sparkfun HX711 User manual

Load Cell Amplifier HX711 Breakout Hookup
Guide
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Getting Started
The HX711 load cell amplifier is used to get measurable data out from a
load cell and strain gauge. This Hookup Guide will show you how to get
started with this amplifier using some of the various load cells we carry at
SparkFun.
What You Will Need:
For this simple hook up guide we will just be hooking up a load cell with the
HX711 amplifier, and showing how you would hook up four load sensors
with a combinator board and the HX711 amplifier. To follow along, you’ll
need:
• SparkFun Load Cell Amplifier - HX711
• Any Strain Gauge Based Load Cell:
Load Cell - 200kg, Disc
(TAS606
)

SEN-13332
Load Cell - 50kg, Disc
(TAS606)
SEN-13331
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If you are planning on using load sensors you will need to obtain or
purchase four units. We recommend our Combinator Board to make it easy
to turn the four strain gauges into a wheatstone bridge type load cell.
(Single strain gauge load cells only have three wires instead of four.)
Suggested Reading
If you aren’t familiar with the following concepts, we recommend reviewing
them before beginning to work with the HX711 Load Cell Amplifier Board.
• Load Cell Basics
• Getting Started with Load Cells
• Installing the Arduino IDE
• How to Power Your Project
• Battery Technologies
• How to Solder
Load Cell Set Up
A selection of different load cells
Depending on the type of load cell you are using, the configuration of how it
should be hooked up to plates or surfaces will change. Below are a few
different types of setups.
Load Cell - 10kg, Straight
Bar (TAL220
)

SEN-13329
Load Cell - 10kg, Wide Bar
(TAL201)
SEN-13330
1
1. [Strain gauges are two wired organized metal foil or wires that are set up in such a way that the resistance changes
when it is compressed or stretched. When a strain gauge is placed on something (usually metallic in nature) its
resistance changes based on the stress experienced by that something. When a single strain gauge is hooked up to a
metallic cell, we are calling that a load sensors, which have three output wires. Load cells usually has four strain gauges
hooked up in a wheatstone bridge formation, which have four output wires. For more information on load cells, strain
gauges, and wheatstone bridges see our tutorial.]↩
Page 2 of 10

Bar load cell between a two plate configuration
S load cell configuration
Possible four disc load cell configuration in something like a bathroom scale
Bar strain gauge based load cells
Usually with larger, non-push button bar load cells you will want to hook up
the load cell between two plates in a “Z” shape, with fitting screws and
spacers so that the strain can be correctly measured as shown below:
Page 3 of 10

Note that only one side of the load cell is screwed into each board. This
provides a moment of force, or torque, on the strain gauge rather than just
compression force, which is easier to measure and much more accurate.
For smaller, push-button or disc load cells, you will want to make sure to
screw in the disc to a bottom plate (or surface you are measuring force
against), and center the beam, plate, or whatever else you are wishing to
measure the force of onto the “button” on the top. Usually another plate with
a hole is used to make sure whatever you are measuring is hitting the same
spot on the load cell each time, but it is not necessary.
Make sure to read the datasheet for the load cell you are using and get the
correct screws to fit into it.
• Note: If you are hooking together four of the SparkFun Load Sensors
using the Combinator board, you should position the four load
sensors equidistant from each other, just like the bathroom scales
shown in this tutorial.
Load cell measurements can be off by +/- 5% due to a range of things
including temperature, creep, vibration, drift, and other electrical and
mechanical interferences. Before you install your scale, take a moment and
design your system to allow for easy calibration or be able to adjust the
code parameters to account for these variations.
Hardware Hookup
Page 4 of 10

