On our main front door at the Stack Exchange offices we have a glass door with a magnetic lock installed. As is common at offices, visitors can ring a doorbell and then our receptionist will press an RTE button to release the door lock and allow them to open it.
The problem we have is that the maglock does not make the typical loud clicking noise from any of it's internal components activating - which is how most people know to open the door. Instead, the door unlocks silently and they just stand there not realizing they can open it.
SO - here's the question: I'm looking for a simple circuit I can build and install in the maglock that will make some sort of recognizable noise whenever the door is unlocked. The lock is normally powered with a 24V feed from a power supply, so the simplest way would probably be something that just connects inline and then activates whenever the power drops out on that 24V line.
The problem is that I don't know exactly what to use to do that. I'd imagine a relay might do just the trick (since there's normally a noise from the internal switch activating), but looking for some guidance/confirmation on that.