A switch that can only latch if power is present?

Question

In a previous post, @Transistor showed hos to create a simple pulse trigger with a relay and a switch: Is there such a thing as a non-latching pulse switch?

Now I'd like to take the problem one step further to deal with power outage conditions.

In my scenario, I have this pulse circuit as shown which means that when the button is pressed, a brief pulse / spike is created.

But... what if

• the switch is glued down
• the source power goes out, and then comes back on at a some random later stage?

The whole point and requirement is that I need human intervention to manually explicitly push the button, make the connection. If the button is glued shut, it's effectively non existent and the pulse is generated when the power comes on.

So, my thoughts are:

• what about specifically the type of switch. A latching switch will require a person to unlatch and relatch - except that when it's latched it's causing the problem I don't want.
• what about a switch that is only latches if the power is present? If in some way the switch cannot latch if there is no power? I can't see how.
• what about a one shot switch like a gas BBQ piezo electric switch. It generates a spark, but you have to release and press the knob again if you want another spark. I can't find any electronic switches like this.

My environment is a mains power distribution board with 220VAC, and DIN rail mounted components, but I'm open to any other ideas.

Answer 1

Perhaps consider a switched capacitor arrangement where you would operate a switch to position ‘A’ where it charges a capacitor and then switch to position ’B’ where the energy from the capacitor energises a relay that is then self-latching. Would that achieve what you want?

Answer 2

Here's a solution.

When the push button switch is pressed, the capacitor charge pulse produces a single pulse output at the relay contact.

When the push button switch is released, the capacitor is discharged through the resistor R1 in order to be ready for the next pulse.

The value of R1 may be equal to the resistance of the relay coil.

Power going off and resuming, with the push button switch in pressed / stuck condition, will not generate any other output.