I need to create an electrical switch on normal net voltage (230V) that sort of acts like an "AND NOT" gate.

All I have at the moment is three copper wires and some connectors. Obviously, when I just connect them, I'll get power on C when A or B are powered. What I need is power on C when A is powered and B is not.

How can I connect my wires to get this, or what do I need to buy in order to end up with what I need?

  • \$\begingroup\$ Are the voltages on A and B in phase with each other? \$\endgroup\$
    – Greg d'Eon
    Oct 6, 2015 at 11:54
  • \$\begingroup\$ Yes I guess so. A and B are controlled by thermostats that cut or pass through net power. \$\endgroup\$ Oct 6, 2015 at 12:01
  • \$\begingroup\$ Don't bet on them being in phase. \$\endgroup\$
    – JRE
    Oct 6, 2015 at 12:09
  • \$\begingroup\$ You can't implement 'not' with wires and switches. \$\endgroup\$ Oct 6, 2015 at 12:22

2 Answers 2


A changeover relay should do the trick, although I have made the following assumptions:

  1. C is allowed to draw power from A.
  2. You have a Neutral/non-earth return path for the coil and B.


simulate this circuit – Schematic created using CircuitLab

  • No power from A? No power to C.
  • Power from B? No power to C.
  • \$\begingroup\$ You've just made a nice 60 Hz buzzer which will destroy the relay in a matter of hours. \$\endgroup\$ Oct 6, 2015 at 12:42
  • \$\begingroup\$ @DmitryGrigoryev How so? A relay with a coil suitable for AC operation is implied in the answer, as are appropriate contact specifications. \$\endgroup\$ Oct 6, 2015 at 12:55
  • \$\begingroup\$ Ok, got it. Sorry for the trouble. \$\endgroup\$ Oct 6, 2015 at 13:00

You could do this with a single change over (C/O) relay, with its coil powered from B - use the normally closed (NC) contacts to connect C to A


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