VolCon

Assembled volcon Rev 1 after re-working the wooden enclosure.

This project began as an experiment when I inexplicably decided to learn how an optical rotary encoder worked (FYI, 2-bit Gray code).

It transformed into a learning experience around USB - specifically the HID protocol. Ultimately I ended up with a simple gadget that now sits on my desk and allows me to control the PC volume.

A salvaged optical encoder was used to detect rotation along with a reclaimed head drum from a VCR which was repurposed as a control knob. I very much like the smooth bearings combined with the heavy mass that let the drum spin forever.

Key concepts for the project were:


Spotted Gum > Stained Pine

After some time (about six years!) of having this device in-use on my desk, I finally got sick of the simple (ugly) stained pine wood enclosure. With some scrap spotted gum and a router (with a Roman ogee and a roundover bit) I made a slightly nicer enclosure for volcon.


Rev 2 - Demo of gray code LED visualisation.

Revision 2

After even more time, I decided to do an overhaul. Revision 2 would use the same head drum, optical encoder sensors and wooden enclosure. The upgrades from revision 1 include:


Quadrature Encoder / Gray Code

The encoder consists of two optical sensor "beams" that are sequentially opened/interrupted by the holes within the encoder disk. The state of each of the sensors can be represented as two bits.

Movement of the knob is simply determined by a change in the state of either of the two sensors. I.e. if either of the bits change, the device has rotated. The spacing of the sensors means only one of the bits will change at a time. Therefore, the direction of rotation is determined by comparing the current state of the two bits to the previous state of the two bits.

Clockwise 00 -> 10 -> 11 -> 01 -> 00
Counter-Clockwise 00 -> 01 -> 11 -> 10 -> 00

# Previous State Current State Rotation Direction Delta
0x0 00 00 No movement. 0
0x1 00 01 Counter-clockwise. -1
0x2 00 10 Clockwise. 1
0x3 00 11 Not possible (error). 0
0x4 01 00 Clockwise. 1
0x5 01 01 No movement. 0
0x6 01 10 Not possible (error). 0
0x7 01 11 Counter-clockwise. -1
0x8 10 00 Counter-clockwise. -1
0x9 10 01 Not possible (error). 0
0xA 10 10 No movement. 0
0xB 10 11 Clockwise. 1
0xC 11 00 Not possible (error). 0
0xD 11 01 Clockwise. 1
0xE 11 10 Counter-clockwise. -1
0xF 11 11 No movement. 0

Note in the table above, if the previous and current states are combined into a single 4-bit value in the order shown, this value is equal to #. So a 4-bit value (#) can be used to determine the direction of rotation.

In the code, any change in the state of either optical sensor triggers an interrupt function (see handle_opto() below) which updates the value of #, then uses a look-up table to determine the direction of movement (delta = -1, 0 or 1).

The look-up table is a one-dimensional array stored in eeprom. The element number corresponds to '#' and the element content corresponds to 'delta' in accordance with the table above.

void handle_opto(void)
{
    // a_b is a 4-bit value that represents previous state (bits 3:2) and the current state (bits 1:0) of the encoder.
    // Static to retain every time this function is called.
    static uint8_t a_b = 0b0011;

    // Encoder lookup table. Three versions of the table for various levels of "sensitivity".
    static const int8_t enc_table [] PROGMEM = {0,-1,1,0,1,0,0,-1,-1,0,0,1,0,1,-1,0}; // Use every pulse.
    //static const int8_t enc_table [] PROGMEM = {0,0,0,0,1,0,0,-1,-1,0,0,1,0,0,0,0}; // Use every second pulse.
    //static const int8_t enc_table [] PROGMEM = {0,0,0,0,1,0,0,-1,0,0,0,0,0,0,0,0};  // Use every fourth pulse.

    // "Remember" previous state of the channels.
    a_b <<= 2;

    // Read in the current state of the channels.
    a_b |= (OPTO_PINS & OPTO_PIN_MASK);

    // Look-up the desired volume delta (-1, 0 or 1) and send to the send_volume(delta) function.
    send_volume(pgm_read_byte(&enc_table[a_b & 0x0f]));
}


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