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.

  • \$\begingroup\$ I'd consider a buzzer. \$\endgroup\$ Commented Oct 24, 2013 at 22:21
  • \$\begingroup\$ @RedGrittyBrick: From the specs it sounds as if the power is lost when door is supposed to open, so it depends. \$\endgroup\$ Commented Oct 24, 2013 at 22:22
  • \$\begingroup\$ I read what you say but does the door unlock when power is removed from the magnetic lock - I guess it does - a kind of safety feature for power cuts etc.. It makes it harder this way round because at the point the when the door is "openable", there is no power to make a noise. This means a battery or energy storage device. Can you confirm? \$\endgroup\$
    – Andy aka
    Commented Oct 24, 2013 at 22:24
  • \$\begingroup\$ Correct, there is no power in the lock when its unlocked \$\endgroup\$ Commented Oct 24, 2013 at 22:25

2 Answers 2


Use a linear solenoid and a bell.

It will work with the same principle holding your door locked. The pull-type solenoid will hold the plunger down while the power is on. When the power is disabled, the plunger is released and sprung forward to strike the bell. When the lock is reengaged, the plunger is pulled down ready to strike the bell again.

linear solenoid diagrambell icon


  • 1
    \$\begingroup\$ Clever solution. +1 for ding! \$\endgroup\$
    – bhillam
    Commented Oct 25, 2013 at 1:28
  • \$\begingroup\$ AC solenoids will buzz loud enough to be heard \$\endgroup\$
    – user30080
    Commented Oct 25, 2013 at 4:22
  • 1
    \$\begingroup\$ @user30080 Not necessarily. My UV exposure unit uses a tiny no-name AC (220 Volts) solenoid for the interlock, and it's silent both when pulled in and when released. \$\endgroup\$ Commented Oct 25, 2013 at 7:28

At some lowish voltage the mechanism will release. You don't have to totally remove power for this to happen. This means that a low voltage detector can still harvest enough energy to make a decent "sound" to indicate the door is now unlocked. A small printed circuit board should be fittable somewhere near the lock that can make this sound.

Of course the power switch that controls the lock needs a small modification to "leak" a bit of current through when it opens the circuit but this is trivial.


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