The good old HC-SR04 SONAR module, and its newer, 3.3V HC-SR04P sibling, have long been go-to distance sensors for hobby robot circuits. But, these modules are distinctively large, with prominent transducer 'eyes', making it difficult to tuck them into smaller robot designs. While my BEAPER Nano and BEAPER Pico robot circuits both include header sockets to mount these SONAR modules on top of their PCBs, I wanted a smaller, LASER-based ToF (Time-of-Flight) distance sensor module that could be embedded inside the BEAPER Bot robot chassis.

A BEAPER Pico robot with an HC-SR04P SONAR distance sensing module mounted on top, and a Laser ToF distance sensor mounted in the oval cut-out on the front of the robot.

A number of common ToF modules exist, including the older and slower single-zone VL6180 and VL53L0X modules, the long-range VL53L1 and VL53L1X modules, newer generation sensors in the VL53L4xx family, as well as the latest, multi-zone 8x8 area sensor modules such as the VL53L7CX and VL53L8CX. Most of these connect to a microcontroller using the I2C bus, and a few of the latest area sensors even include SPI.

Three different ToF distance sensor modules.

For my BEAPER robots I was ideally looking to find a small, accurate ToF sensor with a range of about 1m. After both trying a number of different modules and finding this comparison in a community forum post, I decided the STMicroelectronics VL53L4CD module would be the best fit for my small robots. The VL53L4CD is a fast, single zone sensor, with an update rate of up to 100Hz and a detection range from about 5mm to a maximum of 1.3m.

Connections, but make it QWIIC!

The ToF distance sensor modules I purchased for testing all have a set of 6-pin header contacts to connect them with a microcontroller. Four of the six contacts provide the required I2C connections (VDD, GND, SCL, and SDA), and the other two contacts are for an interrupt output signal and a shutdown/control signal, neither of which I need for my application.

A ToF module (left) connects through socket headers and wires leading to a QWIIC JST-SH connector (right).

To connect one of these modules to the QWIIC connector on my BEAPER robots, I would need some kind of adapter. The simplest method is to use a QWIIC prototyping cable with the QWIIC JST-SH connector at one end and four Dupont-style header sockets at the other end. By soldering a 4-pin header onto the module, I could simply attach the socket ends of the QWIIC cable to the sensor, and plug the QWIIC connector into my robot circuit.

Two different ToF modules plugged into adapter circuits that provide them with QWIIC (JST-SH) sockets.

Instead, I wanted a slimmer solution, and one that could take advantage of QWIIC extension cables with JST-SH connectors on both ends. My first attempt at a solution was to design an adapter board having its own on-board QWIIC connector. The latest version of my QWIIC adapter circuit is shared on PCBWay as the Adapt-2-QWIIC project.

The QWIIC VL53L4CD Design

The front of my QWIIC VL53L4CD prototype circuit showing the ToF sensor module (large, black rectangle), and 0603 SMD capacitors and resistors above a row of header pads.

While the QWIIC adapter board works fine for testing, I wan't happy with the combined size of both the ToF module and a QWIIC adapter circuit. So, I decided to create my own all-in-one distance sensor circuit with a tiny form factor that would allow the sensor to fit into the smallest of spaces in my 3D-printed robot chassis. The semi-circular dimples enable 3D-printed spring clips to hold the module in place, and can also be used for screw mounting without the module having the extended wings that would be necessary to fully enclose mounting holes.

The back of my QWIIC VL53L4CD distance sensor showing the two QWIIC (JST-SH) connectors.

Since I2C is a multi-drop bus, I added two QWIIC (JST-SH) connectors on the back of the PCB – with both of them extending from the same side of the unit. Many of the commercial offerings I have seen put their QWIIC connectors on opposite sides of the circuit boards, requiring extra space either above and below, or on both the left and right sides of a module, making the modules potentially more difficult to mount in tight spaces

The rest of the front of the circuit consists of supporting parts for the ToF module, including the recommended filter capacitors, 10kΩ pull-up resistors for the interrupt and shutdown control pins, and SMD pads for optional I2C SCL and SDA pull-up resistors. There is even a 6-pin header strip at the bottom of the PCB that matches the existing, common ToF modules.

While I designed this circuit specifically for the VL53L4CD ToF module, all of the single-zone STMicroelectronics VL53xxx sensors use the same pin-out and pad layout, so this single PCB design can work with any single-zone ToF distance sensor.

Assembling QWIIC VL53L4CD is not so quick

I assembled the prototype circuits by applying solder paste to the front side (holding the ToF sensor, resistors, and capacitors) using an SMD stencil. After placing all of the front-mounted components (except for the SCL ad SDA pull-ups), the boards went to a hot plate for soldering.

Assembling the back side is a bit more tedious, as I solder the two JST-SH QWIIC connectors by hand. I could probably use a lower temperature solder paste with a hot-air system to solder these connectors, but manual soldering worked okay as I only needed a couple of units to verify their operation. Having PCBWay do the assembly ($29 for up to 20 pieces) is, in my mind, a better trade-off than soldering larger batches by hand.

Measuring distance

Measuring distance using a ToF distance sensor module isn't as simple as writing a few lines of code for a SONAR module. ToF sensors incorporate their own on-device microcontroller and proprietary firmware. This requires your microcontroller to run a driver program to properly initialize and then communicate with the ToF sensor. Each model of ToF sensor is different, and requires its own, specific driver.

For Arduino, search for a VL53L4CD driver in the Library Manager of the Arduino IDE, or look for an Arduino VL53L4CD driver on the internet. As the VL53L4CD is one of the newest single-zone distance sensors, fewer drivers are available for it than for the older and more common VL53L0X and VL53L1X ToF modules.

I found the same to be true for MicroPython, so I created and shared this MicroPython VL53L4CD driver along with example distance measurement programs for BEAPER Nano, which runs MicroPython on its Arduino Nano ESP32 microcontroller. The same MicroPython VL53L4CD driver, along with demo programs for Raspberry Pi Pico, can be found in my BEAPER Pico repository.

A radar-like range display on the LCD of a BEAPER Pico circuit displaying distance read from the QWIIC VL53L4CD sensor module.

The operation of the ToF driver typically involves two steps: initializing the sensor, and then starting distance measurements. Measurements repeat automatically, with polling (or a hardware interrupt) used to determine when new readings are ready – unlike having your program initiate individual readings when using a SONAR distance sensor. Distance from the ToF driver is returned in mm (millimetres), along with other information about each measurement including validity and various signal statistics (most if which is not usually needed, but could help to refine measurement settings and accuracy).

Another big difference between ToF and SONAR distance sensors is that ToF sensors include a programmable measurement timing budget. This timing budget, specified in ms (milliseconds), not only affects the measurement rate, but also the measurement accuracy. Longer timing budgets provide more accurate distance results, while faster update rates sacrifice accuracy for speed. For the VL53L4CD sensor, the timing budget can range from 10-200ms, with the default set at 50ms (or up to 20 distance measurements per second).

Going the distance

My QWIIC VL53L4CD sensor (blue) with some comparable commercial offerings.

While off-the-shelf distance sensors are a quick and easy solution, I'm really happy I decided to create my own sensor module and with the way it turned out. I've now got a very small, accurate, and fast sensor that will be easy to integrate into my robot projects – or any other project that needs reliable, short-range distance sensing. I'm sharing it here in the hope that it might be helpful for you as well!