I want to design a medical equipment that has the capacity to take both your weight and your height at the same time and then calculate your BMI and display the result . I don't just know how to go about it. I know I will need two major sensors for the weight and height, but I don't know what these sensors are and where to order them from because I am so sure we do not have such technology in Nigeria. I have already contacted a programmer but I need help from someone with a hand-on experience in electronics or instrumentation to help me out. I am so confused. I feel so incapacitated when I am not getting the right answers to these questions in my head. do you have an idea? can you please help me out
If you have access to Ebay, then you should be able to get most of the parts fairly easily.
I'm not well versed in the pressure sensors required to measure weight, but others here are and could guide you in the right direction.
An Ultrasonic or Infrared distance measuring sensor would probably a good place to start for height measurement. Have it at a fixed known height above the scale, and as the subject stands on the scale, it would measure the distance from the top of their head (or hair, hat, etc. those will throw off your readings for the calculations you want). Some simple math would then give you the subjects height.
I must ask though, what is wrong with doing it manually? It doesn't take long to measure height and weight, and a calculator would do the rest.
Aren't scales with serial output pretty readily available?
walen_g:
do you have an idea? can you please help me out
For automatic weighing you will possibly need a "weighing scale with RS232 interface".
This is a serial interface that can easily connected with an Arduino board using a cheap "TTL-RS232 Serial adapter".
Automatic height measure of persons may be a problem.
Measuring a bald-headed man is easy: Just place an ultrasonic range sensor above the scale and measure the distance from the ultrasonic sensor to his head. If you know the height of the sensor, you easily know the height of the bald-headed man.
But automatic measuring a women with a pinned-up hairstyle may lead to wrong results with any type of automated measuring, i think.
The most costly part in the system would be the "weighing Scale with RS232 interface". I just looked up and found this one measuring up to 250 kg with an readability of 0.02 kg.
Price: 1,570.40 US Dollar plus RS232 interface plus transport plus taxes plus customs duties.
Compared to that, every other thing needed would be cheap.
I don't know if there might be different scales with RS232 interface available for a cheaper price. Perhaps with less accuracy and less readability.
If you can't afford such an expansive scale, perhaps something do-it-yourself construction from cheap load-cells would be possible for weighing. But it would be a lot of electronic workmanship and accuracy would be probably very bad, even after calibration.
Google "scale with serial output", many are available.
First one google shows is a 400 pound capacity scale for $87 for example.
http://www.globalindustrial.com/site/images/universal/product_extras/300625_Digital_Shipping&Receiving_Scale.pdf
If you are investigating this with a view to investing in a business, put some thought into whether there is real demand or opportunity.
Any doctor I have been to has done all of this manually with the help of a standard weighing scales. Then they refer to a pre-printed chart or a website to do the rest.
They also tend to use waist measurments a lot along with age & gender.
If have seen machines like this in Pharmacies & shopping malls for the general public.
If you are just interested in the project then look into ultrsound or IR proximity sensors over the head to get the height, linked up with a scales that provides output in serial format as has been mentioned already. That sould be pretty easy to capture & calculate. Thereafter its a mechanical/housing problem.
Putting "weighing scales with height bmi" into a google image search will provide links to lots of such devices available on the market
AnalysIR:
Any doctor I have been to has done all of this manually with the help of a standard weighing scales. Then they refer to a pre-printed chart or a website to do the rest.
Well, the scales and height gauge are right, but charts or website? Not actually!
The management program such as "Best Practice" will record the weight and height; BMI calculation is automatic and successive observations are tabulated and can be graphed to illustrate trends. This is not a new development and paper charts are distinctly passé.
AnalysIR:
They also tend to use waist measurements a lot along with age & gender.
Waist measurements are considered to be more strongly and usefully indicative of abdominal obesity and its concomitant influence on diabetes.
@Paul__B
Maybe I have been to 'quacks'....
or more likely mistakenly assumed the management programme was on a website ... 
....tnx for the clarification
AnalysIR:
or more likely mistakenly assumed the management programme was on a website ...
There have been suggestions that they could be.
I for one consider that a particularly bad way to run a practice. At present, losing Internet access is merely a modest inconvenience, but if the business was "in the cloud", well ...
Not to mention the confidentiality implications.