Having a bit of trouble figuring something out about a doorbell

[Byrgg]

The question in the textbook is something like:

Why doesn't the armature in a doorbell vibrate back and forth repeatedly like in a buzzer?

I'm thinking it's related to the fact that the book says that in a buzzer, there's no gong/chimes, etc. Since the doorbell has chimes, is that why it doesn't vibrate back and forth repeatedly?

 

[andrevdh]

Who would want to install a doorbell that will drive them crazy? Just think of all the fun the neighbourhood kids would get pressing it!

 

[Byrgg]

But why doesn't it drive them crazy? What is it that prevents the repeated vibrations?

 

[berkeman]

Methinks a diode is probably involved... Where is the diode placed, and what effect does it have?

 

[Byrgg]

Err, we haven't learned about diodes in class so I doubt that's the answer expected...

 

[berkeman]

Err, we haven't learned about diodes in class so I doubt that's the answer expected...

Oh, okay. Well, have you learned about a solenoid yet? If not, just google it or check out HowStuffWorks.com or wikipedia. It's a coil that you put a voltage across to generate a force to move something. If you put an AC voltage across the solenoid coil, it moves the thing back and forth (that's how speakers work). If you put a DC voltage across it, it pulls something into it and holds it there. How would you make a doorbell out of a solenoid?

 

[Doc Al]

Howstuffworks has an excellent description of buzzers & bells vs. chimes: http://www.howstuffworks.com/doorbell2.htm

 

[Byrgg]

Ok looks like it's because when the switch is closed in a doorbell, the contact remains pulled towards the first chime since the circuit isn't open.

It doesn't do this in a buzzer because if the switch is closed, it self-interrupts by opening the circuit after being pulled towards the gong, and then closes again(gravity or a spring pulls it back), and opens again and so on.

Does this make sense or am I missing something?

 

[andrevdh]

If I remember correctly (I took something like that apart when I was much younger) the doorbell I dissasembled consisted of two metal tubes of different dimensions hanging vertically on both sides of a horizontal electromagnet (or solenoid as berkeman calls it, which is essentially the same thing). The core of the electromagnet, which is a rod of soft iron (magnetizes and demagnitizes easily) is attached to a spring (lets say of the compression type). A pin on one end sticks into the spring (not all the way through). When the solenoid is activated by the push button circuit (current flows through the coil) the iron core is pulled into the solenoid. The pin hits the tube on the other side - Ding! As the pushbutton is released the solenoid deactivates and the spring pushes the iron core back. It hits the opposite tube - Dong!
If you were to hold the pushbutton in on this device you would only get a ding out of it. This should draw the attention of the inhabitans. After a little while release the button and the dong would follow. This would confirm your presence and/or make the inhabitants think that something is wrong with the device all the more reason for further investigation.

 

[berkeman]

Ok looks like it's because when the switch is closed in a doorbell, the contact remains pulled towards the first chime since the circuit isn't open.

It doesn't do this in a buzzer because if the switch is closed, it self-interrupts by opening the circuit after being pulled towards the gong, and then closes again(gravity or a spring pulls it back), and opens again and so on.

Does this make sense or am I missing something?

I don't know about the self-interruption part of your buzzer explanation, but another way to think about it is that the doorbell mechanism needs DC current to operate the solenoid, and a buzzer/speaker converts AC current into sound.

 

[andrevdh]

As one can see from my explanation the normal doorbell circuit do not contain a contact like a buzzer. The pushbutton (normally open) closes and opens the circuit, so it takes over this function. The circuit is closed as long as the push button is held down. This could cause the solenoid to overheat and shorting the coil out if the button is kept down too long.