+/*
+ this holds an hc-sr04 sonar sensor to a 3/16 inch deck.
+ http://fritzing.org/projects/hc-sr04-project
+ it can hold the sonar sensor either facing down, or forwards.
+ when facing down, it can detect if it passes over the edge of a table.
+ when facing forwards, it can detect and follow something in front of it.
+ */
+