It looks like Mr. Murphy has found some time to mess with things again…
The Optocoupler Plug is a little board to let you isolate two I/O pins. When you drive one end with a small voltage to light up a little LED inside, it shines that light onto a sensitive photo-transistor which then starts conducting. It’s effectively a tiny switch, driven by light.
The nice thing about light is that it lets you avoid an electrical connection. So this thing allows you to detect a (small) input voltage without actually making a connection to it. These units support over a thousand volt of isolation – perfect when messing with AC mains coupled circuits, for example.
But the Optocoupler plug is actually two plugs in one, because you can also use it as an output by using it in reverse: then, the two I/O pins on a port can be used as output to turn on the LEDs, and the phototransistors can then control some output circuit without having to actually connect to it.
Here’s the “dual-mode” configuration of the Opto-coupler Plug:
In one mode, it can be used as input (with current limiting resistors on the right), in the other it’s an output (current-limiting resistors on the left, and solder jumpers on the right).
The unit I picked for this board is an MCT62:
Then we recently switched over to a HCPL-2631, without thinking much about it:
Whoops! Different pinout, but also entirely different beast: the MCT62 contains a pair of independent simple “analog” type optocouplers. The HPCL-2631 on the other hand is a “digital” model with some built-in amplification. Which means that this thing needs a power supply, and to stay within the 8-DIP pinout, this can only be realised by tying a common “ground” together and re-using the other as supply voltage.
There’s a lot more to describe about this apparently simple device, but for now all I can say is “we messed up!”. Fortunately, only four people have been affected by this so far, and we’ve contacted each one of them to resolve the problem and send a replacement out. To each of you, my apologies for the confusion and for wasting your time if you’ve been trying to get those faulty Optocoupler Plug kits to work.
With thanks to Jupe Software for reporting the issue, and saving others more grief!
Pssst … this is post # 1111 !