There are a ton of ioniser projects on the internet already which are all more or less based on the same circuit known as the Cockroft-Walton multiplier. So... here's another one! As the circuit is pretty much set in stone, this is just a short page describing my take on the layout and enclosure used. Usually, I would show the schematic followed by a board layout, but here the schematic is the board layout! A lot of commercial designs are made up of 22 stages consisting of 1N4007 diodes, and capacitors with values of between 10n and 100n (I decided to split the difference and use 47n). My layout ended up with 24 stages because this number fits the 5cm x 10cm matrix board I used, perfectly (and conveniently, components often come in packs of 25). In addition to the capacitors, diodes and two 10 megaohm series resistors (the resistors create a safe high impedance output), the board also has a screw terminal block for mains input, another for the high tension (HT) output, and for good practice, a 100mA quick blow fuse is placed in series with the mains live. At first I tried to mount the diodes horizontally side by side but they wouldn't fit comfortably due to their body diameters being slightly too large, so chose instead to mount them vertically with every other one being opposite to the one next to it, which seems to look and work OK

Matrix board

Schematic/Board layout viewed from above (component leads bent and soldered underneath)

A standard double surface patress box with matching blanking plate was used for the enclosure. Not only are these tough little boxes perfect for this project, they can be purchased cheaply pretty much anywhere, though some boxes unfortunately have internal mouldings that are located right where you want to put the circuit board mounting pillars, so these are best avoided (unless you don't mind the board being slightly off centre). For electrical safety, nylon type mounting pillars must be used. The actual ion emitter is just an ordinary darning needle soldered to an eyelet tag and screwed onto an 8mm long M3 nickel plated hex pillar mounted on the top plate. Ions are not easily emitted from blunt or rounded objects so the sharper the emitter is the better (like a lightning arrester, but in reverse!). Although just a single needle was used here, more can be added for a higher output if required

Internal view

For reference, the surface box and blanking plate were from Screwfix (pt.nos. 29047 & 51046). This particular box and plate are made by LAP and have a smoother more rounded appearance than the commonly used square type and it comes with screw caps as well. By design, all external parts (except for the emitter) are white including the PG7 cable gland used for the mains input. If you want to try different types of emitter there is a lot of scope for experimentation, and if the top plate needs replacing it won't break the bank as at the time of writing the cost was only 81p!

Close up of the emitter

In operation the ioniser won't appear to be doing anything but there are several ways to check if it's working:

Carefully place the palm of your hand in front of the needle where you should feel a very slight breeze coming off of it
Carefully put your nose in front of the needle where you will definitely get a whiff of ozone
3) Carefully put your ear near the needle where you can sometimes hear a fizzing sound (this seems to come and go)
4) In a darkened room a tiny purple arc can usually be seen when you place a finger near the tip of the needle
A more technical test is to use a multimeter set to volts. With the ground probe earthed, moving the positive probe near to the needle should give a small negative reading