EE 4770
Homework 3
Solution
Due: 8 March 1999
Problem 1:
The distance from a light source of unknown but steady brightness is to be determined
using a photodetector mounted on one end of a 250 mm rotating arm (with the rotation around
the other end) as illustrated below.
light distance
250 mm
light
arm
photodetector
The light source radiates uniformly in all directions; the photodetector is sensitive to
irradiance and is equally sensitive to light in all directions and is sensitive to no other light sources.
The light source will be outside the circle traced by the rotating photodetector. The photodetector
is connected to a conditioning circuit (already designed) which feeds an 18bit ADC.
The ADC
output is interfaced to a computer. The photodetector and conditioning circuit are linear; the ADC
saturates at 10
mW
cm
2
. The ADC is noisy, so its output randomly varies by
±
1.
The arm makes one complete rotation in exactly 10 seconds. A proximity sensor detects
when the arm is at its 0
◦
angle, at which point a 16bit counter is reset. The counter is clocked at
6
.
5536 kHz and is interfaced to the computer.
An interface routine is to sample the ADC and counter values and determine the direction
of the light source in degrees (with respect to the arm’s zero position), the radiant flux of the light
source in milliwatts, and its distance from the axis of rotation in centimeters.
To do this the
interface routine will have to sample the sensors multiple times (and so will not be able to produce
values the first time called).
The light source can move, the distance and radiant flux values need not be correct while
it is moving or for a short time after it stops.
A shell of the interface routine appears below. Complete the shell.
1
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Given the photodetector and conditioning circuit it is a simple matter to determine irradiance. Let
E
denote
irradiance,
r
e
denote the ADC output, and
E
max
= 10
mW
cm
2
denote the maximum detectable irradiance. Since the
photodetector and all parts of the conditioning circuit are linear,
E
=
r
e
2
18

1
E
max
.
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 Fall '99
 Staff
 Light, Rotation, Return statement, Void type, radiant flux

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