Download This is a report of my findings from experimenting with 2 different

This is a report of my findings from experimenting with 2 different cheap and dirty strain gauge modules that appear
to be available from the usual Ebay/Aliexpress HK/China suspects. The modules come complete with a single
350ohm strain gauge that can be attached to anything you need to measure fine strain fluctuations from.
My primary application is to utilise them for measuring the strain on my pyroless deployment device’s chamber in
an attempt to measure the gas pressure within the chamber without needing dedicated pressure transducers which
tend to be relatively big and heavy for high pressure applications. The idea is to mount the accompanying strain
gauge of the periphery of the chamber’s tube to capture the hoop strain.
Both of the modules – red board and blue board appear to be essentially an identical circuit utilising the same
components and coming complete with an accompanying 350ohm strain gauge. So, I’m assuming one is a copy of
the other or they’re both copied off another module board. Results of measuring both modules demonstrate a
significant drift, especially within the 1st 20-40 minutes from startup before eventually stabilising (if left at a constant
strain anyway). I only ordered 2 of the blue modules and 1 of the red, but from my experience, the magnitude of
stability varies from module to module and I’m assuming the red version is no more stable than the blue in a general
sense or vise versa.
I should point out at this stage that I’m not an electronics engineer nor do I have any particular qualifications or
experience within that industry so don’t go taking any of the assumptions below as gospel and likewise constructive
feedback of errors or corrections will be appreciated. Hence, the report/critique is aimed at the hobbyist or
someone not specialising in this field. I should also point out that I’ve frequently used the term “impedance”
throughout this document and I’ve generally used it in reference to DC resistance of a supply – not so much its more
typical use in AC applications. Feedback appreciated: [email protected]
Going through the schematic that accompanied the product details from 1 or 2 suppliers of the blue module:
At the very top is the LED circuit – the LED current limited through a 1K resistor – simple stuff.
Next (upper left) is what I assume is a bunch of capacitors to smooth the supply power which is pretty important for
circuits that amplify very minute signals into something usable for typical A-D conversion (ie. High gain applications)
hence why there’s a small bank of them.
Next (bottom right) is a TL431 shunt regulator utilised to provide a 2.5V reference voltage. The TL431 is adjustable,
but in the configuration shown in the schematic, it appears to be providing its default reference voltage of 2.5V
labelled as the rather mysterious initialism VFF. This reference voltage appears to be utilised as the supply voltage
for the bridge and as a biasing reference supply for the one multi-turn pot on the module used as a biasing reference
for the final amplification stage. Because this reference voltage is supplying the bridge, it’s probably quite critical
that it’s very stable and accurate. If it’s 1% out, the voltage accordingly across the bridge will be 1% out.
Measuring this output on a cheap data acquisition system appears to illustrate a very steady output of 2.5v. I don’t
have a good enough resolution to identify just how stable it is to the sub-millivolt level, but it appears very stable to
the millivolt level. Certainly not appearing like this is the cause of any drift or nuisance slow transient shifts.
The meat & potatoes:
Which leaves us with the main circuit – the wheatstone bridge and the amplification of its sub mV output.
The bridge is (obviously) a ¼ variety. For those not familiar with wheatstone bridges: there’s basically 3 flavours of
them: full, half and quarter with the level of accuracy and stability reducing respectively from full to quarter. So, this
is the least accurate and least stable of the available options, which is pretty unavoidable for a generic (user friendly)
module like this one. In a nutshell, a wheatstone bridge basically converts the change in resistance of the strain
gauge to a zero offset voltage albeit a very minute one (coz the change in resistance is so minute). An excellent
reference for strain gauges and wheatstone bridges will be listed below. The bridge comprises of resistors R2, R8, R9
and the strain gauge which is illustrated as a light dependant resistor R5 in this schematic. Interestingly, the resistors
are both labelled and measured to be 360 ohm, but the strain gauge itself is advertised and measured to be 350
ohm. I’m assuming this is a result of both parts availability constraints and wanting maximum simplicity? Anyway,
this obviously translates to the bridge being significantly unbalanced, but the 1st stage amp obviously copes with the
16-17mV offset you get with this unbalanced config. If you wanted to convert the bridge to a half bridge by replacing
R8 with another 350 ohm strain gauge (positive-positive tensile strains) or either R2 or R9 with a negative
(compressed) Strain Gauge, you’ll likely need to convert the 2 remaining passive resistors on the bridge to 350ish
ohm each to avoid saturating the 1st stage amp with the (even greater) voltage offset from the unbalancing. One
possible way of achieving this with standard value resistors is to solder a 12K resistor on top of the existing 360 ohm
resistors so they’re parallel ie. R = 1/ ((1/360) + (1/12000)) = 349.51 ohm so it’s close enough to balanced. You can
get closer with a 13K resistor, but a 12K provides a nice positive offset to begin with.
