S-R flip-flop

    |                                           |
    |                                           |
    |                                           |
    |                                           |
    |----| |---------------|/|-----+----( A )---| # "Q" relay
    |     A                 B      |            |
    |           ____               |            |
    |-----------O  O---------------+            |
    |           SET                             |
    |                                           |
    |                                           |
    |----| |---------------|/|-----+----( B )---| # "!Q" relay
    |     B                 A      |            |
    |           ____               |            |
    |-----------O  O---------------+            |
    |           RESET                           |
    |                                           |
    |                                           |
    |                                           |
    |                                           |