Right now my arcade machine has an arcade pushbutton mounted on the top, above the marquee, which toggles the power input to the computer power supply, so I can turn the computer on and off without getting inside the cabinet. I wanted a second pushbutton that would control power to the monitor, amplifier, and marquee lights. However, the pushbuttons are momentary-only, so I'd have to latch the signal somehow. There are lots of complicated ways to do this; here is an elegant solution which also has the benefit that if power to the cabinet is lost, the circuit defaults to the off-position:

At power-up, the gate of Q1 is pulled low, preventing current from flowing through the coil and thus the relay does not switch. Meanwhile, 12V flows through one of the NC contacts and charges C1 to about 3V. When the pushbutton is pressed, the 12V supply flows through R1 and R2 to activate Q1, which in turn activates the coil—and the relay switches. During the break-before-make switch, the voltage on C1 keeps the transistor engaged.

Once the relay is fully engaged, 12V flows through the NO contact and keeps the trasistor (and therefore relay coil) active. However, with the relay active and the pushbutton released, C1 is cut off from a power supply and discharges itself through R3. When the pushbutton is pressed again, it momentarily shorts the power supply into C1, leaving R4 free to drain any charge on the transistor gate, and thus the relay deactivates.

Thus, the momentary pushbutton toggles the second channel of the relay,which in this case is shown switching the hot line of an AC outlet. Why not just install a SPST switch? Because I want the system default to power-off when the 12V supply is interrupted. I can't come up with a simpler solution to meet these requirements. Thanks to Bill Bowden for the inspiration for this.