Can I read a Load Cell value from Electric Imp?

Question

I would like to connect a Load Cell to an Electric Imp and read the weight's value.

I have found quite a few of tutorials where a temperature sensor is attached to the Electric Imp (see: https://www.youtube.com/watch?v=_Z6UdGik3z0) but for some reason when I see examples with Load Cell it seems that I have to use:

• A load cell
• A load cell amplifier (for example: XH711 - Sparkfun - Load Cell Amplifier)
• A board (for example: Arduino mini pro - Sparkfun - Arduino Pro Mini)

My question is, can I just connect a load cell (or maybe a load cell amplifier) directly to the Electric Imp and read the value from there?

Answer 1

A load cell provide the tiniest voltage as an output. This voltage is in the millivolts.

The XH711 is not just an amplifier but also a 24bit ADC that can handle bipolar signals.

If we assume that the reading is 80mV full scale (using the XH711 as an example) and the electric imp has a 12bit ADC and runs on 3.3 volts then in that 8mV scale you have about 99 steps which is 4.1% of the the 12bit ADC range.

You could use the electric imp alone but your reading would be so in the mud it would not be worth it. Further the imp is not capable of reading negative voltages.

Use the amplifier/ADC combo like the XH711. Then read the digital output with the electric imp.

Answer 2

You certainly need an amplifier. A load cell output is typically 1 - 2 mV/V full scale. So for example a load cell, which is capable of measuring maximum 50 kg, with an 5 V ecitation voltage will produce 5 - 10 mV, it is a quite small signal to measure it accuretly.

The Imp's ADC has 12 bit resolution, and runs on 3.3 V. So the smallest step is \$ \frac{3.3 V}{2^{12} - 1} = \small 0.806\ mV \$. In a very very ideal and theoritical, noisless world it would give you \$ \large \frac{10\ mV}{ 0.806\ mV} = \small 24 \$ distinguishable results (~2 kg resolution for the scale). In the real world, nothing useful.

So you definetly need an amplifier to bring up the signal to a level, that the ADC can measure.

1. Now it can be the one you linked. It is an all in one, both amplifier and ADC. The ADC has twice the resolution of the Imp's ADC which is a great advantage. It communicates over TWI ( or I2C) which is supported by the Imp.

2. Design you own amplifier using an Instrumentation Amplifier. Here are some criterias for choosing the right amplifier. Then you can feed the output of the INA to one of the ADC of the Imp, which has only a 12 bit ADC.

You did not tell us what is your desired resolution, which is really important. It matters what do you want to measure. 100 kg full scale with 1 kg resolution or 1 kg full scale with 0.1 mg resolution are not the same.

If you need as much resolution as possible and not familiar with hardware designing choose option 1.. This is the fastest and easiest way. If you have time, do not need as much resolution (so if you are fine with a 12 bit ADC), and want to do some experimenting then choose option 2..

If interested I recommend this article about Wheatstone-bridges in strain gage applications and this reference design about a weight scale.