Encoder

Encoder consists of two main parts: encoder disc and optical sensor

Encoder disc

I have taken this disc from old ball mouse.

This disc have 65 slits, one of them is closed (but it is counted too: 64 opened + 1 closed) - it is zero angle reference mark.
See this drawing:

Number of slits is defined in "config.h" file: "ENCODER_HOLES_CNT".
Diameter of the disc is 15.5 mm.

Optical sensor

I have taken mine sensor from old printer so I can't give exact name of it. It is conventional opto-interrupter, with one LED and one NPN photo-transistor.
Closed opto-interrupter (no current through opto-transistor) is need to produce high level at the sensor output.
Notice that if you are using disc with a small diameter, you need to use sensor with small diameter of the optical beam - usually such sensors have smaller packages.
It is recommended to make sensor fixing adjustable - to change position of the light beam in relation to disc.

Sensor is connected to the PCB using 3 wires: +3.3V (ENC PWR), GND, Signal (connect it to P6 pad).
See "PCB_3D_view_up.png": https://github.com/iliasam/OpenTOFLidar/blob/develop/PCB/PCB_project_v4/PCB_3D_view_up.png
You can use R34 as a pulp-up for opto-interrupter photo-transistor.

This is how signal from the sensor should look at the oscilloscope when motor is running:

Zero mark "pulse" is shown at the center of this image.

Signal from the sensor goes to comparator integrated to the MCU. Comparator reference voltage is 1.2V. See: "encoder_processing.c" and "config.h":
#define ENCODER_COMP_NEG_SRC COMP_InvertingInput_1_2VREFINT

Simulating encoder with Arduino

If you are having problems with your encoder, you can try to simulate it with Arduino compatible board. Use this sketch:

#include <Arduino.h>

#define SEC_TO_US               1000000

//Rotations per second
#define SIM_ENC_SPEED           15

// Number of slits in encoder, including manually closed zero slit
#define ENC_TOTAL_PAIRS         65

//Open-closed pair duration (~1000 for 65 pairs and 15PRS)
#define ENC_PAIR_DURATION_US    (SEC_TO_US / SIM_ENC_SPEED / ENC_TOTAL_PAIRS)

//MCU pin
#define SIM_PIN                 23


void encoder_closed() 
{
  //Closed slit -> no light -> no current -> pull-up to 1
  digitalWrite(SIM_PIN, 1);
}

void encoder_opened() 
{
  digitalWrite(SIM_PIN, 0);
}

void encoder_simulate_pair() 
{
  encoder_opened();
  delayMicroseconds(ENC_PAIR_DURATION_US / 2);
  encoder_closed();
  delayMicroseconds(ENC_PAIR_DURATION_US / 2);
}

//Simulate manually closed zero pair
void encoder_simulate_closed() 
{
  //encoder_opened(); //This slit is manally closed
  encoder_closed();
  delayMicroseconds(ENC_PAIR_DURATION_US / 2);
  encoder_closed();
  delayMicroseconds(ENC_PAIR_DURATION_US / 2);
}


void setup() 
{
  pinMode(SIM_PIN, OUTPUT);
}

void loop() 
{
  encoder_simulate_closed();//zero
  for (int i = 0; i < (ENC_TOTAL_PAIRS - 1); i++)
  {
    encoder_simulate_pair();
  }
}

You need to set number of slits (holes) of your encoder - ENC_TOTAL_PAIRS .
You need to set suitable GPIO pin - SIM_PIN .
You can use any Arduino compatible board - I tested this sketch at ESP32 board with Arduino FW.
Just disconnect motor and encoder from your LIDAR board and connect GND and SIM_PIN to the LIDAR encoder input P6. This input not 5V tolerant.

Add patch to you "scanning_fw" Firmware
In file "dist_measurement.c" in function dist_measurement_recalculate_ref_distance()
add line corr_ref_dist_bins = 50.0f; before the last
dist_meas_zero_offset_bin = corr_ref_dist_bins - true_dist_bin; line.
This is needed to disable Reference Plate distance corrections. I will not get accurate distance measurements in a such way, but you can test that FW is detecting encoder pulses ad laser is controlled OK.

Resulting image: Diameter of the circle is depending on distance to the object.