[Marc] has an old Voigtländer Vito CLR film camera. The camera originally came with an analog light meter built-in. The meter consisted of a type of solar panel hooked up to a coil and a needle. As more light reached the solar panel, the coil became energized more and more, which moved the needle farther and farther. It was a simple way of doing things, but it has a down side. The photo panels stop working over time. That’s why [Marc] decided to build a custom light meter using newer technology.
[Marc] had to work within the confines of the tiny space inside of the camera. He chose to use a LM3914 bar display driver IC as the primary component. This chip can sense an input voltage against a reference voltage and then display the result by illuminating a single LED from a row of ten LEDs.
[Marc] used a photo cell from an old calculator to detect the ambient light. This acts as a current source, but he needed a voltage source. He designed a transimpedence amplifier into his circuit to convert the current into a voltage. The circuit is powered with two 3V coil cell batteries, regulated to 5V. The 5V acts as his reference voltage for the display driver. With that in mind, [Marc] had to amplify this signal further.
It didn’t end there, though. [Marc] discovered that when sampling natural light, the system worked as intended. When he sampled light from incandescent light bulbs, he did not get the expected output. This turned out to be caused by the fact that incandescent lights flicker at a rate of 50/60 Hz. His sensor was picking this up and the sinusoidal output was causing problems in his circuit. He remedied this by adding two filtering capacitors.
The whole circuit fits on a tiny PCB that slides right into position where the original light meter used to be. It’s impressive how perfectly it fits considering everything that is happening in this circuit.
[Thanks Mojay]
That LT1761 is an interesting piece of electronics
I thought I remembered my dad had a handheld light meter that needed no batteries, but I failed to find any such thing when searching recently, so I was beginning to question my sanity.
That’s a selenium cell meter! Cool, but near impossible to repair and no more accurate than a modern meter.
Selenium meters are neat. When I was a wee lad in the 70’s my Grandmother would buy me interesting finds at garage sales. One day I scored a small selenium meter (for maybe a quarter or .75). It was disc shaped and about 3″ in diameter. When you pressed a button, the protective cover would spring up, exposing the meter. Some spring loaded doors on the end would also pop open to expose the cell.
It was really cool, and a lot like a communicator, with a bit of tricorder functionality. Beam me up Scotty!
Now days I finally have one of my ultimate gear lusts, the Gossen Luna Pro SBC light meter. It has the ability to measure much lower light levels than most any other meter, making it excellent for night photography (especially with film). The analog interface – the needle, the dial – is superb. Years ago I might have said it was a bit bulky (and it is), but I have come to appreciate the size of the letters and numbers, especially in low light.
I still have a serious need for a Pentax Digital Spot Meter. I’ve thought a modern version would be a great hack, but it would be very challenging.
I seriously recommend the (much older, and very analogue) Pentax Spotmeter V if you’re interested in film photography. While the 1970s camera I’ve been using has a fully-functional meter, it’s not a spot meter – which is Really Useful for the zone system.
I got mine for ~$75 off eBay. It meters down to ‘Really Quite Dark’. It’s not an ambient-light-meter like the Gossen thing you’ve got (which I’m now also lusting after) so should complement it nicely.
Thanks. I was leaning toward the digital version of the pentax due to the smaller size, and it’s probably a bit more rugged without a mechanical meter. Swoopy lines are also kinda sexy compared to the hard edges of the classic V.
I really do need a good spot meter. Shooting medium format film is expensive, and you only get ten shots before the somewhat involved reloading (doing that in misting rain, blowing dust, or on a sunny mounting peak, not fun). Shooting a film like Velvia means careful evaluation of the scene lighting. And you may be waiting for 10 or 20 minutes for the clouds to shift (or never!), leaving you brief seconds with the light you want. And, if you are going to take two shots to bracket, you sometimes have the “I’m on shot 10 so need to reload for the second shot” problem.
Using a digital camera is a useful crutch but awkward. With a whole bunch of experience you get a whole lot better at metering without using all those fancy tools, but that takes time and care. An ideal tiny digital with a manual mode and good zoom might be better than a spotmeter, because it could also preview exposure and scene. Maybe one of those older canons? It’s hard enough to hike (especially in rare places like Alaska) with a medium format camera, two lenses and a tripod. Add a DSLR to that and oh my back.
One thing I really like about film and careful metering is that you are engineering the photograph using the prevailing constraints of the system components – light, shifting cloud cover, wind that is shaking your tripod – and the particular film medium, DOF, etc. Of course if a bug flies across your lens at just the wrong moment, your image is screwed. I hate the spray and pray of digital, where people take a bunch of blurry and varied exposures hoping for something.
This is a great hack.
Did this camera originally show a mechanical metering needle within the viewfinder? Or was it not integrated with the view finder?
I’m going to guess that this doesn’t have a meter/needle-matching viewfinder. It may be too old even for that.
This is an awesome build! Now, go build a digital back for that camera. (c:
http://camerapedia.wikia.com/wiki/Voigtl%C3%A4nder_Vito_CLR
I remember trying one of these in 1968 when I had a Saturday job in a camera shop. A customer had brought it in to be checked, as he had bought it second hand.
There seem to be more than one version of the meter. The spec at the above link suggests a needle in the viewfinder, but I don remember that. The meter was on the top and I had to set the exposure manually. Nice camera to use. I ran a roll of B& W film through it and the results were very good. the shop had it’s own dark room so we could process film and make prints ourselves. Imagine being able to do that now ;)>
One of the only downsides is that the colour response of the selenium meters was quite close to the response curve of most film. The silicon panels aren’t quite right for it, sadly. It’s why selenium meters still get rebuilt with selenium panels.
Great read and lots of interesting comments. I’m not much of a photographer, but it is always great to see folks getting use out of older gear and lovingly keeping it updated and working :)