Digital Tension Scale, Part IV

Posted 25 December 2018

In my copious free time I have been refining the design for a low power battery operated tensionometer.  In my last post on the subject, I had described the components I had planned to use, and in the ensuing weeks I have been working on implementing this design.  There are several challenges in this project:

Bluetooth Link:

There are a huge number of Bluetooth products out there in the Maker-verse, with varying degrees of Arduino support, and widely varying performance characteristics.  To add to the confusion, there is ‘regular’ Bluetooth and the more recent ‘BLE’ (Bluetooth Low Energy) which are completely incompatible with each other.  As I now understand it, BLE is synonymous with Bluetooth 4.0+ (the iPhone 4S was the first smartphone to implement the new standard).  However, the most common product in use in the Arduino world seems to be the venerable HC-05 ‘regular’ Bluetooth module, available from your local grocery store (well, not quite, but from almost everywhere else!) for not much more than a few pennies

I had no previous experience with BT modules, so this part of the project took some time, and was the last major part to be accomplished.  After receiving my HC-05 modules from China, I used this tutorial to get started.  The real challenge for this part of the project wasn’t getting the HC-05 hooked up to the microcontroller – it was sorting through all the layers of BT-related settings on my Win 10 laptop to pair with the HC-05 device and determine which serial port did what.

  • In Windows 10, I used the ‘Bluetooth and other devices settings’ panel (Settings -> Devices -> Bluetooth and other devices) to find and pair to the HC-05.  The device shows up as ‘HC-05’ and the default pairing password is “1234”.
  • When the HC-05 is paired it automatically sets up at least two serial ports that show up in device manager as ‘Standard Serial over Bluetooth’ ports, as shown below.  However, only one of these ports is actually usable for two-way communication, and it isn’t clear to me why, or how to tell which is which;  I had to experiment with each available ‘SSoB’ port to figure out which to use (so far, it seems like the highest-numbered port is the proper one).
  • After the HC-05 is paired and the com ports are set up, then any serial terminal app (I used RealTerm) can be used to communicate between the PC and the microcontroller via the HC-05.
  • On the microcontroller (I used a Teensy 3.2 with multiple hardware serial ports), I wired the HC-05 to Serial1 leaving Serial (Serial0) available for normal communication between the Teensy and my Visual Studio 2017 Community Edition/Visual Micro add-on for Arduino development platform.

Physical Layout:

The original idea behind this project was to create a self-contained battery-operated digital weight scale that could display weight values on a local display, but could also stream the data live to a remote recording station like a laptop or smartphone. The ‘self-contained’ part requires that all the electronics be mounted on the S-shaped load cell assembly itself, and to that end I designed a housing that connects to the two bolts that hold the arms of the load cell.  The idea is that all the electronics save the LCD display and the battery will be mounted to the underside of the box lid so that servicing would be easier.  Also, by mounting everything to the lid, I can make cutouts for the charger and Teensy USB connectors for easy charging and reprogramming.  After several iterations in TinkerCad, I came up with the following design

Looking up at the underside of the box lid, showing all modules except the battery and the LCD display

Showing the top of the lid with the mounting bracket for the load cell

Isometric view with transparent box walls. The LCD display module is under the battery.  Note the cutouts for the charging and programming USB-C connections

Module Integration:

I had previously tested each module individually, but hadn’t had all of them working at the same time.  I had tested the HC-05 with an Arduino Mega, and I had tested the load cell with both a Sparkfun Pro Micro and with a Teensy 3.2, and I had tested the Nokia LCD display with a Teensy 3.2, but I hadn’t put everything together.  So I wired everything up on my half-size ASP plugboard and got it all working together with a simple program (included below) that exercised the LCD Display, the load cell, the BT module, and the battery charger, as shown in the following photos

RealTerm Serial Terminal Program showing load cell readouts collected wirelessly via the Bluetooth HC-05 modules

HC-05 Bluetooth, HX-711 Load Cell Amp, Sparkfun Charger, and Teensy Microcontroller modules integrated together. Note disconnected USB cable showing that the circuit is running on battery power. The scale is currently measuring 1.8 liters of water in the suspended plastic bag (note the ‘1.8 Kg’ reading on the LCD display)



The software used for the above integration tests is a reasonably complete sketch for day-to-day use of the digital weight scale.  It displays the measured weight on the LCD display, and also sends it to the USB serial port for display on a directly connected PC, and to the HC-05 Bluetooth module for display/capture via a BT-connected laptop or smartphone.  This program is shown below:

However, this program depends on the proper calibration of the load cell, which I have been doing with a separate sketch (also included below):

What I need to do now is to combine these two programs into a single sketch with ‘operating’ and ‘calibration’ modes.  My calibration program already does this to some degree, as it waits 5 seconds on startup for the operator to send the ‘y’ key via the direct-connect serial port. If the ‘y’ character is detected within this window, then the program starts the calibration sequence; otherwise it starts taking measurements as normal.  This behavior needs to be expanded somewhat in that it should accept a calibration command either through the direct-connect serial port (Serial) or via the BT port (Serial1).

Low Power Operation:

I have already done some experimentation on low-power operation of the Teensy 3.2, using Colin Duffy’s fine ‘Snooze’ library, and have determined that I can easily drop the Teensy’s operating current from around 20-30 mA to about 1-2 mA by putting it to sleep during periods of load cell inactivity.  Assuming I get the full 2500 mA hours out of the battery, then I can expect something like 1000 hours or about 40 days between recharges.  However, more work needs to be done to get the low power mode fully operational.


Stay Tuned!



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