Do not do this if you have a wireless or electronic doorbell. This will only work on the old-fashioned doorbells that are simply switches. You could fry electronic doorbells very easily doing this!
I am not liable for any damages incurred through the use of these instructions. Even though it is very hard to hurt people with this, it may freak them out and get you into a legal mess. Use at your own risk!
Step 1: Theory
The purpose of a negative ion generator is to give extra electrons to air molecules, "ionizing" them. Negative ions are good for your health and make you feel better. In order to give molecules extra electrons, the generator must create a very high potential (voltage) between the generator and ground. In the case of this negative ion generator, it creates 15,000 volts.
Now, isn't that dangerous???
In short, no. High voltage doesn't kill people, high current does. A little math:
12v*0.1A = 1.2W
1.2W = 15,000v * 0.00008A
What this says is that if the generator was 100% efficient, it could achieve a maximum current of 0.08 milliamps. 5mA is the minimum current required for any lasting harm to be done, and this is well below that.
You only have to read below if you are very interested in high voltage and shocks.
Most static shocks occur due to capacitance with ground. Basically, every person is a walking capacitor. They are one plate, the ground is the other, and their shoes (or air) is the dielectric. You get charged up due to the triboelectric effect, and when you touch somebody else, you balance the charges in your capacitors. If you touch something metal connected to the ground, you discharge yourself.
This is one way to create a shock. The other way is to provide a direct path for the electricity to go, for example, touching two metal plates at different potentials at the same time. This is what causes most electrocutions. In this example, you are not a capacitor, but a conductor. These shocks are more effective because they are more controllable and more powerful, due to lower resistance.
I tried both methods when constructing this device. Method 1 (human capacitors) works fairly well when you are barefoot, but can barely be felt when you are wearing shoes. Method 2 (human conductor) works amazingly well no matter what.