> >Give me a couple of hours and I will post a circuit. I have to do a
couple
> >of urgent jobs first.
>
> Appreciate that. It is not urgent (I will not be looking at it till Monday
> anyway), so do not bust a gut over it.
>
You mean you are not working 12+ hours a day, 265 days a year. I must be
doing something wrong, it cannot be just age related.

The scanner has decided to throw a wobbly and the Digital Camera is in
Scotland being "evaluated" by one of my Nephews, so I will attempt to
describe the circuit.

Two Identical circuits are require for SDA and SCK. I will describe SDA.

Each Circuit requires
2 off BAT85 or similar (OA47 ?) low volts drop is important, call them D1
and D2.
2 0ff 6N139 optocoupler with Darlington Output. Call them OC1 and OC2

Assume we have two Isolated Circuits called Left Hand (LH) and Right Hand
(RH)

 The Collector of the first transistor of OC1 (Pin 8) is connected to RH
+5volt
OC2 Pin 8 is connected to LH +5v

The Emitter of OC1 (Pin 5) is connected to RH 0v (Earth)
OC2 Pin 5 is connected to LH 0v

The Collector of the Second Transistor of OC1 (pin 6) is connected to a 3k9
resistor, the other side of which is connected to RH +5volt
OC2 pin6 is connected to a 3k9 resistor the other lead of which is connected
to LH 5v.

The Cathode of D1 is connected to the Anode of the Photodiode of OC1 (pin 2)
D2 Cathode is connected to OC2 pin 2

The Anode of D1 is connected to the Cathode of the Photodiode of OC1 (pin 3)
The Anode of D2 is connected to OC2 pin 3

The Anode of D1 is connected to LH SDA
The Anode of D2 is connected to RH SDA

The Cathode of D1 is connected to OC2 Second Collector (pin 6)
The Cathode of D2 is connected to OC1 pin 6

The assumption is made that the mandatory 3k3 pull-up resistor is present on
both LH and RH SDA Lines.

Circuit Operation. A low on LH SDA causes LED OC1 to light (path through the
3k9 resistor). This results in the second transistor of the OC1 Darlington
Pair to turn on taking the RH SDA low through D2 (current through the 3k3
pull-up resistor).

If my description is inadequate Alan, Email me off-list with your Fax number
and I'll send you a copy of my sketch which is currently on the back of the
Marlborough Pack.  8-)

Regards

Chris Carr

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