Fitting a relay.

Yup - that's fine assuming the extra load of the relay coil doesn't upset the warning system. Trouble is a relay will slow down the LEDs coming on which is said to be a safety factor since they normally light faster than incandescent. You could, of course, use a transistor to switch the LEDs - this would switch instantly and not draw significant current from the stop light circuit.

Car relays are not normally polarity sensitive. Some smaller relays designed to be driven off a solid state device have a snubber diode to stop back EMF from the coil damaging the device - but these are usually marked as such.

Thanks Dave. Looks like the relay I have draws about 80 mA. If this does trigger the warning system, by suggesting using a transistor, would that be the same as a solid state relay? Something like the 6 series shown here.

Which only needs 16 mA @ 30 VDC, or are you thinking of something made up using a transistor, plus a few other bits on a bit of Vero board? You can tell I'm a bit lost when it comes to really complicated electronics. :-) Mike.

Might be - but my PDF reader says this needs a password. ;-(

If you use a Darlington pair you can set the input current to far less than that - although you have to be careful about spurious triggering if the input is too sensitive. But mounting it close to one of the brake lights should stop any possibility of this - although you can also use a parallel resistor on the input to set the trigger current.

It would be easy to build one inside a standard relay - the older type with the metal case which can be removed.

If you know the load of the LEDs, I could probably recommend the bits which will cost no more than a ordinary relay.

Strange. I think I got there via the RS webpage. No matter, I've no doubt you're familiar with the type of SSR I have in mind

Had to get on the 'net' to find out what a Darlington pair was.:-) I see what you mean about input sensitivity, when they can be made to be touch sensitive.

It does seem to be a very simple device, but I notice 5 amps seems to be the maximum sort of current they can cope with. Loos like there's 14 LED's in the array. I'll stick a meter on them sometime to see how much current they might need. My understanding is that LED's take very little, but maybe 14 of them will add up to too much.

Thanks for the offer. I'll take you up on it, if the relay I have gives any trouble. I should know tommorow. Unless SWMBO has other ideas, I'll be fitting it in the afternoon.

As an aside. Has the E39 got red lights in the doors that show when they're open? As you know the SD1 does, and so does my Celica. Struck me as strange that a car like the E34, which has upmarket pretensions, doesn't have them. Mike.

Yep, neither of my Saab 900's either the B reg or the J reg had them, just reflectors.

But the H plate GT4 did.

Prompted me to see if there was a later version of my reader for RISC OS, and there was which now works fine on the above.

Yes - very useful device.

Depends entirely on the output transistor.

Maximum for most 'straight' LEDs is 30 mA, so 14 would be just over 2 amps.

It doesn't have those red lights but simply reflective strips. It does have lights in the bottom of the doors to light up the kerb, though.

What a marvellous and innovative idea. I always want to see what I'm stepping onto, or into when I get out of my car at night. I can see I'll have to save up for an E39. :-) Mike.

The Mazda has puddle lights in the underside of the mirrors. Unlock the car with the remote lock, and they illuminate the road.

Richard