I realise that the leds must be used in the correct direction, what happens if I have them the wrong way - will it blow anything or will they just not light?

Hi.You shouldn't blow anything. But then again you never know. ;-)Light Emitting Diods (LED) work pretty much as check valves as other diods and are used as such as well. Letting current flow in one direction and stop it in the other. Hope it helps,

Any semiconductor junction (as LEDs are) can sustain a certain reverse voltage.Go above and you'd blow the junction.If used as light indicators, the fact that you mount them wrong is not gonna do anything: they just won't light up.Severe adverse effects may happen in bad circuits with high currents in place, but usually in cockpits this is not the case.Especially if you use leds with systems like FSBUS, IOCards, EPIC and so on.Should you mount them wrong, just swap their pins and there you go :)

Hehe, I remember one day many years ago when I was fooling around with electronics. I had mistakenly set my variable-volt power supply to 12 volts and then went to test this big fat LED for the first time that was rated for 3 volts. "Wow!" I thought. This thing is really bright! Next thing I know the LED exploded right in my hands like a firecracker. That scared me so much I nearly ###### my pants! Lol...