Algodoo

The automaton has two states HIGH and LOW and reads two symbols H (a stroke
from above) and L (a stroke from below), changing its state accordingly.

With sounds installed and running as described here: http://www.algodoo.com/algobox/details.php?id=67648
you can hear the switch of states. (After installation of sounds, copy the files http://syspedia.de/algodoo/sounds/bell-d.wav and http://syspedia.de/algodoo/sounds/string-g.wav to %USERPROFILE%/Documents/Algodoo/sounds.)

This is the complete state transition table of the automaton:

state symbol -> state' sound
LOW - LOW -
LOW H HIGH high
LOW L LOW -
HIGH - HIGH -
HIGH H HIGH -
HIGH L LOW low