Computer Sound Port Shutter Tester

I  have implemented a sound port shutter tester, which is described later on this page. However I am disappointed with it's inconsistent performance at higher shutter speeds and the inability to make the HW work on different computers. In order to rectify the situation I've designed a shutter tester that interfaces directly to an old Apple ][ computer, and am much happier with that solution.   I highly recommend moving away from the sound card shutter tester to a dedicated HW solution.   See my Apple ][ based tester page for the latest on that design or to Optical Timer/Counter page for my latest micro-controller based design.

At the minimum, a calibrated constant light source is needed in order to obtain consistent turn on and turn off signal slopes with this design.  I never could get good results at higher shutter speeds, as the results tended to show a slower speed than the the exposure is seen with my new design.  If you are having trouble calibrating with this design at high speeds, I would suspect troubles with the sound port shutter tester.

picture of shutter tester
Sound Card Shutter Tester

I will not be held responsible for any damage that results from you looking at or following these ideas and designs. If you don't feel comfortable goofing around with this stuff or the possibility that you could damage something on your computer or camera, just don't do it.

This shutter tester is based on the design at . I thought that I have enhanced that design in several important ways, when just before posting the design to my website, I found this wonderful site, with a very similar design- . Having been involved in computer and networking designs for many years, I have come to the conclusion that if you have a novel idea, it is very likely that someone else has the same idea. Looks like I was right, at least in this case.

Anyway here is what I have done to enhance the original design.

The results of these modifications shows up in the sound capture shown here. You should be able to find a sound capture program to suit the computer that you are using. However be forewarned that not all computer line inputs circuits and capture software are created equal. I tried this design on one PC and couldn't get reliable results. In fact, the capture showed both channels having the exact same signal timing despite the claim that the inputs were stereo. I don't know if that system needed a different circuit design or not, but it didn't work very well.

Measuring Shutter Curtain Speed
You can measure shutter curtain speed by taking the time difference between the first and second channels. For instance, in this case, the second channel trails the first channel by about 5 milliseconds. The distance between the sensors is about 14 mm. Total time for the curtain to cross the entire film plane is then 36mm(actual film plane size)/14mm(distance between sensors) * 5 milliseconds = about 12.8 milliseconds. You can also tell if the first and second shutters are running at the same speed. The total exposure times of both channels will be different, if one shutter is running faster than the other.

Measuring Exposure Time
I measure the exposure time by measuring from the falling edge of the signal to the time just as it begins to rise. In this case, the exposure time is about 18 milliseconds, which is a bit long for a true 1/60th of a second.


The Circuit Diagram
This diagram shows the simple shutter tester circuit along with components used. You can probably use almost any NPN photo transistor in this circuit. I found mine at an Active Electronic Store. For initial testing, I recommend a resistor setting of around 3K ohms.

back of shutter tester

I first made this simple version of the shutter tester on a block of wood. This is pictured above. You should cut a block of wood that would fit well onto the back of the film plane of your camera. I just opened the back of the camera and measured the area to get the size. Then drill two holes 18mm apart to hold the photo transistors. These holes should be positioned so the photo transistors will be centered on the film plane, when the block is placed against the back of the camera. This version has the holes 14mm apart, but 18mm would be better since you can calculate shutter speed by simply doubling the measurement obtained. Drill the holes as small as possible. I actually used 2 different sized drills for each hole. First I went all the way through with a bit that was only big enough to allow the small part of the photo transistor lens to fit. Then I drilled a slightly larger hole from the back only about half way through the block. Once done cutting and drilling you should paint the face of your block black, in order to prevent reflected light from affecting your reading.
block cross section
cross section of wood block showing holes for photo resistors
The photo transistor is fitted into the back of the wood and the small part of the lens is seated in the smaller hole. Solder, screw and glue the other devices together, to the cable and to the block. I used standard 4 wire telephone wire in my implementation, but you may use whatever is handy and has at least 3 conductors. You can use a hot melt glue gun to attach the photo transistors and resistor to the block. You might want to also nail an insulated staple as a strain relief over the cable and to the block.
new version

Later, after looking at the cosmetics and realizing that I wanted to document my efforts on this web site, I decided to package it in a very small project box. That is pictured here. There is no difference in performance of the two versions. The project box version uses a smaller block of wood which has been epoxied to the inside front of the box. I did omit the stepped drilling of the wood and just seated the transistor in the smaller hole, since the box was enclosed and the photo transistors would be protected from abuse. The holes also have been adjusted to 18mm between sensors. I left the AA battery on the outside of the box for ease of servicing. If you wanted too, I'm sure a 1.5 volt N battery would fit inside the box. I also found some thin, black foam padding at a nearby department store and used contact cement to glue a rectangle around the face of the box in order to get a slightly better light seal. At first, I was going to use some felt padding I found at a Home Store, but I felt the felt had too much fuzz and was thicker than necessary. I wanted my sensors to be as close the film plane as was reasonable, without coming into contact with the shutters, themselves.

Using the Shutter Tester
You simply place the camera in from of a light source, open the back of the camera and place the sensor where the film would normally reside when taking a picture. Make sure your shutter tester is plugged into a line level stereo input jack on your computer, start the capture software and take one or more "pictures". Stop the capture software and examine the results. I find my shutter tester works slightly better when I remove the lens from my SLR camera, but it is not necessary.

software window
Shutter Tester Software
Using a standard sound capture program quickly grew old on me. I wrote a piece of software that monitors the sound port, detects when the shutter is fired and displays the data on screen. I also set up a simple save and restore feature, so I could save the shutter data to a file.  Unfortunately, getting a reliable version of the shutter tester software to work on a variety of machines turns out to be pretty difficult.  I do have a version free for the downloading that is available for Mac OS X and might work on some power macs.  I tested this with version 10.2.8.  I believe there may be bugs in the sound driver of earlier versions of OS X, so make sure you have at least 10.2.8 version OS X.  Anyway if you are interested, you can download the software and try if for yourself.

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