Using a Photo Diode as a light sensitive trigger

Other Hardware LEDs and Multiplexing
1

Hi everyone, I have a very specific project I need to build and have very basic Arduino knowledge, so please be gentle with me.

My project is to build a fast actuating light sensor switch to detect small sprocket holes in 8mm and 16mm movie film to trigger a camera shutter (requires a 5 volt trigger for camera) This is to digitise old movie film, frame by frame for preservation purposes.

The film moves like a conveyor past the camera at a rate of around 15 frames per second and there is a corresponding small sprocket hole next to each frame of film. The hole is around 1mm sq (1/16”)

I have spent some time reviewing YouTube videos but can’t find reference to my exact problem, although similar projects have brought the following understanding.

People have used a UV LED (Blue) as a light source on one side of the film and a photo diode on the other side of the film, so when a sprocket hole appears the UV light hits the photodiode instigating a 5 volt trigger pulse that is used by the camera to take a still image.

My understanding so far is that a photo diode seems to have a much faster trigger response than a reverse biased LED, photo transistor or photo resistor, although this may be incorrect.

I would really appreciate if anyone could provide a simple circuit diagram to help me build this project with accompanying code.

One other consideration is being able to dim the UV LED to allow for some adjustment in triggering sensitivity. I have a very limited electronics understanding so please keep it as simple as possible, many thanks.

I have attached a sample image of the films showing the sprocket holes.

MovieFilmTypes.jpg

2

While it is true that a photo diode is faster, the signals you are trying to read are quite slow in electronic terms. I would use a photo transistor as this is plenty fast enough for film transport.
The advantage is you can then connect it direct to an Arduino pin, where as a photo diode will need to use an op-amp to amplify the small change you get from it.

As to sensitivity adjust the LED’s brightness with a PWM signal from the analogWrite function. At 1KHz it is a lot faster than you are trying to count. You might have to put a small RC filter on the input pin to remove this frequency, but it depends on your software.

3

Given Mike's comments, a common IR photo-interrupter would probably do the job perfectly!

4

You are operating at about 15Hz so most any semiconductor photo detector would work. I like the photo diode they are easy to work with. Start with a 10K resistor, connected to the photo diode anode and ground. The junction of the photo diode and resistor becomes the input to the the 74HC14 and the output becomes the input to the Arduino. Connect the anode to the +5. You may have to twerk the resistance but I have used this basic circuit for a lot of years. You can put the emitter and sensor in tubes, this narrows the acceptance angle. This response is to help you get started in solving your problem, not solve it for you.
Good Luck & Have Fun!
Gil