# Tag Info

15

The strain gauge elements come with a positively stress-sensitive portion and a negatively stress-sensitive portion. If you wire them up carefully by flipping them around so the stress sensitive portions unbalance the bridge constructively, you can use all four sensors without any extra resistors. jonk's link to the blog post at http://www.nerdkits.com/...

11

The scale is never really "off". Instead, it just turns off the display and other peripheral circuits such as the ADC and goes into a low-power mode internally. Every now and then (most likely based on a timer interrupt), it powers up the ADC briefly and checks the sensor to see if the weight has changed, and if so, it turns the rest of the peripherals, ...

9

[This is a more theoretical addendum to the answer by @VoltageSpike. ] Active driven guard is used when the signal source (the sensor) has a high output impedance. The amplifier input has a (very) high impedance, so when the signal source also has a high impedance, then leakages to and from the signal lines can introduce a significant and uncontrolled ...

8

Flashing Once you upload your sketch to an Arduino, it does not need to be connected to a computer anymore; the Arduino IDE uploads the sketch into Flash of the Arduino, which will stay there until overwritten again, even when the Arduino is powered off and not connected to any power source. The uploading process is called Flashing (done by the Arduino IDE ...

7

OK, problem solved. The bridge is connected like this. Only one resistance in the load cells is variable, the other is fixed. Why the confusion above? I was measuring resistance of a load cell which came from a different scale. The cells looked pretty similar, therefore I thought they were the same. But they were not! Eureka! simulate this circuit – ...

7

It's likely that your input is simply quite noisy. Applying some form of averaging here is your best bet. The simplest solution is to take n samples and average them together to produce a result. This divides your sampling rate by the number of samples you use per averaging interval. Another option is to use a digital filter; the simplest to implement is a ...

7

I do not know how it is done in arcades, but there are several ways of doing this. A hollow gas filled interior and a pressure sensor to record the pressure change profile when struck An accelerometer (probably the easiest) to record the acceleration profile for a known mass

7

If the objective is to shield from RF, then its only necessary to connect the shield on one end to the ground of the PCB or chassis ground (and not on the sensor end which would make a ground loop). It's not necessary to drive the shield. Driving shields is usually used for very high impedance measurements, when the sensor current is in the nA range or below ...

6

Typical creep (parameter "1") of a Vishay load cell over the stated 30 minute interval. The Vishay document states: OIML recommendation R76 requires a 30 minute test and specifies an error limit for this time period, as well as the last 10 minutes (20 to 30 minutes). NTEP requires a one hour test and specifies an error limit for this period. OIML ...

6

Well, let's start with your range and accuracy specification (and circle back to some other things later): 5 grams out of 200kg full scale is a factor range of 200 / 0.005 = 40000 or put differently, that's 1/40000 = 0.0025%. That's a pretty tall order for noiselessness, so you will need some form of ranging, which you apply externally. Just amplifying the ...

6

The load cell parts you have are most likely arranged as shown in the first diagram below. (This is a typical half bridge.) You should use an ohm meter to confirm the wire colors. In some low cost load cells one resistor (strain gage) will be active (changes with stress), and the other a reference (which can also help temperature compensate the other). ...

6

You don't need to 'remove the spikes' so much as 'give the right reading'. There are at least two good possibilities for what's happening, and they require different software filters. Then there are bad possibilities, that may require a rethink of the mechanical arrangement. The difference between the two that can be handled in software depends on the ...

6

If you dither the signal and average more than 256 samples you can get some apparent resolution (not accuracy) increase, but frankly you’d be far better off ignoring the on-chip ADC and using an external ADC designed for load cell applications. The one you mention (HX711, made by Avia in Xiamen, China) is only about 40 cents (24-bit delta-sigma with on-...

5

According to Vishay Revere, they should be connected in parallel like this: -

5

Simple thing to do would be to use digital pocket scale. This is frequently done on the R/C world. This may be enough for a start. (Source. There are more photos of this setup, as well as other ones.)

5

I think in your schematic that you haven't tied AGND into proper GND: - The above is a sample circuit I found on google that clearly shows that AGND connects to a GND symbol and your circuit doesn't appear to. There may be other errors too but this is the most obvious. Red box and red AGND are my additions to make it clearer to you. Here's another ...

