Computing stuff tied to the physical world

Sensing with an Optocoupler

In Hardware on Nov 27, 2012 at 00:01

The OpenTherm setup keeps me thinking…

I haven’t given up on the OpenTherm Gateway yet, but I’ve also been toying with related ideas for some time to try and just listen in on that current/voltage conversation using a self-powered JeeNode, which then reports what it sees as wireless packets.

It’s all based on Optocouplers, so here’s a first circuit to try things out:

JC s Grid page 47

A very simple test setup, which I’m going to feed a ±10V sine wave @ 50 Hz, just because the component tester on my oscilloscope happens to generate exactly such a signal. The 1 kΩ resistor is internal to the component tester, in fact. Here’s what comes out:


The yellow trace is the voltage over the IR LED inside the optocoupler, the blue trace is the voltage on the OUT pin. VCC is a 3x AA Eneloop battery pack @ 3.75V – what you can see is that the LED starts to conduct at ≈ 0.8V, and generates just enough light at 0.975V for the photo transistor to start conducting as well, pulling down the output voltage. With 1.01V over the LED, it already generates enough light for the output to drop to almost 0V.

In other words: within a range of just 41 mV at about 1V, the optocoupler “switches on”.

So much for the first part of this experiment. My hope is that this behavior will be just right to turn this MCT62 optocoupler into a little OpenTherm current “snooper” – stay tuned…

  1. I was looking at the optocoupler, and I wondered : what if you connect “out” to “in+”. that should make an oscillator. adding a capacitor from “out” to “gnd” should influence the frequency. main question : why is this something I have not seen before ? there must be something wrong with it, I guess. but what ?

    • Not enough open-loop gain, I suspect. I just tried it – the output rises to about 1.1V and stays there.

  2. thanks for testing it, I didn’t have an optocopler handy.

  3. @Ronald – isn’t the loop gain the wrong sign? Looks like a convoluted way to make a -x gain opamp that will just find a stable quiescent operating point.

    There are many novel oscillator circuits out there – this relaxation oscillator dates back to pre-glass triode days!

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