That's basically the idea, but has some rough spots:
1. Threshold depends on temperature, and whatever the trigger current is for the TRIAC.
2. TRIACs are rather inefficient as latching devices (or, SCRs are slightly simpler, but it doesn't really matter either way in this DC application); notice the max. min. holding current is 5mA. It may stay latched down to quite low currents (even sub 1mA at high temperature?), but you can't be sure from a design standpoint. Now, this doesn't matter here as shown, with the LEDs providing that holding current -- but it's not something you'd want on a battery-powered device for example.
3. You might want an auto-restart function, or an electronic reset signal rather than the pushbutton.
#1, and your variable threshold, are handily solved with a voltage reference and comparator. Some can even be used as both in one.
Consider TL431, an "adjustable zener", but considered as a three-terminal device, it's really better called an op-amp with only one (inverting) input, an open-collector (only pulls down) output which is also its VCC, and a freaking massive yet suspiciously accurate 2.50V input offset voltage. This description encompasses its full behavior: it's an amplifier, a control loop -- your warning that its impedance depends on frequency (whereas a zener is ~instantaneous for all in-circuit purposes, TL431 only gives low impedance at low frequencies), that it can oscillate if put in poorly conditioned circuits, that it mostly behaves as an integrator (as most [dominant pole compensated] op-amps do), etc.
Or put another way, you can treat TL431 as a kind of ideal BJT (A = 'E', REF = 'B', K = 'C'): very low "base" current, stable 2.50V "Vbe", "Vce(sat)" about a diode Vf below 2.5V (because there are internal diodes, one which will pull down REF through K*), and a 30V 100mA operating range. The one catch is, "Vbe" is within tolerance for Ik > 1mA (or less for other "improved" versions e.g. TLV431).
*Actually the same is true of a BJT, since B-C is a junction just as B-E is. It's just that, Vce is running out in general (Vce(sat) can be 10s of mV) so you can't really notice it.
Adjustment is simply putting a potentiometer to the REF pin, which allows you to make it 2.5V or higher; to go the other way, use a comparator and reduce the V(REF) with a pot, comparing to the input voltage. (Or divide both to get other ranges, while respecting comparator input voltage range.)
So, instead of a thyristor, a threshold like this (alone or with a comparator as well) can be married to a flip-flop, which can be two transistors back to back (multivibrator) or actual logic (NAND or NOR gates, CD4013 or 74HC74, etc.). You're after an R-S flip-flop, or a type D using the clear/preset inputs. And then reset is obvious -- just pulse the other input to the flip-flop. Which can be a time delay from the output for auto-reset, or a direct input for manual.
There are a few other catches, and of course integrating all that onto an IC helps even more for compactness and low current consumption, but that's the basics you'll want for it.
Tim
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