The bridge is supplied with the reference voltage from the TL431 which is 2.5V. This is to suit the op-amp input
which typically needs to be based around half its rail-rail voltage. Being such a low supply voltage, it does reduce the
level of output voltage which the bridge converts the strain gauge resistance to, which basically translates to more
gain required from the amplification stage(s) and theoretically less accuracy as a result. One virtue with the lower
supply voltage is the amount of power running through the bridge and the strain gauge itself, which can translate to
less thermal concerns from the bridge components (in particular the strain gauge). P = V2 * R
The amplification stage:
The output of the bridge is fed straight into the amplifiers or the amplification stage(s) which is comprised of a
general purpose duel op-amp IC – LM358 of which both op-amps are utilised for the amplification. Being a general
purpose op-amp and not a specific instrument amp, one might be somewhat sceptical of whether it’s up to the job
of an application that ideally requires an instrument amp. However, there is an instrument amp schematic example
in the data sheet and there is a positive anecdotal example here:
https://www.scribd.com/document/14256112/AN002-Op-Amp-LM358-as-Instrumentation-Amplifier
With the experience I have with the module so far, I’m certainly not convinced although I can’t yet pinpoint the
“warm up” deviations I’m experiencing as being caused from the LM358.
So, the voltage produced across the bridge is fed straight into the 1st stage amp (op-amp). The positive (non
inverting) input is connected to the static side of the ¼ bride between resistors R2 and R8. Being the same values and
with the bridge being supplied with a 2.5V reference voltage, the voltage being fed in to the + input of the op-amp
should be 1.25V. The –input of the 1st stage op-amp is fed from the voltage between the strain gauge and R9. This
voltage will vary with varying strains on the gauge ie. More strain = more resistance on the +ve side of the tapping
which translates to a decrease in voltage at the tapping. Again, the amount of strain we’re measuring is SFA, so the
change in resistance on the gauge is SFA and in turn, the change in voltage at this tapping is naturally SFA too.
Okay, so the dynamic (strain gauge) side of the bridge is fed into the 1st op-amp stage –ve input via a 10K resistor
(R6) and fed back from the output of the op-amp through a 470K resistor (R7) in a classic inverting amplifier config.
The gain of this stage can be simply calculated dividing the feedback resistance by the supply resistance so, G =
470k/10k = 47
In my application, a 100Psi pressure in the chamber would put a 0.0001 strain on the chamber tube which of course
translates to that strain on the gauge. The change in resistance on the gauge would be Strain * Gauge Factor (typ 2)
* Gauge Resistance = 0.0001x2x350(ohms) = ~0.07 ohms
So, @100Psi pressure SG now = 350.007 ohms
Translating that to voltage across the bridge: Vsupply*(((R8*SG)-(R9*R2))/((R2+R8)*SG+R9))) =
2.5*(((350*350.007)-(350*350))/((350+350)*350.007+350))) = 0.000125 V = 0.125 mV
Multiply that by the gain of the 1st stage amp 0.000125 x 47 = 0.005875 V
Even though I’ll be generally operating at 600-700 Psi, you can see a gain of 47 just isn’t gonna cut it for anything
short of impractical levels of strain with an impractically low resolution for a typical 10 bit A>D converter.
