--- a/advanced.rst	Thu Jun 16 20:03:25 2016 +0200
+++ b/advanced.rst	Thu Jun 16 20:16:32 2016 +0200
@@ -37,6 +37,34 @@
 help on the Internet.
 
 
+Analog to Digital Converter
+===========================
+
+Our board has only one "analog" pin, ``A0``. That pin is connected to an ADC,
+or "analog to digital converter" -- basically an electronic voltmeter, which
+can tell you what voltage is on the pin. The one we have can only measure from
+0 to 1V, and would be damaged if it got more than 1V, so we have to be careful.
+
+We will connect a photo-resistor to it. It's a special kind of a resistor that
+changes its resistance depending on how much light shines on it. But to make
+this work, we will need a second, fixed, resistor to make a "volatge divider".
+This way the voltage will change depending on the resistance of our
+photo-resistor.
+
+.. image:: ./images/analog.png
+
+Now, we will just read the values in our program, and print them in a loop::
+
+    from machine import ADC
+    adc = ADC(0)
+    while True:
+        print(adc.read())
+
+You should see a column of numbers changing depending on how much light the
+photo-resistor has. Try to cover it or point it toward a window or lamp. The
+values are from 0 for 0V, to 1024 for 1V. Ours will be somewhere in between.
+
+
 Communication Protocols
 =======================