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 opto coupler headers (inputs and outputs) need wider spacing - clearances need to be verified