I got a Time machine

Recently I have restarted a search of a device to test shutter speeds … always good to know if you have done it right. There are a number of devices out there that can do this:

  • Kyoritsu
  • ZTS
  • photoplug
  • phochron XA
  • alvandi
  • vmfoto
  • DIY Arduino

The first two were made for specifically for camera tech’s which are either no longer manufactured, or are extremely expensive. The photoplug uses a computers microphone jack and audio app to record the electrical output wave. The others are self contained devices that detect the light source but display the info as speeds.

I already had a photoplug-like device from vmfoto, but found that using the microphone input a crude method. Using the Shutter-Speed App was almost impossible to read (lots of noise), and it did not work any better on my Macbook using Audicity. I needed something that works … I was intrigued by the Arduino concept with a build/code your own device … but I am lazy … the Phochron XA is one of the best new designed product, but I am too cheap to pay for it.

Long, long ago I was introduced to the time machine. This was when I started High School and took an Electronics course. Our Teacher showed it to us … but we could not touch it. Ever since then I have always wanted one.

An oscilloscope can plot one or more signals as a function of time … it can show the signal of a circuit that changes over time … thus, a time machine.

I found many examples of using a light sensor that can be read by an oscilloscope to read the opening and closing of a shutter.

First I needed to understand how to use one, so I know what kind of scope I need … Martin Lorton has some great videos on understanding and using an oscillosope

So I started searching for an oscilloscope … well, they aren’t cheap either (I kinda knew that already). I did find a blog post about shutter speed testing and they were using a cheap DIY build DSO (digital storage oscilloscope) kit. Looking around on Amazon, I found that there are a large number of these being advertised … now they are not as sensitive, accurate, (or built as well) they can do the simple job that I need it to do.

I decided to get a $50 Kuman Q15001 DSO (JYE Tech DSO150 board) that was pre-assembled (you can get a DIY kit for as little as $21 CAD).

I had a couple of the vmfoto light sensors so I figured I could take it apart and reuse the components to wire up a simple circuit using a photodiode, resistor, and power source … I found the above circuit example on the web … though I am not sure if the vmfoto device used a photodiode or a phototransistor (there is a difference). I did have an breadboard kit that came in handy, so time to experiment.

My circuit ended up with the photo-sensor from the vmfoto (removed the internal resistor) before putting it back in the case (note there is a lot of glue in there) with the mic jack plug cut off, a 1M ohm resistor, and 5VDC power from the USB port on my MacBook. The external light source is a USB LED light.

The photosensor (diode/transistor) acts like an on/off switch … when light hits it, current can flow through it. When the shutter is closed, no light is acting on the photosensor so no voltage is displayed by the DSO. While the shutter is open, the photosensor allows current to flow, so the DSO reads 5+ volts. The DSO is set to trigger (to stop the output display) when the voltage drops to almost 0.

The above shows a test of the Prontor-S shutter that I cleaned up … the shutter was fired at 1/10s. You can see the waveform on the DSO screen of the shutter opening and closing.

The DSO is set so that each vertical grid line is 2V value, while the horizontal is 10ms time. The above wave shows a height of 2.5 grids … 2.5x2v=5V with a width of approximately 8 grids … 8x10ms=80ms … as a fraction that is 80/1000 = 8/100 = about 1/12s. So now I know the shutter is running a bit fast.

Note: this DSO does not offer cursors to measure delta, though there is an rewrite of the opensource firmware that adds this.

I did test the circuit+DSO on a Nikon FE2 focal plane shutter and the mechanical 1/90s was reading about right … one thing you will notice using this method is that at higher shutter speeds the waveform is not as square, which is caused by the photosensors rise/fall time, directional characteristics plus the slight leaking of light before/after the shutter curtain (and the rise/fall timing of the cheap DSO). Below is the Nikon shutter at 1/500s (2ms). The DSO shows 4 grids x 0.5ms + the rise and fall time … so some will read this at the toe when the shutter opens, to the top of the shoulder when the shutter starts to close. The rise delay looks like 0.2ms … so the shutter time is 2.2/1000 = 1/454s if the DSO is accurate.

With a leaf shutter the wave has a more pronounced curvy slope due to how the iris opens/closes.

In the end … it gives a clean enough output that I can actually figure out the shutter speed … eventually I will solder it all together and get rid of the breadboard.

… also an oscilloscope can be used just to simply measure voltage output of a source.

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