Bench Talk

(Source: Mouser Electronics)

One year ago, I designed a Smart Mailbox using the Arduino MKRFOX 1200 board. The board connects to the cloud through the SigFox network. The Smart Mailbox was constructed at the Mouser Electronics Germany branch in Munich, and we have been testing it for the past year. In the following, we’ll review the performance of the board over the previous year.

Background on the Smart Mailbox Project

SigFox allows users to send up to 140 messages per day, with each carrying a payload of 12B. In my application, I didn’t need to send the whole payload. So to save some battery power, I reduced the payload to only 1B. This 1B contained hexadecimal of the remaining battery percentage. The battery percentage is calculated using the equation below:

This calculation is a rough estimate of the battery’s state of charge. Ideally, a battery charging circuit such as this is needed to calculate the battery capacity, which then can be used to calculate an accurate state of charge. Based on Arduino’s website, if you supply the MKRFOX 1200 board using two AA or AAA batteries (1.5V each), the board should operate for up to six months with typical usage.

Update on the Smart Mailbox Project

Our smart mailbox has been sending three messages per day on average. SigFox offers the user the option to pull out received data with a limitation of 50,000 entries. The data is saved in a CSV format, as indicated in a graph (Figure 1). The graph shows the surprisingly impressive power consumption of the MKRFOX 1200 board. The blue line represents the battery state of charge, and the red line is a trendline to simplify the fluctuations of the battery level changes.

Figure 1: The chart shows the Arduino MKRFOX 1200 power consumption. (Source: Mouser Electronics)

The batteries were fully charged when I started building the project, but you can see the graph beginning at an 80 percent state of charge in the chart. This state of charge was when the board was mounted on the mailbox and was operating normally. The 20 percent battery percentage was lost because of testing during the building process. An important point to consider is that the MKRFOX 1200 is programmed to go into sleep mode after the initialization process and only wake from the sleep mode when triggered by the sensor. By this, the Arduino MKRFOX 1200 is operating in low-power mode most of the time. Figure 1 shows that the curve is very similar to a typical 1.5V AA-battery discharge graph. The assumption is that our smart mailbox is reaching the cutoff voltage very soon.  

SigFox Subscription Renewal

The Arduino MKR FOX 1200 includes a one-year free subscription to the Sigfox network. In the following section, we’ll show steps on how renew the subscription.

1. Navigate to https://buy.sigfox.com/.
2. Login using the same SigFox ID you use for the Sigfox Backend.
3. After you log in, click on Buy Connectivity.
4. Choose your preferred plan and proceed to the payment.
5. After you completed the payment, navigate to the SigFox Backend.
6. Click on Device Type.
7. Copy the new subscription contract ID, which is marked red in the image Figure 2.

Figure 2: SigFox backend contract ID. (Source: Author)

8. On the same page, click on your DevKit and click on Edit.
9. Paste the new contract number in the contracts field, and a list will pop up where you confirm your selection (Figure 3).

Figure 3: SigFox backend subscription activation. (Source: Mouser Electronics)

10. Click on OK at the bottom of the page, and your Arduino MKRFOX1200 should be back in operation.

Overall, the Ardunio MKRFOX1200 has been operating very well in terms of power consumption and reliability. The SigFox network coverage is also excellent in my area, but this depends on which area you want to deploy your application.