The following application note was donated by Lex Bolkesteijn.
Lex is using the GP2D02 in his robots but there are many applications
for this part.
The GP2D02 from Sharp is a distance sensor that can measure
distances from 7cm to 1.2 meter.
It uses an optical transmitter and receiver to measure the distance and
outputs the distance in 8 bit serial format.

The datasheet of the GP2D02 can be downloaded from :
http://www.sharp.co.jp/ecg/sys/gp2d02/gp2d02-fea.html

A more in depth desciption of the GP2D02 is also available.
Dr.-Ing. Claus Kuehnel donated one of the chapters of his BASIC
STAMP book that decripes the GP2D02.
In fact the BASIC STAMP book is a good source for interfacing all
kind of hardware.

Download the  .PDF file for GP2D02

'--------------------------------------------------------------
' (c) 1999 Lex Bolkesteijn
'--------------------------------------------------------------
' file: GP2D02.BAS
'
' Sample program to use a Sharp GP2D02 distance sensor
' with a 89C2051. The GP2D02 is non lineair. The shorter
' the distance between object and sensor the higher the output.
'
' The GP2D02 is availlible at Conrad Electronics
' http://www.sharp.co.jp/ecg/sys/gp2d02/gp2d02-fea.html
'
' GP2D02 89C2051
' pin 1 : Gnd -> Gnd +-- connect Vin/P1.0 here
' pin 2 : Vin -> P1.0 (Vcc - 1K --X-- 1K - Gnd)
' pin 3 : Vcc -> Vcc
' pin 4 : Vout -> P1.1 (GP2D02 12K internal pull up)
'--------------------------------------------------------------
'$sim

Dim D As Byte 'distance value
Dim T As Byte
Dim I As Byte
Dim S As String * 20
Config Lcd = 16 * 2 'configure lcd screen

Cls

Set P1.0 'set powerdown for sensor
Waitms 10 'default wait

Do
 Reset P1.0 'Start cycle
 Waitms 70 'time to calculate
 Set P1.0
 Shiftin P1.1 , P1.0 , D , 0 'clock bits in MSB first on falling edge
 Set P1.0 'set powerdown for sensor
 Waitms 2 'minimum delay for sensor
 Cls
 T = D / 16 'we use a 16 char. display (255/x)= 16 max
 Lcd "Distance : " ; D 'display the distance
 Lowerline
 If T > 0 Then 'else string will fail
 S = String(t , 61)
 Lcd S
 End If
Loop
End