So, naturally, for typical ¼ bridge strain gauge applications, you ideally need a gain of at least a few hundred for the
application to be practical.
So we pass the output of that stage to the 2nd op-amp within the LM358 via a 20K supply resistor and a 470K
feedback resistor. This time yielding a gain of 470/20 = 23.5.
This gain is multiplied by the 1st stage to provide the total gain provided by the amplification circuit which = 47 x
23.5 = 1104
If we check all these numbers and assumptions with what I actually measure at say 500 Psi loading (assuming no
axial stress on chamber for simplicity):
Pressing in chamber = 500psi
(p) = 3.45 Mpa (metric conversion) = 3447380 Pa
Outside diameter of chamber tube = 1½” = 38.1mm x 1.6mm Wall (b) = 0.0016m
ID of chamber would then be 38.1-(1.6x2) = 34.9mm and mean radius (r) = 34.9/2 = 17.45mm = 0.01745m
Hoop Stress of tube inside σø = pr/b = (3447380 x 0.01745) / 0.0016 = 37597988 Pa
Modulus of Elasticity of tube material (E) (6061 T6 aluminium alloy )= 6.89x10^10
Hoop Strain on Chamber (εø) = σø/E = 0.00054569
Original Circumference (C) = 38.1 x pi = 119.69468mm
Change in circumference = C x εø = 119.69468 x 0.00054569 = 0.065316mm
Strain Gauge Factor = Typically 2
Gauge Resistance (350 ohms).
Change in resistance of SG = initial resistance x SG factor x strain = 350 x 2 x 0.00054569 = 0.38198 ohms
SG resistance with strain = Initial R + delta R = 350.38198
Supply Voltage to bridge = 2.5V
So Vout of bridge @500Psi loading = Vsupply*(((R8*SG)-(R9*R2))/((R2+R8)*SG+R9)))
= 2.5*(((360*350. 38198)-(360*360))/((360+360)* 350. 38198+360))) = -0.016924V = -16.924 mV
With zero pressure & strain on the gauge the (unbalanced) bridge would be:
2.5*(((360*350)-(360*360))/((360+360)* 350+360))) = -0.0176056V = -17.6056 mV
So the difference from zero strain to strain from 500 Psi pressure = -17.6056 - -16.924 = -0.6816 mv
Multiply that by our amplification gain = -0.6816 x 1104 = ~0.7525 V
Measured Results:
Log of SG Module Output & Pressure Transducer with Leaky
Plumbing and Periodic Repressurising
3.5
1000
900
3
Pressure Transducer Output (Psi)
Strain Gauge Module Voltage
800
2.5
700
600
2
500
1.5
400
300
1
200
0.5
100
Strain Gauge Module Voltage
0
0
0:00
0:07
0:14
0:21
0:28
TIme
h:mm
0:36
0:43
0:50
0:57
So, taking the 2 latest reference points (allowing for the module to stabilise) at 25 minutes when the pressure in the
chamber is ~500Psi and one point after I’ve relieved all the pressure out from the cylinder (say at 46 mins) I get:
Time
Module Voltage
Pressure in Chamber Psi
0:25
0:46
2.524414
3.175049
500.6
3.2
Diff
0.650635
So, 0.65v drop measured vs 0.75v drop theoretical for ambient pressure -> 500 Psi. That’s a fair bit of difference for
someone with limited experience with electronics and I don’t yet know the full explanation for the difference. I’m
assuming the strain gauge factor is 2 which is typical for strain gauges and I have no idea what the tolerance would
be if the factor was nominally 2. There’s actually not as much hoop strain as the equation above results in thanks to
the Poisson effect. Taking this effect into account reduces the resulting amplified voltage swing to 0.715V for my
particular device: the pistons & internal load shaft take most of the axial load, so the effect on hoop isn’t as
pronounced as if the casing was taking all the axial load. Resistors can have up to a 5% tolerance and the
amplification gain is quite large which amplifies deviations with resistor values and also makes measuring the output
at the pre-amplification (bridge) difficult unless you have nice equipment sensitive enough to measure it.
