Hi, I've been given a project at uni to design and build a scale using strain gauges in order to measure up to 2kg and with a resolution of at least 10g . The weight has to be displayed on an LCD display... Any idea please???
thanks ---

T.L.

 

hi,
Do you have to use strain gauges or are load cells acceptable.?

If you have to use a micro controller for the assignment, then:
1. You will have to amplify the output of the strain gauge bridge, the basic signals will about 10mV for FSD on the bridge.
2.Use a PIC to convert the amplified signal [+5V max] to digital using the PIC's internal 10bit ADC [resolution from 0 thru 1023]
3.Connect a 3 or 4 digit numeric LCD to the PIC
4.Design a suitable power supply.
.
5. Program the PIC to perform the required actions.

A starting PIC would be something like a 16F877, get the datasheet from
www.microchip.com.

Or you could use an ic thats can measure mV's and drives a LCD directly.
IIRC a 7106 or 7107 device type.... much simplier than a PIC, but not much learning to add to your personal database...

__________________
Eric
"Good enough is Perfect"

PIC tutorials:
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/

 

Thanks Eric, I get a picture. I strictly need to use stain gauges, I do not know the ratings yet since a proposal has to be handed in first.

About the PIC, I am quite familiar with the 16F741 but I never used the Analog to Digital Converter capabilities of it... I guess this is an occasion for me to give it a try.
My other question is if for the voltage amplification section, can I just use regular 741 op-amps and get a decent output ? And does the sensitivity of the strain gauge depends on the manufacturer of would it depend on how I set the resolution of the PIC's ADC?

Thanks in advance...

 

hi.
The 741 is a poor choice, its 'old' and its limited in its performance.

If you are using/making a strain gauge bridge with a differential output a AD623 instrumentation OPA would be a good choice.
It of course gives a single ended output which can drive an ADC.
Goto www.datasheetarchive.com for the datasheet.

Load Cells, which are most times a bridge circuit made using 2 or 4 strain gauges have a specified excitation voltage, usually 'dc' in the range of 5V, 10V or 12V, it depends upon the manufacturer.
The specification is typically 10mV or 20mV output at full load at the specified excitation voltage.

The 16Fxxx PIC's I use, have a 10bit ADC's which give a value of 0 thru 1023 for a input voltage of 0vV thru +5V.
As you only require 2kg at a resolution of 10gm [1.99] that should be OK.

The main problem in using individual strain gauge elements is the 'fixing' on the member that you are going to use to measure the strain in.

Does this help.

__________________
Eric
"Good enough is Perfect"

PIC tutorials:
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/