Circuitmess Wacky Robot Resistron Instructions Manual page 23

When it comes to adding LEDs, it's a piece of cake! Simply choose
whichever LEDs you'd like to use and put them in Resistron's eyes.
+
Rounded
part
But remember, pay close attention to the polarity of the LEDs as you
connect them. Look for the rounded part (that's the plus side) and
the cut-off part (that's the minus side) marked on the LED.
-
+
Cut-off Rounded
part part
-
Cut-off
part