I'm trying to use an H11AA1 (datasheet) optocoupler to test when my doorbell is pressed (I'm in an apartment building):


simulate this circuit – Schematic created using CircuitLab

It worked when I first tested it, but then it stopped working less than a day later. I have a second optocoupler (which I bought with the idea that I would almost certainly break the first one somehow), and if I swap it in it works. The input part is supposedly rated for 230VAC, so I don't think the issue is there.

I've disconnected it all for now out of fear of breaking the second optocoupler. What did I do wrong?

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    \$\begingroup\$ Missing a current limiting resistor on the led side. Also, you should use the low side driver schematic they have on Fig 15 of the datasheet, rather than the high side driver in your schematic \$\endgroup\$ – BeB00 Feb 27 '18 at 13:13
  • \$\begingroup\$ @BeB00 what's the difference between low side / high side in that case? \$\endgroup\$ – colinmarc Feb 27 '18 at 16:57
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    \$\begingroup\$ Where exactly did you get the 230 VAC rating from? \$\endgroup\$ – CL. Feb 27 '18 at 18:34
  • \$\begingroup\$ On your diagrm, the transistor is between +v and the output (high side switching), whereas on the datasheet and in andy's answer, the transistor is between 0V and the output (low side switching) \$\endgroup\$ – BeB00 Feb 27 '18 at 23:12

What might you be missing: -

enter image description here

The raw input has just internal diodes and these get messed up from anywhere between 1 volts and 3 volts. Use a resistor. 8 V AC is too much to put on the input. +/- 60 mA is the current limit into the device - stay away from this limit.

  • \$\begingroup\$ Just to add, for 8VAC, those resistors should probably be ~1K \$\endgroup\$ – BeB00 Feb 27 '18 at 13:19
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    \$\begingroup\$ That would produce a peak of about 5 mA so maybe you go for 330 ohm resistors or just one resistor (either input lead) of 680 ohms. \$\endgroup\$ – Andy aka Feb 27 '18 at 13:22
  • \$\begingroup\$ Thanks! Isn't the whole idea of an octocoupler to hook it up such that I'm not really affecting the original circuit? I don't want to break the whole apartment doorbell system somehow... \$\endgroup\$ – colinmarc Feb 27 '18 at 16:55
  • \$\begingroup\$ @colinmarc are you connecting the opto in series with your doorbell? That’s wrong. You need to connect it in parallel: doorbell terminal->resistor->opto->resistor->other doorbell terminal. With the resistors, the current draw should be low enough not to be noticed. \$\endgroup\$ – DoxyLover Feb 27 '18 at 19:54

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