I habitually forget to turn my soldering iron off when I finish using it, meaning that it ends up being left on for long periods, without being used. This is bad for the iron, and is a fire risk. So I came up with this.

Step 1: The Basic Idea.

I used the Jaycar kit; 'The Flexitimer' (www.jaycar.com.au/productView.asp), though you can use any sort of timer you want, in which case this is more for inspiration than anything. My requirements for this project were this:
   -The soldering iron can be turned on with the push of a very obvious button.
   -It can be turned off manually as well, with another switch.
   -If I forget to use this second switch, the iron turns off automatically after X period of time.
   -A light tells you that the iron is running.
   -The iron and timer can be separated again with ease.

Step 2: What You Need.

A timer kit/module.
A relay.
A normally open push button
A normally closed push button (I didn't have one, so I used a momentary toggle switch instead)
A light for indicative purposes (choose one to suit its location in the circuit, more on this later)
Any enclosures that you want to put your device in
A power supply to suit your timer and relay
Enclosure hardware (if required) I used:
   IEC socket for power in
   GPO socket for power out
   5 pin microphone plug and socket for cable to switch box

Step 3: The Timer

If you're using the kit I used, now would be the time to assemble it. If not, skip this step.

Step 4: The Relay.

In order to have the timer AND the off switch control the iron interchangeably, and to have the power to the timer cut along with the power to the iron, I used a relay, wired as a latching relay.

To do this, take your normally open switch and your normally closed switch and solder them together in series. Also solder the normally closed contacts of the timer's relay in series, after the normally closed switch. Connect the other side of the timer relay to the coil of your 'latching' relay.

Now, connect the normally open contacts of your 'latching' relay in parallel to the normally open switch.

Step 5: Schematic

In case you're reading this with no knowledge of schematic diagrams, I will explain it to you. NO stands for Normally open, which means you push the button to turn something on. NC stands for normally closed, which means you push the button to turn something off. The same applies on relays.

I have included two possible locations for the lamp, you could use one or both of these. Note however, that if you use lamp A, you will need to make sure it is rated to mains voltage. Lamp B must be suited to your power supply.

The lamp should be placed in one of these two positions for the following reason: The light's purpose is to tell you that the iron is hot and the mains connected to the output. If the light is too far (electrically) from the mains circuit, all it will tell you is that the timer is connected, or the wires connected to the lamp are not broken. If it close to the mains, you can tell that the mains is on, failing that, the relay is a good second option, as it will tell you that the relay is stuck on, or has failed, allowing you to turn the whole thing off before your workshop burns down.

Step 6: Put It in a Box.

So that's the idea behind my soldering iron timer, pretty tricky hey? I'll show you how I made mine, enclosures, hardware etc., but feel free to package it up however you want.

Step 7: A Note on Timer Choice

I used this method of wiring because of the way my timer works. For some reason, the Jaycar timer kit has the relay off as its normal position. To activate the timer, you apply power to the circuit. Then it starts counting down, then turns the relay on. My original idea was based around a timer that turned the relay on, then when the time ran out, turned it off again.

If your timer does what I wanted it to do initially, you may be able to come up with a better system. Play around with it.