Quick electronics question

[Kitwn]

Rob,
Sorry to have impugned your intelligence!

Looking at your diagrams:
If you're dropping nearly 12v across R1 with SW1 open there is something seriously wrong! The collector of Q1 should be at the full 24v. On reflection there should be a pull-down resistor (10k would do) on the base of Q1 to hold it to 0v when the contacts are open, otherwise the base is left floating which should never be allowed to happen. I suspect your emulation software has picked up on that fault and is allowing Q1 to partially conduct.

[cropwell]

otherwise the base is left floating which should never be allowed to happen

I should have remembered the lessons I learnt in the '70's when I was doing circuits with 4066's and leaving inputs floating, with unfathomable results (until the penny dropped). My emulation software is crap and out of date (IC library is woefully lacking), but it is OK for quick checks.
Yes, I put the voltmeter on to show what was happening across Q1 and did wonder why, but with the SW there should not be enough Ib to turn Q1 on. (???)

Even with strapping the base of Q1 to Gnd there is an e-v voltage of 10v with the switch open - curious... Putting a diode in series with the LED puts the emulation right, so I am going to presume that the circuit emulator has a large reverse leakage on the LED.

[Kitwn]

Even with strapping the base of Q1 to Gnd there is an e-v voltage of 10v with the switch open - curious... Putting a diode in series with the LED puts the emulation right, so I am going to presume that the circuit emulator has a large reverse leakage on the LED.

That agrees with LEDs having such a low specs for reverse voltage, usually only 5v.

So we can finally give m_c a definitive answer that might actually work! Strap the transistor base to ground via a large-ish resistor to avoid letting it float with the contacts open and add an ordinary diode in series with the LED.

[Doddy]

Nope, look at the current flow from base to 5V supply - it's modelled the reverse leakage current of the LED. This is the point where modelling with idealised circuit models will give you a somewhat distorted view of reality, the Vce of 6-7mV (on state) will be... unusually low. Similarly the reverse bias current flow through the LED will be highly variable during manufacture and you're likely to see Vc vary quite wildly with component selection and temperature. But the principle of the simulation is correct and shows that the OP pretty much got a workable solution - with the addition of a diode to limit the reverse-bias current flow through the LED. Of course, with this making the 21st post on the subject it would be quicker to breadboard the sucker and just test it :)

EDIT: Dammit - that was looking at new posts and believing post#20 was the latest.