MQTT People counter

This repository contains a program (to be flashed, for example on an ESP32, using the Arduino IDE) which allows to create a sensor capable of detecting people going in and out of a room. It works using the VL53L1X sensor by ST Microelectronics. The passage detection is then shared through the MQTT protocol and the count of the people in the room is done directly on other platform such as Home Assistant (see the dedicated file for a smooth integration in Home Assistant).

Idea behind the algorithm

The code contains an adaptation of the program STSW-IMG10 (developed by ST Microelectronics), written using the Sparkfun library for VL53L31X.

The library used is from SparkFun and is made for their sensor, however the same code can also be used alongside the Pololu VL53L31X sensor.

The sensor uses the time of flight (ToF) of invisible, eye-safe laser pulses to measure absolute distances independent of ambient lighting conditions and target characteristics like color, shape, and texture (though these things will affect the maximum range). Something is detected in a certain zone, when the distance read by the sensor os lower than the corresponding threshold.

The idea behind the main algorithm counting for people is the following: after defining two different zones, a passage (entrance or exit) is registered only when:

1. a person is detected in the first zone
2. a person is detected in both zones simultaneously
3. a person is detected in the second zone
4. no person are detected in both zones

Then, depending on which are the first and the second zone, the movement will be either registered as an entrance or an exit.

Hardware

Sensor

As I previuosly stated, the library used to write the code is built for the Sparkfun Distance Sensor, but also works using the Pololu VL53L31X sensor. It probably should also work with other VL53L1X sensors, however I've never tried it.

Board

The following tables contain the necessary wire connections, when using certain boards.

5V boards

(including Arduino Uno, Leonardo, Mega)

Arduino   VL53L1X board
-------   -------------
     5V - VIN
    GND - GND
    SDA - SDA
    SCL - SCL

The Pololu sensor can also be connetced to 3.3V boards, such as the Arduino Due.

Clearly, to use the MQTT functionalities one has to use a board which supports WiFi connection, such as ESP32: in this case, the connections are the following:

ESP32

                    ESP32   VL53L1X board
-------------------------   -------------
                      VIN - VIN
                      GND - GND
     SDA (pin 42, GPIO21) - SDA
     SCL (pin 39, GPIO22) - SCL

ESP8266

It is also possible to use an ESP8266, using, for example, the following connections: (in that case, use the code specific for this board)

                  ESP8266   VL53L1X board
-------------------------   -------------
                      VIN - VIN
                      GND - GND
                       D2 - SDA
                       D1 - SCL

How to adapt the code to your case

WiFi information

In order to connect to the WiFi, one has to specify the name of the WiFi network, its password, the MQTT broker address and ots port, the MQTT username and the corresponding password. All this values have to inserted at the beginning of the code, in the corresponding lines.

One might also edit the name of the device in the MQTT network: this can be easily done just by editing mqtt_serial_publish_ch and mqtt_serial_receiver_ch. The first one corresponds to the address used when publishing messages, while the second one corresponds to the address for messages sent to the board connected to the sensor.
Important: one also has to specify the informations about the MQTT server, otherwise (using an ESP32) it will be impossible to connect to the WiFi, as noted in #3

Relevant area

In order to find the correct distance, the sensor creates a 16x16 grid and the final distance is computed by taking the average of the distance of the values of the grid; to perform our task, one has to create two zones, by defining two different Region of Interest (ROI) inside this grid. Then the sensor will measure the two distances in the two zones and will detect any presence.

However, the algorithm is very sensitive to the slightest modification of the ROI, regarding both its size and its positioning inside the grid.

In the original code, developed by ST Microelectronics, the values for the parameters are the following:

• ROI_width = 8
• ROI_height = 16
• center = {167,231}

Differently to the the above mentioned program that inspired my code, I positioned the sensor on the side of the door and not above it. In my case, the parameters are very different:

• ROI_width = 5
• ROI_height = 5
• center = {239, 175}

Be careful that both ROI_width and ROI_heigth have to be at least 4. The center of the ROI you set is based on the table below and the optical center has to be set as the pad above and to the right of your exact center:

128	136	144	152	160	168	176	184	192	200	208	216	224	232	240	248
129	137	145	153	161	169	177	185	193	201	209	217	225	233	241	249
130	138	146	154	162	170	178	186	194	202	210	218	226	234	242	250
131	139	146	155	163	171	179	187	195	203	211	219	227	235	243	251
132	140	147	156	164	172	180	188	196	204	212	220	228	236	244	252
133	141	148	157	165	173	181	189	197	205	213	221	229	237	245	253
134	142	149	158	166	174	182	190	198	206	214	222	230	238	246	254
135	143	150	159	167	175	183	191	199	207	215	223	231	239	247	255
127	119	111	103	95	87	79	71	63	55	47	39	31	23	15	7
126	118	110	102	94	86	78	70	62	54	46	38	30	22	14	6
125	117	109	101	93	85	77	69	61	53	45	37	29	21	13	5
124	116	108	100	92	84	76	68	60	52	44	36	28	20	12	4
123	115	107	99	91	83	75	67	59	51	43	35	27	19	11	3
122	114	106	98	90	82	74	66	58	50	42	34	26	18	10	2
121	113	105	97	89	81	73	65	57	49	41	33	25	17	9	1
120	112	104	96	88	80	72	64	56	48	40	32	24	16	8	0

Threshold distance

Another crucial choice is the one corresponding to the threshold. Indeed a movement is detected whenever the distance read by the sensor is below this value. The code contains a vector as threshold, as one (as myself) might need a different threshold for each zone.

The SparkFun library also supports more formats for the threshold: for example one can set that a movement is detected whenever the distance is between two values. However, more information for the interested reader can be found on the corresponding page.

How to invert the two zones

To invert the two zones, one might simply invert the values in the center vector.

Useful links

PDf file with more information about the algorithm: this PDF is an in-depth explanation of the algorithm and contains technical details about the sensor

SparkFun library guide with more information about the functions used in the code

MQTT with ESP32 tutorial

New release

The new version allows OTA updates through the Arduino IDE. For more information about OTA updates in Arduino, one may look at this article