I need a circuit that would stay on after being triggered
Okay. To summarize, then:







simulate this circuit &ndash; Schematic created using CircuitLab

Hopefully, that matches up with what you are now thinking. This assumes that setting CLEAR to ground avoids clearing the FF. You'll need to activate the PRESET line whenever you reset things and start everything up before starting.



It's important to note that the simplicity of the above circuit depends on the fact that your sensor signal is clean and doesn't repeat itself. The Photon makes a good point. So here's another approach that covers his comment:







simulate this circuit

Which you use depends, I suppose. But given your stated interest, I think The Photon's point is more important and that you should use the second circuit and not the first. Just to be safe.