Question: I don't know if self-contained units like this exist, but I'd like to think they do, and I just don't know what they're called, who makes them, or where to find them. 

Please bear with me, for a moment. Imagine a lamp, a simple On/Off latching switch, and a battery connected in series. The lamp is powered when the latching switch is in its *on* position.

Is there another switch-type or device that can be added to the circuit, that if pressed whilst the simple switch is *on*, opens/breaks the circuit until the simple switch is *off*. The new device is effectively a latching normally closed switch, but special because it can reset its own state to closed, if the simple switch changes position to off, and current no longer flows.

Ideally, I'd like something with these characteristics:

* 12V 0.5A
* 10-30mm diameter or width
* panel mountable
* LED indicator (not required, but would be nice!)
* ideally looks like a latching switch but resets mechanically.

Any help appreciated.

Answer: 
The function you're looking for is very common in, for example, a sump-pump controller. If the sump starts to overflow (because the pump has failed), an alarm contact closes, and it usually sounds a very loud buzzer. There's a momentary-action switch called "Alarm Silence" that will shut off the buzzer (so that you can fix the problem in peace), but then it will re-arm itself once the original alarm contact opens.

It is easily implemented with a single SPDT relay, as shown below. S1 is the alarm contact, S2 is the "silence" pushbutton, and the relay K1 keeps the load (buzzer) switched off until S1 opens again and the relay drops out. "Power Source" and "Load" can be pretty much anything, as long as the coil of K1 is compatible with the power source.

![schematic of latching relay circuit](https://i.imgur.com/JL1uM.png)
Well, you can combine some transistors working as keys to achieve this behavior.  

First of all - I recommend you a good web site about transistors: <http://talkingelectronics.com/projects/200TrCcts/200TrCcts.html> 
Please search the page for "push-on push-off" circuit.  

Basing on this page I drew this simple circuit:
![enter image description here](https://i.stack.imgur.com/PP2ml.jpg)

Please note that values of elements are incorrect - I didn't calculate them. Just wanted to show the idea.  

When you turn on SW1 then current flows through normally closed RLY1 and LED is shining. Both transistors are not conducting. When you push switch SW2 then Vcc is applied to R4, some current flows, Q2 opens and starts conducting. Current which flows through R1 cause that Q1 is switched ON, and even you release SW2, the current from Q1 will keep Q2 in ON state. Both transistors will keep each other in ON state. Note that RLY1 is now working, and breaks the current that previously was feeding LED D2. LED stop shining. This situation will remain until you switch off SW1