Okay, finishing up with the circuit, there’s also a multi-turn potentiometer (RP1 500K) connected to the +ve input of
the 2nd stage op-amp with a capacitor (C5). This appears to set the bias for the final stage amp and hence will also set
the offset voltage for the output ie. Adjusting this pot won’t change the gain produced from the amp, it will just
change the output voltage offset so you can set your zero point (no strain on the gauge) at any voltage within the
limitations of the LM358 and the supply voltage you feed it, but the magnitude of voltage drop with an increase in
strain on the gauge won’t differ with this adjustment. So, in the above example (500 Psi in my chamber) the 0.65
voltage drop will be consistent whether we adjust the zero offset to 1.5V or 3V, so if zero strain was set to 1.5V,
@500Psi loading, the voltage output would be 1.5-0.65 =0.85V. At 3V zero, it would be 2.35V at 500Psi loading.
So to actually adjust the gain, you need to actually change the values of at least one of the fixed resistors (R6 or R7
or R3 or R4).
The Capacitor (C5 – 1uf) I think is there primarily for smoothing the signal and generally placed in that spot to also
allow a low impedance path to ground for low frequencies.
…and last but certainly not least is the idiot diode (D1) between the power supply input and the Vcc rail of the
module. Basically there to avoid damaging the LM358 in the event of reversing the polarity – a very easy thing to do
for a 3 pin jumper plug with nothing to stop someone accidently connecting a socket to it the wrong way around.
Testing Results and Deeper Analysis:
I’ve tested 3 of these modules – as mentioned in the beginning 1 red board unit and 2 blue board units. Whilst the
circuits are similar (practically identical) and are comprised of the same components, it’s worth mentioning the red
board module’s component labelling doesn’t reflect the schematic that appears on some sites. The resistors at least
are numbered/labelled in a different order.
As has also been detailed previously, the gain of the modules’ amplification appears to represent the theoretical
gain within a generous size ball park and I won’t elaborate any further regarding that. I might have mentioned, the
modules are configured to reduce the output voltage with an increase in strain on the gauge ie. With an increase in
gauge resistance. Normally, you’d expect an increase in voltage with an increase in whatever you’re measuring, but
so long as you’re aware of it, there’s no particular issue with an inverted output.
The fundamental issue with the modules I’ve experienced from the testing I’ve conducted is the stability of the
output and in particular the drift in output voltage over time.
What appears to be a universal attribute with all the modules is the voltage ramp after power-up which can take
between 10-20 minutes to stabilise. The curve appears typical with that of a capacitor charging up. However, more
concerning is the drift upwards from when a significant strain is applied to the gauge which appears to take longer to
stabilise. Again, this drift could also be consistent with a capacitor charging/discharging.
Here’s an example of a module’s (blue board) output from power up with different levels of strain exposed to the
strain gauge via different (regulated) chamber pressures in my chamber:
And the same data with the voltage offsets removed to illustrate the consistency of the voltage drift you experience
at power-up for different levels of strain:
Ie. These lines should be horizontally flat if there’s no drift.
So, where the ^*%@ is this drift or voltage hysteresis coming from?



As illustrated, its profile is consistent at different levels of strain.
Its profile shares close similarity with a typical capacitor charging.
All the modules tested share the same attribute although at different magnitudes.
So, the plan of attack is to 1st concentrate on the caps of which there are 8 in total for one of these modules.
I reckon we can discount the 1st 4 caps used to smooth the power rails. No resistors or meaningful impedance of any
description, so they’re theoretically going to be charging fully within a very minute fraction of a second. I think it’s
general wisdom to assume a capacitor is fully charged at 5 times RC, but let’s be ultra conservative and work on 100
RC ie. 100 x impedance in ohms x capacitance in Farads = total change in seconds.
