Load Cell appears to lock up DigOle Display

Hardware
1

This may be a hardware issue, at this point I’m not sure. I built the 3Kg Scale project (using the HX711) and it works great. I wanted more of a Kitchen scale with a Display than something that logged a keg level so I added the DigOle OLED12864, in UART mode. I am using the pins 189 for the SPI with 2 for the data collection bit. UART57 is used to connect the display. This works really well but I need to weigh more than 3Kg so I am trying an SPL330 10Kg cell.

The SPL330 seems to be working predictably, giving repeatable results. The problem comes if it gets anything of a tough knock on the load cell - it appears to lock up the display. The Imp is not locked up and is still performing server.log statements but the display stops updating. If I unplug the display board and plug it back in, it will usually but not always resume function. If it does not, reconnecting the 3kg cell then power cycling the display brings it back. No Imp reload.

I am probably missing something obvious, this is what I have tried:

  1. made sure pins assignments are correct, well connected. Verified ground pins are at 0.0
  2. put a good VOM on the 3.3 looking for voltage drops. The HX711 and 3Kg cell draw 5-7 ma, the 10Kg & 711 use 8-12. Never below 3.27.
  3. added 1uf decoupling caps. Looked at the 3.3 line for spikes/drops with oscilloscope. Nothing to report but then, I don’t know what I’m looking for.
  4. added a 22uf electrolytic to decouple caps on breadboard.
  5. shortened the 10Kg wiring to be identical to 3Kg scale
  6. powered the 10Kg unit and HX711 from a separate 3.3 source, details to all grounding.
  7. diode across HX711 E+ to E- to dampen possible “flyback”.
  8. checked the 10Kg unit for shorts, internal & to the case.

Through all this I get the same results. It works for hours but the first non-gentle load cell tap locks the display, even if their 3.3 volts comes from separate sources. The code running on the Imp is not interrupted and will resume when the external hardware is reconnected. Even if the load cell is crap, I’m expecting it may be, I can’t see how it can disturb the display so predictably.

Dave

2

Do you server.log your weight measurements? Do they look ok?

Are there any initialization commands for the OLED? Which boards get subjected to the shock, is it just the imp and load cell, or the display too?

3

It is just the load cell that sees the “clunk”. There are short wires from the cell to its HX711, not even it sees the vibration. There are init commands for the OLED but I include them in the current display loop so that unplugging and reconnecting the OLED gets the scale running. The Imp continues to display the server.log values. The Imp appears to be very tolerant of having things disconnected and reconnected. I was certain that I was getting something of a spike through the power, hence powering the load cell from a different 3.3 - no change! Again, using the 3Kg cell makes for a robust device so the breadboard thing is not really a suspect.

This morning I tried powering the display from 2 AA batteries and that seems to be better - but I’ve thought that a few times. I’m still puzzled how the one device can knock another off, even with decoupling present. My 'scope goes to 10M which normally sees me through Guitar amps, etc but isn’t giving me much of a clue here.

Dave

4

How good is the ground between the display and the rest of the system?