Interface the HX711 to Pi

Building a Raspberry based electronic scale using the DCT Electronic HX711 board is as easy as Pi!

The DCT board comes with a metal shield and two sets of male headers: straight and right angle.

Required Parts

HX711 board without driver chip

There are several HX711 boards available. We have been testing the brown boards that have a metal shield. There appear to be two versions of those available. Some only have one chip, some have two.

Prepare the HX711 board

First prepare the HX711 board by soldering the shield and connectors.

Installation

Method 1: Connect HX711 to P1/J8

HX711 with screw terminals and metal shield installed connected to P1/J8 on the Pi.

P1/J8 close up

This will work with all models of the Pi: A, B, A+, B+.

Pins 2, 16, 18, 20 on P1 (Model A and B) or J8 (Model A+ and B+)are used:

P1/J8	Pi	HX711	Color
Pin 2	+5	Vcc	Red
Pin 16	GPIO28	DO/RX	White
Pin 18	GPIO30	CK/TX	Green
Pin 20	Ground	GND	Black

#define CLOCK_PIN 24
#define DATA_PIN 23

Method 2: Connect HX711 to P5

Two DCT Electronic HX711 boards with metal shield.

DCT Electronic HX711 board plugged into P5

You will need a 1x4 female header (2.54mm Pitch Straight Single Row PCB Female Pin Headers) and load cells.

1. Order the board for $6.99 on Ebay
   This one for $2.17 should work, too.
2. Download the software from gitHub
   git clone https://github.com/ggurov/hx711
3. Solder a 1x4 straight female header in P5 on Pi (bottom side of board) and a 1x4 male header on the DCT board
4. Solder the load cells to the DCT board.
5. Plug the DTC board into the Pi.
6. Change the GPIO pins in hx711.c from 30 and 31 to 28 and 30. This way they are all on one side of P5:

P5	Pi	HX711
P5-1	+5	Vcc
P5-3	GPIO28	DO/RX
P5-5	GPIO30	CK/TX
P5-7	Ground	GND

#define CLOCK_PIN       30
#define DATA_PIN        28

Temperature Compensation

Temperature drift using the HX711 and CZL635. This is a 37.9 lb piece of railroad track

Temperature Sensitivity

From the CZL635 Spec Sheet link below:

Temperature Effect on Span 2.5 g/°C
Temperature Effect on Zero 5 g/°C

From the CZL602X Spec Sheet link below:

Temperature Effect on Span 0.02%F.S/10℃
Temperature Effect on Zero 0.03%F.S/10℃

Full Scale = 50 kg 

.02% F.S/10℃
.0002 * 50 kg = .01 kg / 10℃
              = .001 kg / 1℃
              = 1 g/ 1℃

So the CZL602X are 2.5 times more stable than the CZL635

HX711 Spec Sheets

http://hivetool.org/w/images/d/d2/Hx711_english.pdf

Load Cells

Spec Sheets

1. CZL635 Spec Sheet (from Phidgets.com)
2. CZL602X Spec Sheet

Theory and Application Notes

1. LOAD CELL TECHNOLOGY IN PRACTICE by REVERE TRANSDUCERS APPLICATION NOTE 07/6-13/01