Fingertip laser light sensor

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Original Assignment

I will provide you with a laser that emits a sheet of laser light and a phototransistor for detecting this light. You will investigate their use in developing a fingertip position sensor that senses distance to the finger based on reflected laser light. Your job is to design a sensor circuit and determine the analog voltage readings you get as a function of (1) your finger position and (2) the sensor's location relative to the emitter. You will likely display your results by communicating with a PC or by writing to an LCD (see, e.g., C Example: Serial LCD or C Example: Parallel Interfacing with LCDs).


Experimental Setup







First Experiment:Finger in front of the laser and phototransistor (Resistors and Y distance from finger to laser/transistor varied)

Resistor Selection.jpg

Distance calibration using 19.5 K-Ohm resistor

Distance Calibration.jpg

Transistor next to laser (For certain Y distances, X coordinates varied)

Varying Horizontal Position.jpg

Transistor in front of the laser (Y distance from transistor and laser varied)

Varying Phototransistor Position.jpg

Transistor next to laser (For certain X coordinates, Y distance varied)

Varying Phototransistor Position Horizontally.jpg

Limitations and Environmental Factors

There were a few environmental factors that affect the sensors:

1) Ambient light: The overhead light saturated the phototransistor and so the laser was pulsed(the pic was programmed to strobe the laser) in an effort to provide a noticeable difference between the ambient and laser light. Before the laser was pulsed, a finger in front of the transistor cast a shadow which had a larger effect (due to the blocking of the ambient light)than the actual blocking of the laser. Any shadow cast on the sensors by a finger or head will affect the ambient light sensed by the phototransistor and should be taken into account.




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