The HX711 Load Cell Amplifier accepts five wires from the load cell. These
pins are labeled with colors; RED, BLK, WHT, GRN, and YLW. These
colors correspond to the conventional color coding of load cells, where red,
black, green and white wires come from the strain gauge on the load cell
and yellow is an optional ground wire that is not hooked up to the strain
gauge but is there to ground any small outside EMI (electromagnetic
interference). Sometimes instead of a yellow wire there is a larger black
wire, foil, or loose wires to shield the signal wires to lessen EMI.
Four strain gauges (SG1 through 4) hooked up in a wheatstone bridge
formation
Here we have a large black wire, some loose wires, and foil and loose wires
respectively as EMI buffers
In General, each load cell has four strain gauges that are hooked up in a
wheatstone bridge formation as shown above.
The four wires coming out from the wheatstone bridge on the load cell are
usually:
• Excitation+ (E+) or VCC is red
• Excitation- (E-) or ground is black.
• Output+ (O+), Signal+ (S+)+ or Amplifier+ (A+) is white
• O-, S-, or A- is green or blue
Page 5 of 10

Some load cells might have slight variations in color coding such as blue
instead of green or yellow instead of black or white if there are only four
wires (meaning no wire used as an EMI buffer). You might have to infer a
little from the colors that you have, but in general you will usually see these
colors.
If the readings from the HX711 are opposite of what you are expect
(for example the values decrease as you increase weight) simply
reverse the O+/O- wires.
Once the load cell is is hooked up to the amplifier, you can hook up VDD,
VCC, DAT, CLK, and GND to a microcontroller such as a RedBoard or
Arduino board.
Note VCC is the analog voltage to power the load cell. VDD is the digital
supply voltage used to set the logic level.
PRO TIP: In many cases, you can just short VCC and VDD together. If
your microcontroller uses 3.3V logic however, you'll want to connect
VCC to 5V and VDD to 3.3V.
Load cell wires hooked up to the HX711 Amplifier board
The example code has DAT and CLK hooked up to pin 3 and 2
respectively, but this is easily changed in the code. Any GPIO pin will work
for either. Then VCC and VDD just need to be hooked up to 2.7-5V and
GND to ground on your microcontroller.
Fritzing diagram of HX711 amplifier connected to a RedBoard
Now, if you would like to set up four single load sensors with our
combinator board and amplifier, connect the five pins labeled RED, BLK,
WHT, GRN, YLW to the matching pins on the HX711. Next, connect each
of the four load sensors to the following pins:
• Red →+
•Black→-
•White→C
Page 6 of 10

The combinator board also has room for an 8 pin RJ45 socket, which can
be used to connect your project via Ethernet cables for long distance
applications.
Another nice thing about our combinator board is that most home scales
use four single strain gauge load sensors, so this is a handy board for
hacking your own scales at home!
Hacked home scale’s four load sensors hooked up to our combinator board
Example of a single strain gauge, or load sensor. Here RED is the center
tap
For load sensors, there isn’t a set color coded standard. Comparing the
scale pictured above with the load sensor schematic, while the black wires
matched, the red and white wires were swapped. Also, only two of the four
sensors used a white wire for the ‘center tap’ of the load sensor, the other
two used green. I connected the black wires to “-”, the red to “+”, and the
white and green wires to “C”.
To determine how to hook up your single strain gauge load cells to the
combinator, measure the resistance between the three wires. You should
find a larger resistance (close to double) between a pair. In our example the
resistance between red and black was 1.6 kΩ, and the resistance between
white/green and red was 800 Ω. Therefore, the center tap to the strain
gauge is the white/green wire. The center tap or center pin of your strain
gauge connects to the “C” pin on the combinator. The larger resistance
wires (red and black in this example) connect to the “+” and “-” pins on the
combinator.
Page 7 of 10

The combinator board hooks up the four load sensors in such a way that
two resistors in the wheatstone bridge configuration are constant values
and the other two are variable in this way:
To hook up the combinator board to the HX711 match the RED, BLK, WHT,
and GRN pins
Once you have the combinator board successfully soldered to the twelve
wires, you can now connect it to the HX711 amplifier board via the 4
standard load cell wires. You can use short jumper wires or if your
electronics are a long distance away from your scale consider using an
RJ45 connector and an ethernet cable to connect the combinator to the
HX711 amplifier.
Arduino Code
Now that you have your load cell, amplifier, and microcontroller hooked up,
you can add your code and start calibrating your setup.
You can download the most up-to-date code and libraries from the link
below.
GITHUB REPOSITORY
If you have never worked with downloading Arduino libraries or need a
quick reminder you might want to take a look at our tutorial on Installing
Arduino Libraries.
Or you can easily get started and running with everything in Codebender!
The first thing you will want to work with is the calibration code:
“SparkFun_HX711_Calibration”:
Page 8 of 10