5

Well, the really low tech way to do this is to use a seismic mass (i.e. a chunk of iron:D) attached to a spring or a pendulum and some kind of sensor to detect the movement. Since we are literally punching the device you need something sturdy. You can also use a load cell if it's fast enough. Most standard cell are quite slow and may damp the peak. Really, ...

4

Taking Nick's idea a step further, how about 2 plates with a slightly compressible dielectric between them. Then you've got a pressure controlled variable capacitor. Build a simple RC oscillator and measure the frequency of the output. Drive the whole thing with a microcontroller that calibrates itself when it's first turned on so it self-adjusts for the ...

4

A few things to be aware of first: With only 10 bits your 1 bit resolution will be (Full Range)/1023, in this case 40kg/1023 = 0.0391kg (39.1g). The 1 bit noise level (39.1g) at the Arduino analog input is 5v/1023 = 4.89mv, (assuming a 5v Arduino system). At the amplifier input the 1 bit voltage value is further divided by the amplifier gain. So any system ...

4

Virtually "any old voltage" will do if the "any old voltage" is used for both bridge excitation AND ADC reference voltage. Basically you will get no errors theoretically. This is called a ratiometric measurement.

4

In an ideal in amp the equation you have is true. In a real in amp we have to deal with something called offset voltage. This will be the real difference between the inverting and non-inverting inputs, as opposed to the 0 found in an ideal in amp. Looking at the datasheet we see that it has a worst case value of 125uV. Multiplying this by the gain of 381 ...

4

The regulator is the problem - you are exciting the bridge with 12 volts and this means that the typical voltage on the bridge output (to ground) is half this value at 6V. You are running the chip from 5V and so I suggest you feed the bridge power connection from 5V. The next problem you will have is that the offset (reference) pin of the INA122 is ...

4

The one important spec you are missing is the voltage range out of your load cell, with whatever resistor you might connect to it. From that you can tell the maximum gain without clipping against the A/D range. Since precision is the most important spec, you should use a delta-sigma A/D converter since these can go to 20 bits or more. Note that 20 bits is ...

4

A load cell with a fast ADC and FIR filter would do the trick without any problems. You would require a trigger input to store the result at right moment. The dynamic properties is conditioned by the mechanical construction of the cell itself. I have experince with HBM FIT1 digital cells, where the cycles are 30k per day within the last 10 years. You can ...

4

The load is applied to the single high spot on the top bracket, (in between the two rivets). On the dimensioned drawing the load is applied in the same direction as the 7.8 mm dimension. The outer frame of this load cell is mounted to a flat surface. In a weighing scale product the outer frame is often held in place with a matching slot. You could ...

4

I've opened a lot of bathroom scales some months ago to 'steal' some knowledge for my projects and I had the same question. Most of them are using a 'vibration sensor' to detect when people step up on that scale. Here you can see how a 'vibration sensor' is working: http://www.sunrom.com/p/vibration-sensor This is much more efficient and simple to ...

4

I have also found many documents related to Junction Box or Summing Box for load cells that state that connecting the bridges in parallel is the way for averaging the signals, but I found no one that provided an explanation of the working principle for this. Motivated by what Phil G pointed about using Thevenin equivalent circuit and as I found some ...

4

This ended up being a software problem. The HX710 has two modes: A) differential input B) DVDD-AVDD measurement. You set the mode for the next conversion period by the amount of PD_SCK trailing pulses in the current conversion period. This is shown on page 5 of the datasheet: In the driver code, I neglected to disable interrupts during the data clock out ...

3

What you have is probably two strain-gauges in each package forming a half-bridge. The wire colours could mean anything so what you have to do is take a multimeter and measure the resistance. You should be able to figure out what you measure from this: - simulate this circuit – Schematic created using CircuitLab Between two of the three wires you ...

3

It's likely a flex sensor, or force sensitive resistor. These are long devices that change resistance according to the amount of flexure (flex sensor) or force applied (force sensor) along the tape. If this is the case, you can measure the resistance, and should find that it changes as you bend or put pressure on the tape. Breathing and movements aren't ...

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