1) it has sensitivity at the wave length of your chosen light source
2) it has the reaction time fast enough for your application
3) it has the current carrying capacity necessary for your application
4) it is in a shape that can easily be handled (ie. not a surface mount type)
5) is an NPN type instead of PNP type
If you happen to have PNP type, you just need to reverse the C and E on your schematic.
Since your application is not anything ultra fast, super sensitive, or high current, I would imagine bulk of what is being offered at Mouser would fit your application. But, going beyond making this general statement, it would involve going over data sheet for each which I have no access.
The basic circuit that you refer to is OK except for one oversight: the loop for the current closes through the sound card input, which is normally high impedance (47k_ohm is standard). Unde those conditions, the response of the circuit might be slower than optimum. You should move the 5k (actually 4.7k wil be easier to find) from where it is on the schematic (replace it by a direct connection), to being across (in parallel with) the two wires that go into the sound card. Other than that, any photo transistor would do provided (as pointed out by tkamiya, it is NPN, as drawn on the schematic; but it is probably hard to find one that is /not/ NPN.
Regarding the more recent comment also by tkamiya I wouldn't put too much trust in something like this. Following this instructions, one is trying to measure a rise in DC voltage with something (sound card) that doesn't respond to DC voltage. Long story short, it might give you some indication but you have no ways to know if the results are accurate.
That is not a major issue. True, sound cards (at least those I've seen) suppress the DC component. So, the signal you will see in Audacity (or other) will not be a faithful representation of light intensity versus time (especially visible for long shutter times). But that is not the goal. You will still see clearly two spikes: one at shutter opening, the other at shutter closure. Intensities are not accurate, but times are.
Feel free to send me an Email if tha does not make sense. I could also send you screenshots taht illustrate what I wrote.
Instead of using the sound card, I used an old oscilloscope (old analog scopes are fine as the phosphor does stay lit long enough for you to see the trace well and even a lowly 20Mhz units are plenty fast for the job). Using an ociloscope eliminate the problem of dc voltage input and response time. Even a slowest 20Mhz scople response time is plenty fast enough for the job.
The problem is still with the photototransitor. I found that they are fast enough to measure a 1/125 time with less than 10% error but with higher shutter speed there is problem with phototransitor response time.
I used a prebuilt sensor from Banner Engineering. It has a response time of 50microsec (1/20000) and uses an opposed fiber optic system of about 1mm. It works well but I can't test shutter speed in auto mode.
Last edited by Chan Tran; 02-21-2010 at 04:04 PM. Click to view previous post history.
I found that they are fast enough to measure a 1/125 time with less than 10% error but with higher shutter speed there is problem with phototransitor response time.
It is NOT a problem with phototransistor response time. Phototransistors have response times of microseconds. Maybe picoseconds! Your circuit as a whole may have a slower response time however.
The DC blocking is not really a problem. Either the sound card has input coupling capacitors or it can handle the short DC spikes. As already mentioned, the test is still perfectly valid because all you care about is the time between the opening and closing spikes. When I made my shutter tester, I used a capacitor so that I did not send DC to the soundcard.
Practically any phototransistor or photodiode will work. Any silicon photodiode is going to have roughly the same IR response; no matter, they still respond to visible light. I use a flashlight. You can get IR photo detector/emitter pairs at radioshack. The detector is a silicon phototransistor that will work fine.