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I am trying to know if an air conditioning is ON/OFF (Daikin with only Wired Remote Control). I want to read its LED. The circuit is fed by a simple 17.8 V two wires from the machine. I've read the base of the LED respect to the GND, and both extremes are at 6.13 V when OFF and at 1.9 V one over the other when ON. I don't dare to interfere the circuit at all, but simply tell the state to a Shelly 1 relay to expose it into a HomeKit system.

In my really poor knowledge about electronics I understand 741IC, yet a bit old, could make the work, as it doesn't drain any current from the Vin and could simply connect the pin 7 (V+ pin) with pin 6 (Output pin) which I would directly connect to the SW pin in Shelly 1.

So the final scheme would be:

enter image description here

Would it interfere, could damage the AA circuit?

Transistor
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Juan Falgueras
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    1) This is a "hookup", not a proper **schematic**, include a proper schematic if you want a decent answer. 2) This circuit is **NOT** going to do anything useful. You're treating the 741 as if it is a "ready to use" amplifier, it is not. You should a) not be using a 741 because it is ancient b) study opamp circuits to see how other use opamps and then do the same. If you, as a beginner, try to make your own circuit my guess is that you will destroy components and be frustrated that nothing works. So study what others do, then do the same. – Bimpelrekkie Apr 03 '21 at 12:58
  • You should first read: http://web.mit.edu/6.101/www/reference/op_amps_everyone.pdf Then read this: https://electronics.stackexchange.com/questions/304521/reasons-not-to-use-a-741-op-amp to learn why no one should not use the 741 anymore. – Bimpelrekkie Apr 03 '21 at 13:01
  • You're right. I'm learning… quick and dirty programming has always worked for me… It seems I need a deeper change of habits – Juan Falgueras Apr 03 '21 at 13:16
  • *quick and dirty programming has always worked for me* Sure, for programming that can work, there's nothing to damage either. For hardware design, it is more complex. – Bimpelrekkie Apr 03 '21 at 13:55
  • I would try to either isolate myself from the circuit with an optocoupler, either on the input as per Transistors suggestion below or on the output. The 17.8 V you mention, is that available at all times in the remote control? Can you hook into it? If yes, I would place a comparator there to trigger on the LED voltage and drive an optocoupler on the output. Transistors suggestion has fewer parts though if you are willing to solder and/or cut into the existing remote. – winny Apr 04 '21 at 09:56
  • Great! powering of the wired remote (on a wall) is always on, as it displays permanently the AA state. So I think I can draw a few mAmps from it to source the Shelly (it's less than 1w always… with a ESP8266) and the LED sensor. The switch of that shelly will be used to short 0.5secs the on/off button connectors of the remote Thanks a lot for your idea, and the the CircuitLab link! This is really passionate! – Juan Falgueras Apr 04 '21 at 10:59

1 Answers1

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schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Eavesdropping with an opto-isolator.

Inserting an opto-isolator into the original LED circuit provides the best means of doing this as it provides complete electrical isolation between the two devices. It does require minor modification of the source but it should be a trivial exercise. The opto-LED will drop about 1.4 V but it seems that you have plenty of voltage to spare.

D3 can, of course, be connected as shown, above R2 or below D2. If you can lift a leg of R2 that would be the easiest.

Transistor
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