conversions, if anyone's interested in further reading, they can follow the links.

Anyhow, here's the link to the information, source code, tutorials, etc. I'd appreciate any feedback, and if you want to adapt it to your particular camera, I'd be happy to help w/ that as well.



Doug K
03-27-2009, 09:59 PM
Wow, that's impressive, and you've taken all the work out of me having to figure this stuff out. This looks like a future project for me, and it's quite handy that SparkFun is near me. I work in controls, though as a maintenance grunt, but this is definitely something I think I should be able to do. I'll definitely ask questions when I get cracking on this. And, I'd definitely be interested in using this on a DSLR as well since I do cross over to the digital world.

03-28-2009, 10:18 PM
This reminds me of this (http://www.freewebs.com/johnfletcher/). I might like to try building this for the 11x14 pinhole im building, right now I have a speed-i-o-scope for the camera, so I'll need to utilize a linear actuator for it.

c.a. church
03-30-2009, 10:46 AM
Willrea - interesting - I didn't know that was out there. It's neat that he added lookup tables for film-based reciprocity failure. As the original project was for dSLRs, I hadn't even considered adding such a thing. (They don't suffer from it.)

The best case scenario when using a linear actuator is to choose one that does not require power held to remain in position. This will help reduce power consumption in the field. What I really meant to say when saying "linear actuator" was "solenoid" - in my circle, we've tended to call them the same thing, but there are motor-driven linear actuators that would be more expensive, although would probably work just fine.

Doug - when you get started, I would be more than happy to answer any questions you have!


03-30-2009, 12:58 PM
Yeah, the reciprocity failure data would be quite helpful.

Doug K
03-31-2009, 10:21 PM
!c. thanks. I appreciate it, and I also appreciate the work you've done to get this done. I'm thinking the control scheme will be easy, since you've done all the work, but the creative end for me will be the output side. I'm employed in an industrial world, so the parts I'm familiar with are large and durable. I don't get to see the more interesting experimental side with components you've used here. I think shutters with a "t" setting would work well with this since you could fire the output solenoid once at the beginning of the exposure, and once at the end. Would indeed save battery life. Plus, that's how my Nikon works in the bulb setting with the wireless remote. Thanks again, you've got me thinking.

c.a. church
04-01-2009, 10:43 AM
No problem Doug - you can call me "Church" btw, everyone else does =)

It should be very easy to trigger a solenoid -- just remember that it's going to draw more mA than the arduino pins or a 4N28 opto-isolator can handle, so you'll need a transistor on the solenoid-end of the opto-isolator, or just replace the opto-isolator with a solid state relay (which combines both).

If you want to use reciprocity failure lookup tables (as mentioned above), without changing too much code, I'd suggest buying one of the new arduino boards with a 328p chip instead of the older 168p. The reason is that they have double the SRAM, and the sketch I provided pushes the 128 to the very edge of its memory capabilities. They're the same price, effectively, although some vendors have dropped the price on the old ones to try and move them out the door.


04-01-2009, 01:12 PM
Here's (http://f295.f295.org/uploads/Blah.pl?m-1154706328/s-all/)an old thread on F295 about an electronic shutter project.

And here's (http://f295.f295.org/uploads/Blah.pl?m-1143527112/s-new/#num9) an old thread on F295 about a homebuilt CdS meter project.


Doug K
04-07-2009, 09:24 PM
I got my arduino board and taos chip today from Sparkfun. I did get the 328p chip. I won't have time for a week or two to work on this, too many outdoors activities to distract me with the weather turning nice. Well, it's often nice here in Colorado, but you catch my drift. I'm looking forward to getting going on this though. I have an idea in my brain for an output that is modular. So I'd have one that works with the wireless remote on my Nikon, and one with a solenoid to operate a mechanical shutter. Looks like fun!

05-04-2009, 11:57 PM
Absolutely brilliant. This is the correct approach using today’s technology. The idea of non-linear look-up tables to handle reciprocity correction is intriguing.

01-12-2010, 10:13 PM
Anyone get this working?