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Googling for "soft latch" brings up many results, and on this site 12 out of 232 questions tagged latch contain the phrase "soft latch". However, there seem to be no references to a "hard latch" other than relating to breastfeeding and doors.

What is the difference between a latch that is soft and one that is not? What is the term to refer to the opposite of a "soft" latch?

user218354
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  • Can you give some context to your question? Why were you searching for "soft latch"? – Transistor Apr 13 '19 at 22:32
  • Because when searching for "latch" I frequently came across the term "soft latch" which led to me searching for what it meant. It seems to be a casually used term, but there's absolutely no reference to a "hard latch" so I'm wondering if there's a meaning to the "soft" or if it's some etymological quirk that caught on. – user218354 Apr 13 '19 at 22:33

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Have a look at Soft latch power switch circuit. The article states:

Dаvid Jones through his Youtube channel EEVblog described in detail how to design a cheap soft latch power switch circuit, using one push button switch to toggle your circuit power on and off with the following design requirements:

  1. Zero power when off.
  2. One on/off switch.
  3. Standalone(no MCU).
  4. General components only (Diodes, Transistors, ..ect).
  5. Minimal parts.

Not all those features are required to satisfy the "soft-latch" requirement but I think the important one is number 2 and possibly that the circuit will reset to off after a power cycle.

Examples of a hard-latch would be a standard toggle switch (as opposed to a momentary push-button style), a toggling relay (which will stay put until electrically kicked the other way) or, perhaps, an electronic controller with some kind of memory that will power up in the same state it was in before the power cycle. (This last one is trickier than it seems. There have been several questions on this site on the topic of a 1-bit memory for similar purposes.)

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Transistor
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    Excellent answer! That's exactly the kind of implicit knowledge that I think should be made a little more explicit. No rep to upvote, will wait a while to accept to encourage other answers, etc. And an extra theoretical +1 for EEVblog. – user218354 Apr 13 '19 at 22:45