Vacuum Pump

You don't need to do that as if you seal the jar by turning the valve to off and disconnect the pump completely. There is still silence and as you open the valve, they can hear the hiss as the air goes back in and then the sound of the buzzer/bell can be heard - it even gets louder as more air enters the jar.
The LED was a trade off as one member of Teaching staff commented "ohh you can't see the arm moving like you can with the bell so how do the students know the buzzer is still ringing"