# Resettable latching switches

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.

• +1 for Please bear with me, for a moment. – perilbrain Oct 4 '12 at 19:29
• Are you talking about radio buttons? – morten Oct 4 '12 at 20:08
• @morten, not radio buttons. grzegorz's circuit emulates desired functionality, I'm just hoping a component manufacturer has the whole circuit in one package. Maybe it's too much to ask for, maybe there are simpler approaches. – jon Oct 5 '12 at 9:47

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.

• Thank you, apologies for my delayed response, I was embarrassed that I hadn't thought to use a relay like this :) – jon Oct 12 '12 at 12:10
• +1. Nice electrically-latching relay. "Any fool can make something complicated. It takes a genius to make it simple." -- Woody Guthrie. – davidcary Oct 14 '12 at 3:51
• +1, very clever. Here is a live simulation: everycircuit.com/circuit/6650754331312128 – Mahmoud Al-Qudsi Apr 29 '16 at 14:38

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.

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

• Thank you very much, your circuit has exactly the functionality I require from a single component. I will have to revert so something like this if I can't find a simpler solution, so this might be very useful. thanks again. – jon Oct 5 '12 at 9:37
• I roughly transcribed your circuit into the falstad circuit tool to help me understand it, see here:bit.ly/OGVEvu. – jon Oct 5 '12 at 9:41

I am unable to get you clearly but I think you are asking about push button type switch, a switch through which I wrote this answer(my keyboard). It will remain on if pressed and goes off when released.