1. Abstract
This project has the goal of providing pulses of light from a laser pointer, and detecting information about the surrounding environment using data gathered from the reflected light. A photoresistive cell was successfully used to measure the magnitude and timing of the light returning from diffuse reflection of the laser, and an op-amp and diode circuit correlates that signal with the laser signal itself. However, in this case the timing and detector circuits were not sufficient to measure practical distances based on timing alone.
2. Introduction
The pulses of light are emitted from a laser pointer, and detection of the returning light gives some information about the surrounding environment. A CdS (Cadmium Sulfide) photocell will be used to detect the reflected light and measure some of its characteristics. Of particular concern will be how those characteristics relate to the timing of the laser.
3. Experiment
The following diagram illustrates the circuit design:
A. Timing Circuit
A 555 IC will function as an astable multivibrator. The output will drive a pair of resistors to -6V, which will complete the current flowing to the laser pointer. Varying capacitor and resistor values in the circuit can change laser pulse timing and duty cycle.
B. Changes in Photoresistance
To permit the circuit to function in a variety of ambient lighting conditions, the detector circuit will measure changes in light level, rather than attempting to deal with the absolute level. Aluminum tubing is used to block most incoming rays, so that the detector will act in a more directional manner. A photoresistor, in series with a 470k resistor, acts as a voltage divider and supplies a voltage which is related to the light intensity within the photocell's spectral band of sensitivity. This is voltage connected to the positive input of an op-amp.
A slight delay in this signal is induced by an RC network, and subsequently provided to the negative op-amp input. Acting as a comparator, the open-loop op-amp will signal whether light level is increasing or decreasing. This allows the inherent latency of the photoresistor to indicate whether or not the reflected laser light has entered the tube. Choosing a laser pulse rate that sufficiently fast will, within a certain detection range, keep the detector voltage moving and block most high-frequency noise on the output side of the op-amp.
4. Results and Discussion
Since more distant targets will yield longer pulses, a multimeter can be connected to the circuit to sense approximate distance to a target. Alternatively, by using a carefully aligned mirror as a target, the reflection is specular so the transducer could be used over a much greater range (similar to the Johns and Webster experiment referenced below). Additionally, the aluminum tube in the project could be replaced with a small telescope, allowing the photodetector to assess a relatively greater amount of light, thereby boosting the detector range.
Use of a follower LF411 op-amp (amplifying the detector's voltage divider) was also attempted to improve the detector responsiveness of the open-loop op-amp. In the experiment, this had the effect of greatly reducing the dependence on reflected light's intensity so that the sensed laser-to-detector delay was much more uniform over varying distances.
As a result, the delay was on the order of microseconds, whereas the shifts of a few nanoseconds are expected over the transducer's range. The variance was not markedly visible on the oscilloscope, given the small amounts of electrical noise present. However, additional controls on noise and comparator circuit redesign could potentially overcome that problem.
5. Conclusion
As of today, much of the pulse timing is caused by latency in the detector circuit. Hence, the photocell, the comparator characteristics, and the inverse square law are magnifying (by at least a factor of 103) the actual pulse duration that should result due purely to the speed of light.
As a result, distance measurements are dependent on intensity of the reflected light (ie. darker objects have similar electronic signatures as more distant objects). Limitations of this circuit yield a detection distance of about 50 cm, with distances of a few meters possible under ideal conditions.
6. References
Will Johns and Med Webster (Vanderbilt University)
Speed of Light (September 1, 2003)
http://www.hep.vanderbilt.edu/~webster/classes/p225lab/speed.pdf
Patrick, Dale R. and Fardo, Stephen W. (Eastern Kentucky University)
Understanding Circuits and Op-Amps
Prentice Hall, Englewood Cliffs, New Jersey, 1989
