Yes, that is a practical solution. Note that if you send only the "increment" message as a tone, you have reduced reliability that the display at the far end will be synchronized. If there is a communication problem, the receiver may miss a tone and not increment the counter. Or if the link gets interrupted, it might possibly be interpreted as TWO increment commands. Of course, whether this is a problem or not depends on what the application is (which we don't know). [/quote]
I think at this stage I'll use the 556 for first working prototype and if it does seem unreliable I will have to change it.
[/quote] Also not clear how you could reset/zero the counter/display (or maybe that is not necessary?)[/quote]
Surely for the reset can't you send a different frequency in order for the Arduino to acknowledge it as the 'reset' option?
Many Thanks