---
title: "Arduino laser distance meter"
date: 2013-07-27
---

<div style="float:right">
image:ut390b_front.jpg[Uni-T UT390b laser distance meter front view]
</div>

## Uni-T UT390B

UPDATE 2: Serial commands and timing information are now available [here][1].

UPDATE: I've posted code to parse the serial datastream
[here][2].

This is a tutorial on how to get serial data out of a cheap handheld laser
distance meter, which can be fed into an Arduino or a PC. You can use the
distance measurements in all sorts of projects like robots and quadcopters.
There are vendors who sell conversion kits that sniff the distance measurement
off the LCD and output it over serial. The problem with those kits is that they
cost more than the laser meter itself. Luckily I managed to find a laser
distance meter that outputs distance measurements over serial directly: the
UT390B from Uni-T which sells for about $56 from online retailers. Laser
distance meters use precise electronics to measure the length of time it takes
a laser beam to reflect back from a target. Handheld units like this one have a
range of about 0.2m - 40m with a precision of 2mm.  They're a great replacement
for HC-SR04 ultrasonic sensors if you need long range measurements.

## Preparing the unit
You can see the unit's debugging port underneath the battery cover. To take it
apart, remove the 2 visible screws.

image:back.jpg[UT390b device with battery cover removed showing debug pins]

Now you can pry off the cover.

image:cover.jpg[UT390b with front cover open]

This ribbon cable connects the unit's keypad. Scrape off the glue and unplug it
for now.

image:ribbon.jpg[The ribbon connecting the two halves of the UT390b device]

Next, remove the 2 screws with washers that hold the rubber piece at the front.
If you want to remove the screen, it's connected by 4 clips to the circuit
board. It's probably a better idea to leave the screen connected, so you can
verify that the unit is working properly while you're testing your serial
parsing code.

image:lcd.jpg[Removing the screen from ut390b device]

**DON'T** remove these 3 screws. They connect the laser diode to the circuit board,
and if you loosen these screws you risk breaking your laser.

image:screws.jpg[3 screws on circuit board which should not be removed]

The easiest way to access the debug port is by desoldering the battery
connector as shown below. Note: this photo shows the 3 screws removed. **DO NOT**
do this, as it's unnecessary and could cause the unit to break in two.

image:dont.jpg[Removing the ut390b circuit board (don't do this)]

Finally you can solder some thin wires to the debug port. The pinout is as
follows, seen from below the board.

image:pinout.jpg[Pinout of ut390b debug pins. from left to right: GND, RX, TX,\
BOOT0, VCC (2.7V)]

We can replace the original keypad with a microcontroller which can simulate
button presses to control the unit. The keypad pinout is below. The ribbon
connector is 8 pin, 1mm pitch. If there's enough interest, I will assemble and
sell a small interface board to break out the necessary signals.

image:lcdpinout.jpg[Pinout of ut390b keypad ribbon connector. From left to \
right: Unit, n/c, Func, On, GND, Mode, n/c, Off]

## Decoding the serial output
This unit's serial port runs at 115200 baud (8N1). On bootup, it outputs the
following text with `\r\n` line endings:

```
ldpara:395
curent ver:420411
Year:Jan 21 2013 Time:13:53:10
ldpara:395
Iint OK
APDMIN=136 APDMAX=167
BIASVOLMIN=2307 BIASVOLMAX =1718
```

Notice "APDMIN" and "APDMAX", which are likely calibration values for the
avalanche photodiode used to detect the reflected laser beam. A few sloppy
printfs as well.. To turn the unit on, connect the ON signal to GND for about
300ms. Once it's on, the ON signal is also used to take a measurement. After a
measurement is taken, the unit outputs the following:

```
Dist: 2827,curtemp =21
V2.0
nDist: 2827,tempDv=0
```

The Dist and nDist values are in millimeters. As far as I can tell, the two
measurements are always identical. If there's a measurement error (Error 154,
out of range or sensor error) the unit will output:

```
OUT_RAN dist = 30
```

If the unit can't determine the average distance (Error 160, sensor shaking too
much or Error 155, signal too weak) the unit will output one of the following
lines:

```
MEDIUM2 AND THIN NOT MATCH
MEDIUM1 AND MEDIUM2 NOT MATCH
THICK AND MEDIUM1 NOT MATCH
MEDIUM1 AND MEDIUM2 NOT MATCH
```

When you turn it off, it outputs:

```
SysPowerOff!
WriteTestData TRUE
Serial input?
```

I'd like to find a way to control the unit over serial. So far all I've found
is that sending 0x23 ('#') or 0x73 ('r') will cause the unit to power down. It
seems to ignore every other byte, as well as some common english words and
modbus commands. If you manage to find any other byte sequences, [email
me](mailto:qartis@gmail.com) or leave a comment below. If you liked this
article, you might also like DORI, my robot project. DORI uses a slightly
different laser distance measurement strategy. You can learn more on the
[project homepage][3] and in [my thesis][4].

[1]: /laser-distance-meter-update-serial-commands-timing-measurements/
[2]: /parsing-laser-distance-meter-serial-output/
[3]: http://dori.qartis.com
[4]: http://arxiv.org/abs/1304.5568