Simpler than the World's Simplest Softlatch (re: EEVBLOG #262)

[Fungus]

I think the gate capacitance will be really important, too.

[Zero999]

It's possible to do this with a relay. This can be handy inside industrial control systems.

It works because the relay has hysteresis. The relay won't turn on, when power is fist applied, because the voltage across it is too low, thanks to R1. C1 charges via R2. When the switched is pressed (W3) C1 discharges a bit into L1, causing the relay to turn on and connect R2 to 0V. C1 discharges when the switch is released. When the switch is pressed again, C1 charges via R1, briefly pulling L1's voltage down to 0V and turning it off.

R1 should be a bit higher than the relay's coil resistance and R2 about double R1. C1 should be large enough to soak up enough charge to turn the relay off. Normally getting it to turn on is easier, than off.