Click the input switch or type the 'c' bindkey to control the circuit and watch the resulting behavior. The following explanation is cited from the Flip Flop Fans website: Start-up is with everything off. When the button S1 is first pressed, relay#1 coil is activated through the normally-closed positions of relay#3's switch R3a and relay#2's switch R2a. This closes relay#1's normally-open switch R1c on the third leg, activating relay#3. In the meantime, looking at the relay#1 leg, R3a has opened but R1a in parallel has closed taking over from it, on the second leg R3b has closed for future use but R1b opened preventing relay#2 from activating, and on the third leg relay#3's 'no' switch R3c has closed, taking over from R1c and latching the relay#3 coil on, so providing a path to the load through the 'no' switch R3d. Releasing the button, relay#1 coil is deactivated, its switches return to the 'normal' state, but relay#3 remains on. With a second press, current can now flow to relay#2 coil through R3b and R1b, the relay activates breaking switch R2c and releasing the latch on relay#3.
Note that the circuit draws no power when the flipflop is in the off-state, and only the third relay is drawing power while the flipflop remains in the on-state. The first two relays are only activated while the input button is pressed and during the subsequent state transition. This makes the circuit an attractive solution to control power to a slave device without wasting any power at all during standby.
The first two relays can be low-power variants, because each of their switches only carries the load of a single relay coil. Naturally, a high-power (high voltage and/or current) switch might be required for the third relay, whose fourth contact is used as the actual flipflop output.
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