I recently built a unisolder soldering station and am struggling to understand how the detection of the soldering iron within its holder is implemented. The unisolder board itself provides a 3-pin connector with +3.3V, GND (floating / not referenced to PE/chassis ground), and a signal pin that's weakly pulled high to 3.3V. The iron is detected as being in its holder when the signal pin is externally pulled to GND.
The following circuit is what's connected to that 3-pin connector via U2:
J1 is connected to a conductive part of the holder and makes contact with with a part of the soldering iron handle that is directly connected to protective earth. The circuit works in practice, but I don't yet understand why.
Here's what I think I understand so far:
- C3 is a decoupling capacitor for Vcc
- C2 is there to filter high-frequency noise on the signal (P1)
- BSS138 is used to pull P1 low when the iron is present
- R2 limits the gate current and also forms a low-pass filter together with C4
- U1 is a quad NOR gate, and only the J output and A+B inputs are actively used
- the one used NOR gate is configured to invert the logical signal on the anode of D1
- R1 weakly pulls J1 to 3.3V and forms another filter together with C1
What I'm still really confused by is the role of D1. How does it provide the right input on the inverter to detect whether or not J1 is connected to protective earth? Why is it OK to effectively leave the inverter input floating when the diode is reverse-biased? Why doesn't connecting 3.3V to PE through R1 cause a current to flow that'll trip a ground fault circuit interrupter? Are there perhaps other things about the circuit I've misunderstood?