The circuit is extremely easy. Let's go though it in detail!
1) If we place our 4x AA or AAA batteries in series, we're going to have a voltage of roughly 6v, which is perfect. You can use a 9v battery, but you have to make sure that your buzzer is designed to withstand 9v. If not, you may need a 5v LM7805 regulator. In this case, place the batteries in series.
2) As you can see, we've got a 10k ohm resistor connected between the positive battery lead and the base of the NPN transistor, which is also connected to the first post. Note that the second post is connected to the negative lead of the battery pack. For those of you who don't know much about transistors, we are using it as a switch. When power is applied to the base of an NPN transistor, the power at the collector will drain through to the emitter, which is connected to ground. We're going to get back to this portion of the circuit in just a second. Please note that I've included a bunch of images that give examples of how to use NPN transistors as switches.
3) We have the positive lead of the DC buzzer connected to the positive lead of the battery. The negative lead of the DC buzzer is connected to the collector of the transistor. Since the emitter of the transistor is connected to the ground line (negative lead of the battery), then power is applied to the base of the transistor, power is applied to the piezo and drains to ground from the collector to the emitter (Completing the circuit). When there is no power tied to the base of the transistor, the buzzer circuit is not complete. In other words, the buzzer will not sound. When power is applied to the base of the transistor, the buzzer circuit is completed, and the buzzer turns on.
4) Let's have a look at the nail posts. We have one nail post connected to the base of the transistor and the protective resistor. The second nail post is connected to the ground line. When we connect our magnet wire loop between the nail posts, this connects the base of the transistor to ground, which means that there is no power applied. As soon as that magnet wire breaks, power from the positive lead of the battery pack is applied to the base of the transistor, and the buzzer will sound until you can either turn the power off, or reconnect the wire.
5) You can loop the wire pretty much as far as you want. As long as your magnet wire is intact, the buzzer will not activate.
IN BASIC TERMS:
- Power is required for the circuit to work. We are using a 6v battery pack (4x AA or AAA batteries in series)
- Until power is applied to the base of the transistor, power at the collector (connected to the negative lead of the DC buzzer) will not connect thorugh the emitter of the transistor and connect with ground.
- The 10k Ohm resistor acts to protect the base of the transistor, as well as limit the current to the circuit. Without it, there would be absoluately no load, and the battery pack would short circuit.
- When the magnet wire between the posts is connected properly, the power at the base of the transistor is sunk to ground (0v)
- When the magnet wire is broken, the path to ground at the base is broken When this happens, the base of the transistor receives power, and the DC buzzer circuit is completed. the result?BEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEP!!!!!