# Wheatstone meter

Discussion in 'General Electronics Chat' started by Pyrdon, May 30, 2010.

1. ### Pyrdon Thread Starter New Member

Mar 17, 2010
12
0
Hey,

I have a wheatstone bridge formed out of strain gauges according to the attached file. When in use, this is fed with a supply voltage of +E/-E.
Normally it is connected to various amplifiers and such but I would want to find a simple way of just measuring the bridge itself.

So I want to build some device. What would be good is to run it from battery and just plug it in on the load cell and simply see the milivoltage output it gives.

I am not specifically experienced in electronics but have the feeling I would need something like this.
* voltage regulator
* operational amplifier to amplify the signal output from the bridge.
* adc to convert it to digital
* display (lcd/7-segment?)
* a lot of digital logic for the correct display of the voltage.

I was thinking I could use some microcontroller, since I then would get adc and the digital logic for free. Would that be a good approach or is it silly to use that for such an 'easy' task?

Is it possible to generate a stable say... +/-5V supply voltage from a 9V battery? What current output can I get from such?
There is no need for precision. If I would get results in terms of mV, that would be enough.

Any ideas?

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2. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
What is the purpose of R1 & R6? It is easy to simply apply one voltage to the bridge and attach the E- point to ground. The output is the difference between R2 & 4, and R3 & 5.

If the gauges are standard 350 ohm ones, then 10 volts is a safe excitation voltage to apply.

It is common to use an instrumentation amplifier to amplify the differential voltage from the bridge. That voltage is in the range of millivolts, and so a bit small to convert unamplified.

A nine volt battery is just too weenie for such usage (bridge excitation and the amplifier). Think about a wall transformer with a 12 volt output and a 7808 regulator for a stable supply. An INA114 instrumentation amp can run with a single supply (but is not rail to rail, so a supply higher than 5 volts will be needed to let it swing to +5).

I always point people to Omega Engineering, as they have lots of information on strain gauges and application notes.

3. ### Pyrdon Thread Starter New Member

Mar 17, 2010
12
0
R1 and R6 are added for temperature stabilization. Is there now possible way of using batteries to power such small device?

Is the use of a micro controller feasible?

Do I even need to digitize it and present on a 7-segment display or are there any other analog devices I can use for such presentation? Like in a galvanometer? In that case, can it just be amplified and fed to something like that?

4. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
Nah, use the IA to amplify so the change in bridge voltage is related to the applied strain and a meter will display engineering units. They used strain gauges somewhat before A to D converters.

5. ### Pyrdon Thread Starter New Member

Mar 17, 2010
12
0
What kind of meter and where could I find those? Is that available in any electronics store?