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First of all I am a beginner in electronics / electricity. I am currently trying to connect a coffee machine to a Raspberry Pi powered relay.

No problem with that, but as I am going to put apart and rewire a coffee machine, pretty dangerous heating device after all, I wanted to fully understand its wiring diagram beforehand.

That's where I'm stuck. This is the diagram I drew of it.

coffee machine wiring diagram

I could understand that the three pin device in the middle is a rocker switch. The thing I don't get is the use of the third pin in such a device. What if there were only two pins int the switch, and the wires connected to the upper right one were just fixed together ?

Thanks in advance!

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    \$\begingroup\$ It's probably an illuminated switch so the top-right pin provides a path for the lamp and a convenient tie point for the appliance manufacturer. \$\endgroup\$ – Spehro Pefhany Jun 28 '15 at 15:36
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    \$\begingroup\$ Is the switch lit? \$\endgroup\$ – Ignacio Vazquez-Abrams Jun 28 '15 at 15:36
  • \$\begingroup\$ It actually is an illuminated switch. I still don't get one thing : when the switch is lit off, i.e. the left and the top-right pin are connected, isn't there some kind of short circuit ? \$\endgroup\$ – arnaud Jun 28 '15 at 17:21
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As said in the comments, it's most likely a circuit like this:

schematic

simulate this circuit – Schematic created using CircuitLab

Due to this design, the light will be on, whether the thermostat is on or of (if it exists in the circuit)

So yes, shorting the right terminals would give a nice short circuit.

I've also seen switches with four terminals. Two for a pure switch, and two for a pure light.

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  • \$\begingroup\$ After some thinking, and doing some more testing, I think you're right. \$\endgroup\$ – arnaud Jul 4 '15 at 20:43
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The normally open side of the relay is thermostatically pulled down to the closed contact position to allow current to the hot plate / brewer element; then when the t-stat opens, the n/c side of the relay completes the circuit to the green light that lets you know that the brew is ready.

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