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Here's what I try to do:

There is these common soft latching circuits using BJTs or MOSFETs or a 555 timer

enter image description hereenter image description here

I made those circuits and they're working very well. The buttons used are always Normally-open push-buttons.

However, I've got plenty of Normally-closed push-buttons (I am working as a fire alarms technician, and Pull stations almost always uses N-C buttons to trigger the alarm, and I got plenty from used ones)

I want to make that kind of latching circuit but using N.C. button.

I tried just to replace the N.O. by an N.C. button but apparently it is not that easy.

I can use an inverter using for example a BJT like so:

schematic

simulate this circuit – Schematic created using CircuitLab

But I don't know how to make it fit in the common circuits as those that I've shown above.

There is this interesting 555 based Active-Low trigger circuit:

enter image description here

It triggers for an amount of time determined by the threshold capacitor. but it is not a latch.

I tried to modify that circuit as well, without finding a proper solution.

What can I do?

To resume: I want to make a latching switch circuit using one single Normally-closed button.

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  • \$\begingroup\$ oh yeah just forgot it. I made this schematic very quickly. But you're right \$\endgroup\$ Commented Jan 5 at 14:17

3 Answers 3

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Here is a circuit that does the job with a relay and a normally-closed button. The trick is to use the normally-closed contact of the relay, which inverts the button logic.

You could also use a solid state relay of course.

NE555 trigger circuit with NC button

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  • \$\begingroup\$ ok thank you ! i will check that \$\endgroup\$ Commented Jan 5 at 14:22
  • \$\begingroup\$ In simulation, it works, but in real test, for now, all i get is a slight pulse on the output. \$\endgroup\$ Commented Jan 5 at 17:35
  • \$\begingroup\$ Pls show your real circuit. \$\endgroup\$ Commented Jan 5 at 17:57
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    \$\begingroup\$ I started again from scratch, and it works perfectly ! \$\endgroup\$ Commented Jan 5 at 19:18
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Here's the LTspice sim of Stefan's 555 circuit using a CD4066 CMOS switch to invert the PB logic:

When the PB voltage (green trace is high) the PB SW is open.

enter image description here

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  • \$\begingroup\$ what software is this ? \$\endgroup\$ Commented Jan 5 at 17:13
  • \$\begingroup\$ The simulator software is LTspice, free from Analog Devices. \$\endgroup\$ Commented Jan 6 at 2:59
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You could replace the switch in your first two examples with the output side of a MOSFET opto-coupler. This is essentially a very small solid state relay. You need a MOSFET type because in your circuits, current can go either direction through the switch contacts depending on the logic states.

Bias the input LED from the power input to the circuit and connect the switch across it so that the NC switch contacts short out the LED input. When the switch opens, the LED wakes up and the opto output "closes".

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