The next most likely culprit (to my mind) would be C5 in the schematic. It’s charged through a potentially high(ish)
impedance source (500k pot) and it would make sense that a variation in the voltage on pin 3 of the LM358 over a
period could vary the output significantly. Final amplification stage gain = (470/20) = 23.5
Problem is, even if the pot was wound out to its max (500k) the theoretical charging time of C5 should be:
0.000001F x 500,000ohm x 100 = 50sec using our ultra conservative x100 RC charge formula
That’s not really too close to the 10-20 minute time period we’re experiencing. Nevertheless, we have to measure
these points coz the source of the drift isn’t immediately obvious.
Now, there is some variation there, but the swing is … say… 5-7 microvolts and not really consistent with the output
profile we’re looking for. Let’s assume this swing was 10 microvolts – multiply that by the gain of the final stage and
you still have only a output swing of 235uV which is a small fraction of the say 250mV swing we’re experiencing at
power-up ie. >1000 times less. So, I’m crossing C1, C2, C3, C4 and C5 off the list of candidates.
C6 is the next candidate to focus on. Again, theoretically, it should be supplied with a reasonably low impedance
supply – R2 which is 360 ohms and maybe R10 which is supplying VFF which is also 360 ohms. It should theoretically
be charging relatively instantly:
0.000001F x 720 x 100 = 0.072 seconds (again using the ultra conservative x100 factor)
Let’s measure to verify (in parallel with the output of the module):
Smoothed Trendlines of Voltages of Module Output & 1st Amp
+Input (Pin 5 of LM358 dual op amp) in reference to ground
1.147
Voltage of Pin 5 (reference to Ground)
1.1468
3.12
1.1467
3.1
1.1466
1.1465
3.08
1.1464
3.06
1.1463
3.04
Pin 5
1.1462
Module Output
10 per. Mov. Avg. (Pin 5)
1.1461
1.146
00:00
3.02
10 per. Mov. Avg. (Module Output)
3
01:26
02:53
04:19
05:46
07:12
08:38
10:05
Time mm:ss
Ah ha, there’s smoke in the air! The input to pin 5 (+ve input of 1st stage op-amp) does have a resemblance to the
output profile albeit a much more noisy one. There should be no reason why the input to pin 5 isn’t a flat horizontal
line. The bridge is complete with static resistors (inc dummy strain gauge resistor) and the excitation voltage
supplying it should be rock solid – fed by the TL431 circuit.
Again, the C6 cap hanging off this input line should charge very quickly given the low(ish) impedance feed so, as I
see it there are 3 evident (to me) potential causes of this voltage swing:
(1) The feed impedance is much higher than what the numbers tell me and C6 takes time to charge.
(2) The voltage being supplied from the TL431 circuit isn’t entirely stable.
(3) There’s significant enough high frequency AC within the feed to affect the impedance (capacitive reactance) of
C6 to act as a low impedance source to ground through it (C6).
Potential cause (1) seems most unlikely – the feed impedance would need to be multiple orders of magnitude higher
than what I’m seeing in the theoretical numbers to be the primary cause. Not that difficult to measure.
Potential cause (2) seems quite unlikely – I can’t believe someone would design a circuit with an unstable voltage
source exciting the bridge and other fundamental voltage reference inputs. The whole point about including a
regulator is generally to provide a regulated & stable voltage source. If the issue was isolated to a single module,
then maybe a faulty component or assembly can be potentially come into play but…
Voltage of Module Output (ref to Ground)
3.14
1.1469
Potential cause (3) – well, there’s no obvious signs of any significant high frequency AC along the input path and it
would be very coincidental that this (primarily) could consistently trigger a voltage drifting profile that’s so similar to
a cap charging.
Potential cause (1) seems most unlikely – the feed impedance would need to be multiple orders of magnitude higher
than what I’m seeing in the theoretical numbers to be the primary cause, but it’s the only scenario that would
provide a theoretical profile that closely matches what’s measured on both the output and pin 5 of the inputs. Not
that difficult to measure and it’ll be the mechanism I’ll be focusing in on 1st with some empirical data acquisition.
Yet more empirical sampling required 