What is it?

A digital control knob providing 128 unique results evenly spaced around a full circle. It is designed as a control panel knob and includes a nut and washer, but can be adopted for other uses. The module communicates via the I2C bus with choice of 16 addresses available.

This is identical to my existing Absolute Encoder Module except it has two Sparkfun QWIIC compatible connectors instead of the standard header and Grove connectors.

This is an alternative to using a potentiometer and analog pin, allowing full-turn and multi-turn operation, and is not impacted by temperature variations.

The software includes optional automatic saving of logical zero and multi-turn offsets so that the system will automatically remember these settings after power down or reset.

It differs from the more common incremental rotary encoder which has only two or four values in a rotation and is designed to measure full rotations and direction. This measures angles.

The Bourns ACE-128 Absolute Contacting Encoder is, as far as I know, the smallest and cheapest gray code absolute encoder available today. From the datasheet

Until now, the choice of an absolute encoder meant an expensive, and larger sized product. Through the use of combinatorial mathematics, the absolute code pattern of the Bourns® Absolute Contacting Encoder (ACE™) is placed on a single track for a very economical, energy efficient and compact product. Bourns® ACE™ provides an absolute digital output that will also retain its last position in the event of a power failure. An intelligent alternative to incremental encoders and potentiometers, the Bourns® ACE™ is ideally suited for many industrial, automotive, medical and consumer product applications.

The product listed here combines this sensor with an I2C backpack and library so multiple of these little wonders can be included in your next Arduino or Raspberry Pi project.

NEW: Backpack only is available as an option. In case you already have the encoder itself.

NEW: Units with integrated I2C EEPROM are now available.

You may prefer this to using a potentiometer and analog pin on Arduino in applications where:

• temperature sensitivity of the potentiometer is causing the value to drift. As the device uses mechanical contacts it is not susceptible to environmental variations and will retain its value when powered down.
• you need full turn angular measurement
• you need multiple turns for a wider range
• you need precise control - with approx 3 degrees between positions and a very slight tactile feedback you can get the number you want without overshoot.
• you need this on the Raspberry Pi which doesn't have analog pins.

You may prefer this to using a rotary encoder where:

• absolute position is important (rotary encoders sense relative movement - typically full rotations and direction only)
• you need the system to remember the position when powered down
• you need finer resolution than incremental rotary encoders (most measure 90 degree increments)

The module includes:

• Bourns ACE-128 Absolute Contacting Encoder EAW0J-B24-AE0128L soldered to the pcb
• nut and washer (9mm mounting hole)
• PCF8574 or PCF8574A chip (3.3V and 5V compatible)
• solder jumpers to change the address. Address pins have pull-down resistors
• two Sparkfun QWIIC compatible connectors
• locations for through-hole resistors for I2C pullup (resistors not included)
• shaft is 1/4" round (slotted end)
• optional 24CW160T-I/OT 2kB I2C EEPROM

The software includes

• the ability to automatically save the logical zero position and multi-turn offset, so it remembers where it is when powered off
• read the values out as 0 -> 127, -64 -> 63 or -32768 -> +32767 (multi-turn - 512 turns)
• Arduino library
• Raspberry Pi Module

The following options will be available:

• address range 0x20 - 0x27 (ships with 0x20) (I2C EEPROM optional)
• address range 0x38 - 0x3F (ships with 0x38) (I2C EEPROM optional)

In addition to the completed modules, the backpacks will be available without the sensor, and the sensors will be available by themselves. This QWIIC version of the product is only available in a small quantity at this time, so only the 0x38 assembled unit (no EEPROM) is in stock for now.

Why did you make it?

The original prototypes were made for a head-following mechanism for my Dalek dome and eye, using hand-made pcbs.

Two were used to convert wiper motors into high powered servos

A third was used to detect the operator's head rotation in a motion control headset

To support the ACE128 I wrote an arduino library to handle the I2C communications and gray code translation. The device itself generates gray code which has to be converted to normal numbers before use, and that takes some binary math - especially if you connect the pins in a different order from the datasheet. This has now been ported to Python for the Raspberry Pi

My hobby is progressing to manufactured PCBs and SMD soldering, so I pulled this design out and made some up. The current units are made with lead-free solder using a steel stencil and Controleo2 derived reflow oven. Each unit is individually tested.

What makes it special?

The ACE-128 is the smallest and cheapest absolute encoder available today. It uses some really ingenious mathematics in its design. This library and package are a unique solution for easily including precise digital knobs into Arduino and Raspberry Pi projects.

About state storage

By default Arduinos with AVR controllers e.g. Arduino Uno will use the internal EEPROM. SAMD21 controllers e.g. Arduino MKR do not have state storage available by default as they do not have internal EEPROM. An I2C EEPROM can be used on either type by enabling it with a flag in the library.

Raspberry Pi saves to disk - I2C EEPROM support coming soon.

If using I2C EEPROM you only need one EEPROM per project, not one per module. If you already have an I2C EEPROM in your project you can use that, likewise you can use the chip in the encoder module to store other data, not just the module state. Storage uses 3 bytes per module, and there are 2048 bytes available in the optional EEPROM chip.