It’s that time of year again… a mistake, courtesy Mr. Murphy: the recently-added Opto-Coupler Plug this time.

Triggered by a post on the forum, I created a little test setup. The idea is to feed some outputs in on one end of the plug, and then read them back out. First the outputs, using ports 3 and 4:

The plug itself is hooked up to port 1:

Here’s the test sketch (see opto_demo.pde):

What it does is toggle the ouput in all possible combinations, and then report the input signals it’s reading. Here is the correct output:

I had to create the EAGLE component for the MCT62, since it wasn’t available. Here’s what I made, using some existing part and modifying it:

Now if it were correct, then the Opto-coupler Plug would be fine. But it’s way off. Here is the datasheet pinout:

Silly me. The emitter is attached to “+”! – and the cap is also useless… there is no + on the chip, it doesn’t need a power supply.

I don’t know what I was thinking at the time, but it clearly doesn’t make any sense. The test setup I had on the breadboard worked fine, of course. It’s just that I went completely astray on the EAGLE design side of things. And then during the pcb test, I must have made a mistake and only tested one side.

Oh well. Fortunately there is a way to fix these plugs, but it ain’t pretty:

One copper trace from pin 8 on the bottom has to be cut, and a short wire has to be connected between pins 8 and 5 to attach the photo-transistor’s emitter to ground.

I’ll be fixing the (luckily small) batch of opto-coupler plug kits currently in stock here. And I’ll get new PCBs made ASAP – but this will take a few weeks, as usual.