This post summaries the test I made to produce a mail presence sensor. It is aimed to be wireless, connected to the Pilogger logging station. It is interesting to use a load cell in order to know the weight of the object in the mailbox. But we will discuss the difficulties to implement a reliable solution.
It could be found very cheap load cell on Ebay, with a small board featuring a 24-bits ADC. The ones I used for this test is a 10Kg cell with a HX711 board.
As the principle of sensing is based on the physical deformation of the metallic rod, the system is affected by temperature change. So I decide to use a thermistor (NTC), placed directly within the metallic rod, by placing it in a small drilled hole. (Figure 1, on the left)
The HX711 is an interesting chip for its price range. It is design to measure very tiny resistance change using a Winston bridge. It have two channels, one with two gains (64 and 128) and another with a fixe 32 gain. While the first channel is connected to the load cell, the second one is perfectly usable for measuring the thermistor. To do so we have to do another Winston bridge including this thermistor. (Figure 2, on the right)
I chose to use the same micro-controller and RF link as the previous wireless temperature probe. In other words, a PIC16F1825 and a nRF24l01+. The difference is the HX711 module and a voltage regulator. The HX711 needs a stable 3.3V, and then I use an analog input of the PIC to have a battery voltage information.
The micro-controller code is rather simple as well, it sleeps for around 8 seconds, then sequentially get 8 values of HX711 channel ‘A’ and average them, the same for channel ‘B’, then get the ADC value of the battery, and finally send the data with the nRF24l01.
The XC8 C compiler file for the PIC16F1825 is here in Github.
I fixed one end of the load cell directly to the mailbox bottom, and the other end to an aluminium plate of approximately the surface of the box. For the tests, I just fixed the proto board on a side with tape, and the battery below.
Tests and data.
The system is surprisingly sensible, a letter is clearly visible on the weight data.
But sensitivity is not precision. And things start to be more complex when I try to figure out how to calibrate the system. There is a link between temperature and weight data, but not only. The entire mechanical system plays a role.
Figure 8 shows a complete month of temperature and weight data, with arbitrary units (ADC output). While points marked with ‘1’ are the time where a letter was on the plate, the others are interesting. Point ‘3’ shows that a heavy parcel was kindly placed in the box by the postman. But after that, the weight base line did not recover to the same value. Apparently a mechanical bias appeared, it could be a permanent distortion of the load cell.
I could not fully understood the events marked as ‘2’. These drops should reflect a resistance change, so I imagine it could be linked with humidity and water condensation. It seems it disappeared once I cover the HX711 pins and copper tracks with varnish.
I tried to establish a correlation between the temperature values and the weight values in order to compensate the changes. As the event ‘3’ put an offset I have two data groups. I agree the data looks funny, and it is hard to see a true correlation. (Figure 9&10) We could barely see a kind of exponential curve such as Weight = a + b. Temp², but not enough to compensate correctly.
To summarize, the concept is clearly working. I first though the battery would not last so long, but event not fully charged, the 4000mAh li-ion is still alive after a month. (Figure 11).
The HX711 used strait out of the box without shielding gives quite good results. On the 24bit resolution, around 19 to 20 are out of noise, which is remarkable for such a draft system.
As it is, I have at least an idea if there is something present (and roughly how big) in the mailbox, and this everywhere I could have an internet access. To be continued !