 
SparkFun_HX711_Calibration (https://codebender.cc/sketch:123175?
referrer=sarahalmutlaq) by sarahalmutlaq
(https://codebender.cc/user/sarahalmutlaq?referrer=sarahalmutlaq)
ÈClone & Edit
(/?
referrer=sarahalmutlaq)
3Download
(https://codebend
e
referrer=sarahalm
u
[Edit
Please select a board 
/
*
Example using the SparkFun HX711 breakout board with a sc
a
By: Nathan Seidle
SparkFun Electronics
Date: November 19th, 2014
License: This code is public domain but you buy me a beer
This is the calibration sketch. Use it to determine the c
a
outputs the zero_factor useful for projects that have a p
e
Setup your scale and start the sketch WITHOUT a weight on
Once readings are displayed place the weight on the scale
Press +/or a/z to adjust the calibration_factor until t
h
Use this calibration_factor on the example sketch
This example assumes pounds (lbs). If you prefer kilograms
calibration factor will be significantly different but it
Your calibration factor may be very positive or very nega
t
and the direction the sensors deflect from zero state
This example code uses bogde's excellent library: https:/
/
bogde's library is released under a GNU GENERAL PUBLIC LI
C
Arduino pin 2 > HX711 CLK
3 > DOUT
5V > VCC
GND > GND
Most any pin on the Arduino Uno will be compatible with D
O
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

 
To program your Arduino from your browser, please use
Google Chrome (http://www.google.com/chrome/)/Chromium
(version 41 and above on Linux) or Mozilla Firefox
(http://www.mozilla.org/en-US/firefox/) (32bit only on Windows).
Dismiss
Once you have calculated your calibration factor of your load cell set up,
you can move on to other code, such as the simple scale output example
code, “SparkFun_HX711_Example”:


 
SparkFun_HX711_Example (https://codebender.cc/sketch:122922?
referrer=sarahalmutlaq) by sarahalmutlaq
(https://codebender.cc/user/sarahalmutlaq?referrer=sarahalmutlaq)
ÈClone & Edit
(/?
referrer=sarahalmutlaq)
3Download
(https://codebend
e
referrer=sarahalm
u
[Edit
Please select a board 
/
*
Example using the SparkFun HX711 breakout board with a sc
a
By: Nathan Seidle
SparkFun Electronics
Date: November 19th, 2014
License: This code is public domain but you buy me a beer
This example demonstrates basic scale output. See the cal
i
specific load cell setup.
This example code uses bogde's excellent library: https:/
/
bogde's library is released under a GNU GENERAL PUBLIC LI
C
The HX711 does one thing well: read load cells. The break
o
based load cell which should allow a user to measure ever
y
Arduino pin 2 > HX711 CLK
3 > DAT
5V > VCC
GND > GND
The HX711 board can be powered from 2.7V to 5V so the Ard
u
*/
#include "HX711.h"
#define calibration_factor 7050.0 //This value is obtaine
d
#define DOUT 3
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

 
To program your Arduino from your browser, please use
Google Chrome (http://www.google.com/chrome/)/Chromium
(version 41 and above on Linux) or Mozilla Firefox
(http://www.mozilla.org/en-US/firefox/) (32bit only on Windows).
Dismiss
Check out the other example code in the Github repo, or Codebender for
powering down the HX711 (github, codebender) and known zero startup
(github, codebender).
Page 9 of 10

Resources and Going Further
Want to know more? Check out this tutorial if you haven’t already:
Need even more? Check out this awesome article wheatstone bridges and
load cell types.
Getting Started with Load Cell
s
J
UNE 11, 201
5
A
tutorial defining what a load cell is and how to use one.
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https://learn.sparkfun.com/tutorials/loa
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