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Digital Fat Calliper Sensor

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Hi John,

If I choose potentiometer as my variable resistor, IC chip as my A/D converter, and add a microcontroller, do you think it will work?

Vivian
 
It can certainly be made to work that way. But you already have a very nice pincer and encoder. Why not use that? The mechanical part will probably be the hardest part to make (excluding programming the microcontroller). Using what you have avoids that hurdle.

A single microcontroller with enough pins is all that you would need in addition, plus a display/PC interface, of course.

Is there something I don't understand about this project?

John
 
Hi John,

Based on this photo, PC212530.JPG
Do you know what is PBO-1 and PEO-1? :confused:


Vivian
 
The pattern suggests the following:

PEO-1 is the multiplexed output for the 8 ranges to four wires. Using charlieplexing, 4 wires could read 12 positions ((positions = n(n-1)). I suspect it is charlieplexed.

PBO-1 has the eight wires that read the fine divisions withing each range.

Although that pattern seems likely, I would do some testing with an ohmmeter to confirm it. Set the meter to diode test, if it has that function.

John
 
Hi Vivian,

As John has stated, the PCB multiplexed. It is simple to connect a microcontroller to. Have you decided what you want to do with the value once you have read it from the caliper?

Doug.
 
Hi Vivian,

As John has stated, the PCB multiplexed. It is simple to connect a microcontroller to. Have you decided what you want to do with the value once you have read it from the caliper?

Doug.


Hi Doug,

I not very understand what you mean by the value get from the caliper.

Vivian
 
The pattern suggests the following:

PEO-1 is the multiplexed output for the 8 ranges to four wires. Using charlieplexing, 4 wires could read 12 positions ((positions = n(n-1)). I suspect it is charlieplexed.

PBO-1 has the eight wires that read the fine divisions withing each range.

Although that pattern seems likely, I would do some testing with an ohmmeter to confirm it. Set the meter to diode test, if it has that function.

John


Hi John,

Thanks for the explanation. I will perform the diode test by tomorrow.

Vivian
 
I not very understand what you mean by the value get from the caliper.
I mean the number that the microcontroller derives after reading the position of the calliper. The number that relates to the position of the calliper.
 
I mean the number that the microcontroller derives after reading the position of the calliper. The number that relates to the position of the calliper.

Hi Doug,
The reading get from the pincher will substitute into the body density equation and finally will get the percentage of the body fat.

Vivian
 
Hi John,

I have perform the diode test, and I found the PEO-1 is same as what you are mention in #26. The connection of the four wires are show in the figure.Untitled.jpg
But for PBO-1, I found that it is not the fine divisions withing each range, i suspect it is connected with the capacitor and resistor circuit as show in the figure below.
Untitled1.jpg

Vivian
 
The circuit in post 32 is likely a charlieplex, but there may be other ways to multiplex 8 switches to four wires using 8 diodes. I am neither an expert in multiplexing nor in charlieplexing. The way I look at the circuits is to focus on a single unit (shown on the right), then use that unit to connect different pairs of pins/wires. My concern whether your device is truly charlieplex is related to whether your switches share a common wire.

Since 8 diodes (4x4) will allow 12 switches to be reduced to 4 wires, and you only need 8 switches for the circuit you have, there will be different permutations of how the wires are connected to each switch. With the schematic I have attached and your ohmmeter, you can figure out exactly how each switch pad is connected to each wire.

charlieplex.png

BTW, do you have EAGLE installed on your PC? Eagle is a free cad program from Cadsoft specifically for schematics and PCB design. The advantage of using it here is that ETO supports downloads of its .sch and .brd files. That way, someone could attach a file, and you could manipulate it, instead of having to deal with an image.

John
 
Hi John,

Thanks for your reply. That means I only need 4 input that will be connected with the PIC microcontroller, right?


Vivian
 
Hi John,

Just want to know your opinion, do you think is it suitable if I choose PIC16F887 as my PIC microcontroller? Thanks.

The PIC16F887 features:
256 bytes of EEPROM data memory, self programming, an ICD, 2 Comparators, 14 channels of 10-bit Analog-to-Digital (A/D) converter, 1 capture/compare/PWM and 1 Enhanced capture/compare/PWM functions, a synchronous serial port that can be configured as either 3-wire Serial Peripheral Interface (SPI™) or the 2-wire Inter-Integrated Circuit (I²C™) bus and an Enhanced Universal Asynchronous Receiver Transmitter (EUSART).



Vivian
 
Last edited:
The 16F887 is popular and will certainly handle what you seem to want to do.

There are lots of chips that will do what you seem to want to do. Even the 16F628A could do it. The 16F628A is often used in tutorials. It can have 18 pins in the the DIP and SOIC versions or 20 pins in the SSOP version. Basically, you need 12 pins for data (in the simplest version), two for power, and at least one for output (if serial). So, almost anything with 16 pins or more will work.

As I recall, this is your first microcontroller project, and I would suggest that you get a DIP version of whatever chip you decide upon for ease of handling. The main downside to the more capable chips is the setup. That is, all of those capabilities need to be addressed in the initiation part of your program. That sounds harder to do than it is in practice. First, there is lots of help on ETO to help you get through that, and second, there are lots of examples on the Internet.

So, in sum, just get something to get started. It is hard to conceive of any choice of chip you could make that has at least 16 pins that would be a complete dead end and require you to start all over.

You will need a programmer. The PicKit 2 and PicKit 3 are popular and relatively low cost. A step up is the ICD3, which Microchip markets as being a more professional version compared to the hobbyist PK's. If you are affiliated with a school or non-profit, both versions can be had at a discount.

John
 
Hi John,

In my circuit, I will add 4 LEDs light, 1 buzzer, 1 on/off switch, 1 enable switch to get the measurement, input and output. Can I know what is the 12pins for data that you wrote? If I not mistaken, the charlieplex only need 4 pins, am I right?


Vivian
 
I don't think those four "charlieplexed" pins/wires carry all of the data. The other eight data pins to which I refer are the 8 wires off on the side. I know they go to some resistors and capacitors on the other board. That could be a way to reduce the number of inputs to the MCU, e.g., the on/off signals could be converted to different voltage levels and read by a single ADC on the MCU. It could also just be for filtering electrical noise. There are a lot of unknowns. With a chip like the 16F887 you could read all 8 wires as a single port.

John
 
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