Monthly Archives: January 2019

Digital Tension Scale, Part V

posted 09 January 2019,

In my previous post on this subject I described the components I planned to use for my Digital Tension Scale project, and also the design for a box that would mount directly on the S-shaped load cell assembly.

This post describes the ‘final’ (to the extent than anything I do can be considered final) assembly of the completed system into my 3D-printed housing, and the results of some initial battery-powered testing.

As shown in the following photos, the major components (Teensy 3.2 microcontroller, HC-05 Bluetooth Module, HX-711 load cell amp/A-D, and Sparkfun ‘Basic’ LiPo Charger) were mounted on perfboard, which was then in turn attached to the box lid via a set of custom-printed standoffs.   A short piece of ribbon cable connects the Teensy to the LCD display.   The general idea behind this physical layout is to allow easy access to the electronics for troubleshooting, and to allow for battery charging and/or Teensy programming without having to open the box.

3D-printed housing. Note the glow from the Sparkfun charger LED

View of housing showing the access port for supplying USB power and/or programming the Teensy

View with the lid and electronics board removed. The LCD display is face down in its cutout

Exploded view showing all system components

Showing connections from load cell to HX-711

Top view showing how load cell attaches to the housing

Closeup showing load cell lead routing and power/programming port

End view showing charging port

 

Preliminary Testing Results:

At this point I have everything running on battery power only inside the box, and I have been able to demonstrate remote data capture on my PC using the HC-05 BT link.   The following image shows the data taken from my rowing machine, and a short video demonstrating the setup.

Complete Code:

Here is the complete Teensy 3.2 program as it stands today.   As you can see if you inspect the code, I have the Teensy low-power stuff turned OFF for the moment (that’s the purpose of the ‘#define NO_SNOOZE’ statement.

 

Schematic:

Future Work:

  • Do some more work to reduce power consumption to extend the battery life.   I got the ‘Snooze’ feature to work on the Teensy, but that only reduces the Teensy’s power consumption; it does nothing directly to reduce the power consumption of the other components.   I tried using a MOSFET to turn the HC-05 BT module on & off, and found this to be impractical, as then the module loses its connection to the remote data collection device.    I have also tried removing power from the LCD module, but that also turned out to be problematic.

Stay tuned,

Frank