How to get a serial reading from an Ultrasonic Rangefinder on a Raspberry 3 B+?

Question

I'm trying to set up a Maxbotix Ultrasonic Rangefinder (LV-EZ0) with a Raspberry Pi 3 B+.

I've tried all the tips I could find:

raspi-config

disable serial interfacing for shell, but keep it on for port hardware

/boot/config.txt

I made sure that those lines were there or I added them myself:

enable_uart=1
dtoverlay=disable-bt (also tried dtoverlay=pi3-miniuart-bt)

# services
Stopped / Disabled bluetooth, serial-getty@ttyAMA0

I also tried to: sudo cat /dev/serial0 > catReading.txtin one ssh window and sudo echo -en "test\n" > /dev/serial0in another one, but catReading.txt stays empty because I get a "Permission denied" to write anything to serial0.

Command outputs

sudo dmesg | grep tty => [ 0.000901] console [tty1] enabled [ 1.023127] 3f201000.serial: ttyAMA0 at MMIO 0x3f201000 (irq = 81, base_baud = 0) is a PL011 rev2

ls -l /dev/* =>

lrwxrwxrwx  1 root root           7 Jan 31 16:57 serial0 -> ttyAMA0
lrwxrwxrwx  1 root root           5 Jan 31 16:57 serial1 -> ttyS0

cat /boot/cmdline.txt =>
console=tty1 root=/dev/mmcblk0p7 rootfstype=ext4 elevator=deadline fsck.repair=yes rootwait

Wiring

• Sensor VCC to 5V pin on raspi
• Sensor GND to GND on raspi
• Sensor TX to TX on raspi (I tried to plug it to Raspi's RX as well, because I simply don't know what I'm doing here 😁. When I did that, I can read a few bytes, none of them starting with 'R' once decoded to utf-8, so probably not data sent by the sensor).

I looked up to see if there was any way to read a PWM input on the Raspi 3, but it's apparently not possible unless I add extra hardware.

Any suggestion from there?

Answer 1

First of all, sensor TX pin is transmit output so it must be connected to RasPi RX pin which is a receive input - Connecting TX to TX is usually wrong. But in this case is not that simple, so don't do that just yet.

The sensor TX output voltage levels are same as it's supply voltage, so it uses 5V logic levels. RasPi RX pin does not tolerate 5V logic levels, it must use 3.3V logic levels, so some conversion is needed. Fortunately the output might be so weak that the RasPi RX pin can tolerate it, but this is definitely not good to drive 5V signals to things expecting 3.3V signals.

Finally, the sensor TX output is actually directly compatible with RS232 voltage level receivers, which means it must be inverted before connecting it to a UART without RS232 voltage receiver such as RasPi. This is the reason you don't receive correct characters.

Solutions are to connect a USB-RS232 adapter to RasPi, or add a voltage-level converting inverter in between. For example a 74LVC04 is an inverter, it can be used with 3.3V supply voltage to drive RasPi RX input with 3.3V logic level, and it can tolerate 5V logic levels on inputs.

However, those are not the only solutions. The sensor can be used with 3.3V supply voltage, so you can use the sensor and inverter both at 3.3V, so any inverter that runs with 3.3V can be used, such as 74HC04. You could also get a RS232 to 3.3V TTL/CMOS level receiver board that runs at 3.3V to do the inversion and level conversion.