The Scientist and the Geek

Using LV-MaxSonar-EZ Sonar with Picaxe

Recently I’ve had to develop a circuit using a picaxe 08M2 with MaxSonar-EZ1, and had a bit of difficulty understanding the output values from the sonar analog pin. Although the sonar have a really cool manual that comes in the package, it was still a bit confusing, specially if you don’t read the bellow quote carefully.

"Outputs analog voltage with a scaling factor of  (Vcc/512) per
inch.  A supply of 5V yields ~9.8mV/in. and 3.3V yields ~6.4mV/in.
The output is buffered and corresponds to the most recent range data"

It’s quite straightforward, but can be a bit confusing because the analog reading from picaxe is 0-255 range, which means 5v/255

The code for picaxe would be something like this:

main:
readadc C.1, w1
debug w1
goto main

and the board would look like this:

When is all connected in the debug window you’ll see some values of w1 in the 0-255 range, so by using math rule of three, being X the sonar output voltage :

which is the same as:

example being w1 = 3:

So the output is 0.059v, but we know that 1 inch is vcc/512 (in this case vcc = 5v , but could be 3.3v) so is 5/512 (0.0098) and not 5/255 like what we got. So we do again a simple rule of three:

So we get 6inches which is the minimum value readable by the sonar. If we need centimeters we just multiply by 2.54 and get 15,3cm.

so the code will be something like this:

symbol range = w1
main:
readadc C1, range
range = range*5/255*98
'range = range*254/100 'convert to cm
debug range

The values will be different mainly because we are working with integers here, so everything is rounded up.

Don’t forget that if your vcc is 3.3v you should change the constant 5 to 3.