Solenoid-Powered 6" Brass Bell

I mentioned in an earlier post that I was working on a project that would help us to hear the doorbell out on the back patio. The problem is that the doorbells themselves are upstairs inside the house and difficult to hear. No matter how many times we tell guests to just walk in and come to the back patio, they still try to ring the doorbell and we don't hear them. This is also another write-up on a product for the kind folks at Element 14 ( What item did they send me this time around? The 12-volt solenoid (Newark SKU: 20M1816) that's going to ring my bell! This solenoid is powered by 12 volts DC. It's super-easy to hook up: Just two wires and BOOM! They're inexpensive and quite rugged. The hole through the center is large enough and the coil is long enough that I was able to stick a spring inside. I think I'll be ordering some more of these. They will come in quite handy on other interactive projects.

Newark has a product on the Ledex stuff:

This project is going to be remotely fire a solenoid under a 6" brass bell mounted on the wall outside the back patio door. The solenoid will be triggered by a simple 5-volt pulse from an AVR microcontroller that will receive commands remotely over Xbee wireless from the doorbell (the doorbell side has yet to be worked out).

Here is a video of the solenoid and the bell triggering from a 5-volt button push:

The circuit is super-simple: A TIP41A NPN transistor is used to control the 12 volts that power the solenoid. Any 5-volt input can fire the solenoid and hold in the armature. I added a spring to the inside of the solenoid that loads up when the solenoid is activated. When the 12 volts is gone, the spring shoots the armature out and the heavy end of it strikes the inside of the bell. Couldn't be simpler.

Here is the circuit sketched out:

Solenoid bell circuit sketch

Here is the circuit on the breadboard:

Breadboard solenoid bell circuit labeled

As I figure out more of the entire project, I will continue to add to this post.