And how would one do an alarm that is kept on until it is manually reset ?
I assume it should be do-able with a single 555 timer chip but I'm having trouble coming up with a schematic to wire it together.
My setup currently only (briefly) triggers the alarm for as long as the beam is interrupted. I would like it to stay on until I use a switch ....
An alternative might be the latch (Build a Simple Laser Tripwire Alarm | Make:) combined with a 555 in astable mode as suggested by UnoDueTre but that would also require 2 555 chips.