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[on:]

i have been trying for a few days and searching this forum but i cannot seam to solve what should be a simple setup

i have 2 half bridge  loads cells (http://www.sparkfun.com/products/10245)

and this op amp (http://www.national.com/ds/LM/LM741.pdf)

i am trying to get this to register in arduino. from what i know so far is that since the load cell is a half bridge i can use two  to make a full Wheatstone bridge. but i either got the wrong amp, i read a lot of talk about using an ina125p, is that better?

here is a diagram, can someone help me fill it in.

[Reply #1 on:]

You're going to need a couple of more op-amps to do this right, since you need an instrumentation amplifier circuit to do this right -- it's a basic op-amp application circuit and you need three op amps to build it. Also be aware that the output is proportional to the excitation voltage as well as the load -- that's why the sensitivity and so on are defined in terms of mV/V -- the V term is the excitation voltage. The output is 1 mV/V full scale. So use a precision reference if you want good results.

Wire the + of one sensor to the - of the other and vice versa. If you do + to + and - to -, you will get no output -- the resistance of both bridges will shift together and there will be no difference signal to measure. This anti-parallel arrangement also has the nice effect of doubling the output to 2 mV/V. If your excitation voltage is 5V and you'd like to map the output onto 0-5V (i.e. an Arduino ADC pin), you will need to set the instrumentation amp up with a gain of 500.

The INA125 that you've seen mentioned elsewhere combines the instrumentation amplifier and the excitation source into one convenient package. It only requires a single external gain resistor and a bit of power supply filtering to use.

[Reply #2 on:]

The 741 op amp is hopelessly out of date - it used to be the state of the art 40 years ago and requires supply voltages in excess of +/-5V, ie it  doesn't work at all from a single 5V supply rail.

For high accuracy requirements an 'instrumentation amplifier' is needed which is usually a clever circuit using 3 op-amps.  The INA125 is all of this on a single device - you just add a gain-setting resistor (which should be a high-precision type).  The datasheet shows how to use it for a bridge circuit.

[Reply #3 on:]

ok i use the INA125 sounds good. next question. is there a way to use the 3 wire load cell by itself with out the need for a second reference one. if so how do i hook up the 3 wires, there seam to be no diagram of a 3 wire config.

[Reply #4 on:]

If this load cell is half a wheatstone  you could just add two R with same value as each one in the bridge to create compensation for temp errors and proper signal level for your INA. Put all R as near each other as possible(with bridge). Remember, wire lengths are crucial so use sense wires when feeding bridge. I would also recommend a calibration with a known weight when everything is put together, including your cables. Try to make at least a three point calibration for linearity check.
(When building by yourself i prefer this method before shunt calibration.)

thank you

[Reply #5 on:]

I am experiencing similar issues.  I have a load cell (0-1000lbs) that I am trying to attach to the ArduinoUno.  I know I need to use some sort of OpAmp for amplification but I am not sure if I am wiring it correctly.  I have attempted to wire it with several different amps.  What is recommended for this set-up?  I have wired it several ways and get values to output using the serial port but the value never changes when pressure is applied to the sensor.