Measuring shutter speed with photo-diode - help understanding results please

mr.datsun · Mar 21, 2013

Measuring my Nizo Pro's shutter speeds. This one attached is a single frame taken at the 18fps setting and should hence be 1/43. Here is the output using a photodiode. I seem to have more peaks than is expected so not sure where I should take the start and end of the shutter opening time.

I'm using a a circuit like the one below (sans capacitor) and using a Bowens flood light. The photodiode is inside the camera body right behind the shutter gate. Recording to minidisc and opening in Soundbooth. And the PDF below also tells us how to measure the time from the wave form.

I've added an A, B, C and D to help identify the points I should be using.

Dead Link Removed

DWThomas · Mar 21, 2013

I think you would be better off using the capacitor, as it isolates the DC bias from the audio input. There is probably a capacitor in the audio interface but it is likely much larger and swamping some of the effects you're looking for. About two years ago I bought some parts to build one of those doohickeys but still haven't gotten around to it, it could be I really don't want to know!

Prof_Pixel · Mar 21, 2013

Why did you not include the capacitor?

mr.datsun · Mar 21, 2013

Mr Thomas and Prof. Pixel:

Ok. I'll rebuild tonight!

I left the capacitor out because I couldn't get it to work and didn't understand why I needed it. I saw a version without one. But now I'll try it again with a capacitor.

Thank-you for your help.

Prof_Pixel · Mar 21, 2013

The little blips you should be seeing are a result of charging and discharging that capacitor.

tkamiya · Mar 21, 2013

The capacitor is there because audio input of a sound card cannot see DC. By putting a capacitor there, it will cut out all the DC component, and only the AC component, the change (such as a sudden rise in voltage when diode sees the light and NOT see the light) in voltage, will be apparent. In essence you are making a differentiation circuit. That's why the peak goes to the opposite direction - charge and discharge. No, it isn't charging THAT capacitor actually....

You should be seeing more changes though, even without a capacitor.

mr.datsun · Mar 21, 2013

Prof_Pixel said:
The little blips you should be seeing are a result of charging and discharging that capacitor.

Thank-you. Just saw the LED blip. So maybe it's working - i'll try the recording otherwise I'll buy some more components to get a better match with that circuit. I'm trying to make do with stuff in my kit.

mr.datsun · Mar 21, 2013

Sourcing the correct parts after all your helpful comments.

But I can only find one photodiode in Maplins if I am to buy one tomorrow - the SFH300. Will that work in place of the BPW40? Otherwise I'm stuck until I can find one. I have some photodiodes that I am using but I have no idea what they are.

This is the Maplins one:
a 'http://pdf1.alldatasheet.net/datasheet-pdf/view/45637/SIEMENS/SFH300-2.html

limnidytis · Mar 21, 2013

I built one a couple of years ago -
https://dl.dropbox.com/u/16474348/web/ShutterSpeed/Photography/Shutter_Speed_Tester.html
I found the schematic I think on the large format forum. My EE friends helped pick out the components. I think it's generally accurate at slower speeds, but I don't have a way to independently test the device. At faster speeds the signal becomes stretched out and it's hard to pick specific on an off points. Lately I made a small slit mask that I put on the front of the box to try and limit extraneous signal and produce more abrupt on and off signals. It may have helped some, but is also let me measure the speed of a Zorki shutter at the beginning and end of the shutter travel so I could set the tension on the drums.

paul_c5x4 · Mar 22, 2013

mr.datsun said:
But I can only find one photodiode in Maplins if I am to buy one tomorrow - the SFH300. Will that work in place of the BPW40?

The SFH300 phototransistor will work just fine - I used one for a shutter tester wired up to a parallel port without any problems.

mr.datsun · Mar 22, 2013

paul_c5x4 said:
The SFH300 phototransistor will work just fine - I used one for a shutter tester wired up to a parallel port without any problems.

Excellent news, thank-you. I just bought one.

But I wired up the circuit and can't get anything out of it. Don't know how to debug it but I'll try again later. At least i know all the parts are now correct.

The original suggested using a laser pen to shine through the lens. I'm using a Bowens flood light. Will it still work?

paul_c5x4 · Mar 22, 2013

Try reversing the connections to the phototransistor - Chances are, you have wired it up round the wrong way.

mr.datsun · Mar 22, 2013

I bought a multimeter and found a dodgy connection inside maplin's battery holder!

The new phototransistor didn't work but an old one I had was perfect.

I have measured the various shutter speeds of my camera. Have to say they are pretty much spot on at least within the limits of the measuring system.

Just from random samples 1/41, 1/59 and 1/111 (for 18, 25 and 54 fps). Pretty close to the spec.

That's a good result. Thank-you for all your comments and help. Back to film tests with confidence in the camera system!

Chan Tran · Mar 23, 2013

I need a responsive phototransistor as I don't use audio recording software but rather a digital storage oscilloscope to view the signal.

Measuring shutter speed with photo-diode - help understanding results please

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