Brake light relay.

Ahh. Pity - they're wonderful things and can do all sorts of tricks.

A simple single pole on/off. Called SPST Single pole single throw.

Easier to get one from Halfords etc - they certainly do them. It will have a fixing bracket and normal car 1/4" spade connectors. Even some with a normal car fuse sticking out the top which might be a useful safety feature.

Or Maplin LJ68 at 1.49 - this is exactly what you need. It's described as SPNO which is 'single pole normally open' - in other words the switch makes when the coil is energised.

RS might well do something similar - you can do the searching. ;-)

There are thousands of different designs that would do the job, but a pukka car type like the one above is easy to mount and connect to. Most industrial types need to be mounted on some form of PCB.

Thanks for the info Dave. I understand the principle involved. The how and why of their use. It's the when and what type I am never sure of. :-) There seems a bewildering choice, apart from that of just voltage and current carrying capability. Mike.

There's certainly a vast choice for electronics, but on cars (apart for special apps) there's a fairly standard type like the one I've described. They come in two common types, SPST, and SPDT - the DT being double trow where a common input feeds one terminal at rest and the other when made. They look the same externally, though, apart from one having an extra terminal. The terminal layout is thus:-

_____________ | | -----|-----Normally open | | | Coil-------|--| ___ |--|-----Coil | | \ | | | ___ \ | |______|__ \__| | \ Common \Normally closed

For your application you use the common and normally open, if using a SPDT relay. The coil connections aren't polarity sensitive.

There are also some oddball relays around using the same pin layout, so just be aware if buying from a small spares shop.

Most automotive relays have the terminals numbered with following convention:-

85 - Coil + 86 - Coil - 87 - Normally open contact (connected to terminal 30 when coil is energised) 87a - Normally closed contact (connected to terminal 30 when coil de-energised) 30 - Common contact. Fused supply for consumer connects here.

Coil polarity is sometimes specified as some relays have a protection diode across the coil. If the polarity to the coil was reversed, the diode would cause a permanent short circuit.

A standard automotive relay typically has a contact rating of 10A, enough for 10 LED brake lights.

As you have the bulb monitor, ideally the relay should be connected into the brake circuit before the monitoring circuit, usually just after the brake switch itself. This can be a pain, having to run wiring all the way to the front of the car from the boot, so it is worth trying to wire the relay across one brake light to begin with. Connect 85 to the wire supplying the exisiting brake bulb, and terminal 86 to a good earth on the cars body. Connect terminal 30 to a fused permanent feed (the live side of the boot light is a good place), then finally terminal 87 to the positive wire coming from your new LED brake light, and the other wire to earth.

HTH

Anthony Remove eight from email to reply.

I've also made a mistake in the diagram - common and n o are reversed Should be :-

_____________ | | -----|-----Common | | | Coil-------|--| ___ |--|-----Coil | | \ | | | ___ \ | |______|__ \__| | \ Normally open \Normally closed