There seems to be a run on this BMI thing as another thread seems to be after the same thing. That thread
can be found here. Eventually there may be good material in both threads or a moderator may see cause to merge them.
Now I am no BMI guru but we need two sets of data to calculate the BMI. We need the weight and the height of our subject. With that in mind we now need a sensor to get the weight and a sensor to get the height. This can be as simple as placing our subject on a scale (get weight) and then using a ruler or tape measure to measure the subjects height (get height). Using a pencil and paper we record the data and do some calculations:
English BMI Formula
BMI = ( Weight in Pounds / ( Height in inches x Height in inches ) ) x 703
Metric BMI Formula
BMI = ( Weight in Kilograms / ( Height in Meters x Height in Meters ) )
There we are, we have the BMI.
Somehow I don't think something that simple is what you are after with this project/assignment. Since this is an electronics forum where electronics help is available I figure you want an electronic approach. This brings us back to sensors, a sensor to measure weight and a sensor to measure height.
Starting with weight. At the heart or front end of just about every electronic scale is a tiny device called a strain gauge. You may want to Google "strain gauge" but
this link should provide an idea of what they are all about. Most electronic weighing systems are based on using a strain gauge and they can get expensive.
Newer electronic basic bathroom scales use strain gauges. If you take a look
at this link you will see a good picture of a bathroom scale opened up. Note the little red, white and black wires extending to the sides. Those wires are connected to strain gauges. The electronics downstream merely signal condition the output of the strain gauges, and scale it to usable information. Thus for projects like this, I figure hacking a cheap bathroom electronic scale is a good starting point to get weight. Hey, just my take on it.
Height is another animal requiring yet a different sensor. Measuring height is basically measuring distance. Distance from head to toe sort of. There are more ways to measure distance then weight. You can use a $3,000 USD plus LASER Interferometer, an ultra sonic transmitter/receiver, or something less. I suggest the use of a string pot sometimes called a yo yo pot. A string pot is a very accurate linear potentiometer (variable resistor) generally in a multi-turn design. They can be very accurate and very expensive but can also be home made for a reasonable cost.
What if I took a 10,000Ω ten turn potentiometer and placed a wheel on the shaft. The wheel has an outside circumference of exactly 12 inches (sorry but I use inches despite the fact they make no sense as a unit of measure). Now each revolution of the wheel would be 12 inches so if I wound string on the wheel each revolution would be 12 inches of string. Each revolution would also be 1 KΩ of resistance. If I place 10 volts across the pot then measure the wiper output each revolution would be 1 volt. So we now have 0 to 10 volts equals 0 to 120 inches of distance (height).
From this point we can take an analog output signal from our sensors and scale it in engineering units to Lbs or Inches. For those across the ponds metric units of Kg or mm I guess.
Since we don't know your level of understanding electronics it is hard to say more. You need to really explain what you want to do in detail. What we have is two analog signals. Let's say we have 0 to 500 Lbs = 0 to 10 volts and 0 to 120 inches = 0 to 10 volts. Now what do you want to do?
Ron