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I recently built a unisolder soldering station. One of its features is a sleep mode based on whether or not the soldering iron handle is present in its holder. A SLEEP pin is provided that's weakly pulled up to 3.3V and needs to be pulled to GND to indicate that the iron is in its holder.

Part of the holder I'm using is conductive and makes contact with a part of the soldering iron that's directly connected to protective earth. I'd like to be able to detect when the holder is connected directly to protective earth through the soldering iron handle when it's inserted and pull the SLEEP pin to the controller's GND appropriately.

However, the controller's GND is not referenced to earth by design, and because of that I'm unsure about how to design a circuit to detect the presence of the iron while keeping protective earth isolated from the controller's power-supply.

How could I go about building such a circuit?

This is the schematic of the unisolder board I'm using. I'm supplying it with 23V AC from the secondary of an isolated transformer.

enter image description here
Source: https://github.com/sparkybg/UniSolder-5.2/blob/main/schematics/5.2C/UniSolder5.2C_back.pdf

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  • \$\begingroup\$ Please post a jpg of the circuit (or use the circuit tool to draw an example schematic) \$\endgroup\$
    – Voltage Spike
    Jun 17, 2021 at 17:55
  • \$\begingroup\$ @VoltageSpike is the edit I made what you suggested, or were you asking for an additional schematic showing how the transformer is connected and how the soldering iron tip is grounded? \$\endgroup\$ Jun 17, 2021 at 19:20

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The grounds are tied together here:

enter image description here

The grounds are not isolated, they are tied together through the same terminal of resistor. If you have a sensor, reference it from SGND which should be the same ground potential as the controller, but odds are they are both close to the same potential (the only difference being the trace resistance).

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  • \$\begingroup\$ In the part of the schematic you quote I do see GND and SGND being tied together (though to me it looks like they're tied together directly and not through a resistor). However, I think neither GND nor SGND in the schematic are references to protective earth / chassis ground. The resistance between GND/SGND and the tip of the soldering iron that's connected directly to protective earth measures in the kilo-ohms. Do I read that part of the schematic incorrectly, or did I perhaps miss something elsewhere in the schematic? \$\endgroup\$ Jun 17, 2021 at 20:41
  • \$\begingroup\$ Thanks for that previous edit - I see how I might've misread the resistor part. I'm still not sure what "If you have a sensor ..." means. My intention was to build a sensor circuit to detect whether or not the holder is directly connected to protective earth while not directly connecting protective earth to the controller's GND / SGND. \$\endgroup\$ Jun 17, 2021 at 23:25

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