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+Opto-isolated inputs
+--------------------
+
+The C8051F320 needs a current of at least 50 uA to drive an input low.
+The current transfer ratio of the opto coupler is 50% or better.
+Thus we need an input current of at least 100 uA.
+
+If we allow for a loss of at least 100 mV (yielding a minimum input
+voltage of 1.5 V), the protection resistor can be at most 1 kOhm.
+
+If the maximum power dissipation of the protection resistor is
+limited to 1 W, we can allow up to 30 mA into 1 kOhm +/- 10%.
+
+The maximum input voltage would thus be 30 V.
+
+
+Future changes for higher voltages
+----------------------------------
+
+The circuit is currently not designed for voltages exceeding about
+30 V between any two points !
+
+To allow the use with mains voltages, be it across the relays or be
+it as a difference in "ground" potential of separate loops, at least
+the following changes will be necessary:
+
+- air gaps must be 2 mm or larger (6 kV)
+- conducting elements closer than 2 mm must be isolated if high
+  voltage may exist between them
+
+In particular, this means:
+
+- the PCB must be coated
+- opto couplers may not share the same socket
+- the front opto coupler outputs need wider spacing
+- clearances need to be verified