Hi all, newbie here.
Just found this web site and think that some Sparkfun products will help with my project. I want to continuously monitor the weight of the keg in my kegerator to estimate the amount of beer remaining.
The HX711 Load Cell Amplifier (SEN 13879) looks perfect. But, I’m afraid the gain seems too low. I’ve hacked a bathroom scale, connecting the four load cells as a wheatstone bridge. With 5V excitation, I only read about 6 mV across the bridge when I stand on the scale. I’m 185 lbs, about 20 lbs heavier than a full keg. The HX711’s highest gain setting provides a FS reading with +/-20 mV differential input. So, at 5 or 6 mV, I’m leaving a lot of bits on the table.
Has anyone tried something similar? Does my bridge output seem too low? Any ideas on a part with higher gain?
Thanks for any help.
Greg
Hi Greg,
Keep in mind that the HX771 is a 24-bit converter. To find the number of bits that would change over your 0-6mV range: Log2( (2^24)*(6mV/40mV)) = approximately 21.26 bits. Since you can’t use fractions of a bit, 22-bits will change over your 0-6mV range. If your connections and the Sparkfun board are built well, you will most likely be able to get an effective and accurate 16-bit conversion out of your system. After you acquire a sample, the two MSBs can be ignored (or used for error checking) since your range will never toggle them and the 6 LSBs can be dropped which will be mostly noise. The remaining 16-bits is the converted value that can be treated like any other number assuming you shift it correctly. For the purposes of weighing your keg, this is way more resolution than you require. If the signal is still noisy, you can remove more LSBs at the expense of overall resolution.
24-bit A/D output: xxxx xxxx xxxx xxxx xxxx xxxx
Bits you’re interested in: xx11 1111 1111 1111 11xx xxxx
-Bill
Thanks Bill. I came to more or less the same conclusion after thinking about it some more. Even 10 bits of resolution is fine for my application.
Greg