One possible solution (I have not thought this one through) but what about enabling the circuit only when the input is sensed, that way you could achieve enormous energy savings. I am thinking of something like an optocopupler driven by an output to enable the constant current generator, the current generator then drives another optocoupler for an input.

Excellent idea, Erik!

Something like this could work:



By using a diode bridge at input, detection can even be simplified, means 89C52 needs to activate its opto for only a period of about 2...5msec. Also, DC signals can be applied with reversed polarity.

Kai