Computing stuff tied to the physical world

Opto-coupler doesn’t couple

In Hardware on Sep 27, 2010 at 00:01

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:

Dsc 2000

The plug itself is hooked up to port 1:

Dsc 2001

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

Screen Shot 2010 09 26 at 15.55.52

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:

Screen Shot 2010 09 26 at 15.32.33

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:

Screen Shot 2010 09 26 at 16.29.27

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

Screen Shot 2010 09 26 at 16.30.20

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:

Dsc 2003

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.

  1. If you don’t mind my asking, who do you use for your prototypes? I do a hobbyist pcb service and my next order goes out in a week if you’re interested…

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