# measure voltage/current by using PIC

Discussion in 'Embedded Systems and Microcontrollers' started by aimiazri, Jan 7, 2010.

1. ### aimiazri Thread Starter New Member

Jan 7, 2010
6
0
hello everyone.. can u help me to measure current by using PIC 16F8777A?? actually, i m going to design a device to measure fatness in our body.. by using a formula which including our weight,height,sex(men or women),age as well as resistance in our body.this we call BIOELECTRICAL IMPEDANCE ANALYSIS.. so, it came out to my mind to measure the resistance by using voltage divider.. any of u have done this project??

Last edited: Jan 7, 2010
2. ### beenthere Retired Moderator

Apr 20, 2004
15,815
282
You will need to research the topic further.
The impedance implies a frequency sensitive component, and therefore a source of that frequency.

You need to find out how the measuring is actually done. Using DC techniques probably will not work.

3. ### Dragonblight Active Member

Aug 19, 2009
35
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This project sounds awesome. Let me know if it pans out all sweet-like.

4. ### t06afre AAC Fanatic!

May 11, 2009
5,939
1,222
such instrument is available on commercial basis. I have been tested with it on my gym. As I understand it uses a AC sweep. And then calculate how much fat and water (among other parameters) your body is composed of. I am almost 100% sure it does not measure resistance but Admittance. Admittance just like impedance, is a complex number, made up of a real part (the conductance, G), and an imaginary part (the susceptance, B)

5. ### aimiazri Thread Starter New Member

Jan 7, 2010
6
0
i have found all the info about that.. and i found the formula of the BIA.. one of the link i found was http://www.brianmac.co.uk/fatbia.htm. futhermore i also have found the circuit. but not really complete. the info that i have found all in the PDF below. but I am sure this not really complete.. First is how to make the current flow in our body is constant at around 0.8mA (60 Heartz). second, since this device use battery which is DC how to convert it to AC? 3rd, what is the value of the all the resistance in circuit accept Rx(body resistance)? However, to measure the resistance of our body, i can get an idea by using differential voltage in before n after our body resistance,Rx.

if u dont want to download the PDF, here i have the picture.(refering the PDF is better since it also tell how the circuit running)

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6. ### Dragonblight Active Member

Aug 19, 2009
35
0
I've read up on effects of electricity on the human body quite extensively, and it looks like this device is pushing it's own current through the body to measure resistance. It seems that it's going to measure the current from finger to finger on one hand to get a base resistance reading on each side of the device, human skin has ridiculous levels of varying resistance, and this one seems to take care of that via a form of on the fly base calibration before pushing a small current from hand to hand across the body. Really, you shouldn't be tripping on this AC/DC thing. This measurement seems to have nothing to do with the body's natural electrical potential.

As an added addendum, in the patent, it notes that there is issues with measuring via current scales that use the electrical index to measure body fat. It's pretty much only going to measure the lower body fat percentage (i'm going with legs and MAYBE lower torso). This seems to attempt to alleviate this by measuring the body fat percentage between the hands, and this issue may bring in the same issue as to only calculate the top half and not the bottom half. The only real solution seems to to be utilizing an electrode on each extremity for a set of six calculations that could be averaged out.

Last edited: Jan 11, 2010
7. ### aimiazri Thread Starter New Member

Jan 7, 2010
6
0
Actualy, yes there is about the potential difference.. since the PIC only can read voltage.. so the only way i can do is to measure the voltage drop when through the body and then only can measure the resistance by devide the voltage drop by constant current that flow in body.