Also, it might well be that my control card will pull sufficiently to ground to negate this resistor.

Check the voltage on the collector of Q2 with the controller not connected.

 

I put a 22k resistor (R102) between collector and GND of Q2 but only by reducing it further to 4.7k did the LED turn off. However there is still 1.2v across the resistor when the LED is off but if I reduce the value any further, the resistor gets very hot.

Try putting the 22k resistor directly across the LED. There's a leakage current problem and the LED is emitting light at a very low current.

 

1.2v across 4k7 = ~250uA which isn't unlikely if Q3 not turning off fully. Try a 10k from base of Q3 to ground to make sure no noise (mains hum etc) is turning it partly on when the base is disconnected.

Thank you Irving - I just tried that and still get the same 1.2v - I still have to force my brain to allow resistors going from +ve to GND withouth something else like a bulb/motor (yes, I know bulbs are resistor but resistors are not bulbs!!).

 

Thank you Irving - I just tried that and still get the same 1.2v - I still have to force my brain to allow resistors going from +ve to GND withouth something else like a bulb/motor (yes, I know bulbs are resistor but resistors are not bulbs!!).

Are you saying it gets hot when supposedly off? or just when on?

 

Try putting the 22k resistor directly across the LED. There's a leakage current problem and the LED is emitting light at a very low current.

That drops it to 1.1v.

 

Are you saying it gets hot when supposedly off? or just when on?

I tried down to a 1k resistor (R102) to get the voltage down - that's when it got very hot when the LED was off!

 

That drops it to 1.1v.

Try a smaller resistance. Or a pull down resistor on the NPN.

 

Check the voltage on the collector of Q2 with the controller not connected.

I haven't yet (dared) to connect this circuit to my card (I'm in UK so hard/£££ to replace) so the test LED is taking its place as a load (the card is 25mA max).

 

A 1k resistor will draw 2.4mA on 24v and dissipates 0.6W, so not surprising it gets hot when ON, if you're using normal 1/8 or 1/4w resistors, but shouldn't when off.

 

Try a smaller resistance. Or a pull down resistor on the NPN.

Is that a pull down on the base? I still have 1.2v when pulling down the base.
The smaller resistance is when things get hot (when the LED is off).

 

A 1k resistor will draw 2.4mA on 24v and dissipates 0.6W, so not surprising it gets hot when ON, if you're using normal 1/8 or 1/4w resistors, but shouldn't when off.

The pull down (R102) only gets hot when the LED is not on - presumably because the current is flowing through the LED & its resistor when the LED is on. Which makes me wonder of the PNP (Q2) is not being fully turned off by Q3?

 

I tried down to a 1k resistor (R102) to get the voltage down - that's when it got very hot when the LED was off!

That doesn't make any sense at all. Are you sure you have the correct transistor for Q2 a 2N3906?

 

Is that a pull down on the base? I still have 1.2v when pulling down the base.
The smaller resistance is when things get hot (when the LED is off).

A 2.2k across the LED is now at 0.5v - presumably because it's now inline with R201.

 

That doesn't make any sense at all. Are you sure you have the correct transistor for Q2 a 2N3906?

Yes - I just pulled it out & checked (that would have been really embarrassing!)

Just as a sanity check in case my schematic doesn't match reality, I've attached an image of my breadboard. The top rail is 5v, the bottom 24v. The green wire is the 5v switch.

 

Check the voltage on the collector of Q3 with the transistor ON and OFF.
Are the 2 negative rails connected together?

 

Check the voltage on the collector of Q3 with the transistor ON and OFF.
Are the 2 negative rails connected together?

The voltage between GND and Q3 collector is 0v when the LED is off and 23.9 when 0v when the LED is on. The negative rails are definately connected (my new breadboard caught me out earlier because the rails are split half way).

 

The voltage between GND and Q3 collector is 0v when the LED is off and 23.9 when 0v when the LED is on. The negative rails are definately connected (my new breadboard caught me out earlier because the rails are split half way).

That's not right. The LED should be ON when the Collector of Q3 is 0v and OFF when the collector is 23.9 volts.

 

Is that a pull down on the base? I still have 1.2v when pulling down the base.
The smaller resistance is when things get hot (when the LED is off).

Don't see how that could be the case. Even when the LED is on, dissipation in a 1k resistor would be 6mW.

 

I've attached an image of my breadboard.

You need to get one of these:

Here's a breadboard with 3-4 different circuits.

 

Don't see how that could be the case. Even when the LED is on, dissipation in a 1k resistor would be 6mW.

The pull down on the PNP gets hot, not the one on the NPN that you suggested.