For xmas a couple of years ago my parents bought me a nice temperature-controlled soldering station which I've been using to build all my half-completed projects. It works great, but it has one small problem: the LCD display isn't backlit so, at a glance, it isn't obvious that the power is on. This isn't normally a problem but if I'm working on something that means I'm soldering something and then going back and forward to the computer to program an Arduino or whatever, it's possible to forget the soldering iron is on.
And then go to bed.
And then go to work.
And then come home and find out that there's been a 50W desk-mounted heater on for 24 hours and somehow we still have a home and not a smouldering ruin.
The solution to this problem is trivially easy - mount an LED on the front to make it obvious that the iron's on. First thing we need to do is open up the soldering iron and find a power source to hook our LED up to.
This is the main PCB for the iron, and at the top left is an LM7805 linear voltage regulator that supplies the 5v to power the control circuity. Perfect and has long leads that we can solder to. If you're looking at the front of the LM7805, the three leads are Vin - GND - Vout.
A quick check with the multimeter to confirm.
I come to a small problem here - how am I going to solder on the inside of my soldering iron? I briefly consider using the soldering iron itself and then remember that I have a butane powered soldering iron for just these sorts of occasions:
I dig out a 5mm red LED and find my LED calculator
to work out what current limiting resistor I need. Looks like 150 ohms will do the trick, which is one of the several values I have. 10 minutes and some dremel action later and we have this:
I should have used some heatshrink, but I couldn't find any. It should be ok like this though. The LED fits in pretty snugly but for peace of mind I might stick some hot glue on the back to hold it in. But does it work...?
Success! And all done before breakfast on a Saturday. Quite good fun in the end, and if it stops me from accidentally burning down the house, well that's just a bonus.