On board light indicator - would this work?

[TheBeastie]

I am minded to try and see if I can cobble together an on-board lighting indicator to let me know when I have lost a bulb. I am not sure if this would work - what do you think?

I am almost mind made up to run a separate earth cable back to battery for each of the four front and back side/indicators and have pretty near finished separate earth to headlights and spots. The headlights and spots earth is quite heavy but the others would only need quite thin cable I think.

My thought is to piggy back a thin lead off the earth cable when it gets back to the battery box and run it up to the dash where I am thinking of mounting a circuit board with LEDs (single LEDs as on a PC front) on it in rows - four for front side and indicator, four for headlamps and spots, four for rear side and indicator. I could set up the circuit board so I could connect all the incoming wires (which although earth would actually be my +ve and then common earth all the LEDs to give circuit)

The logic is that if the bulb is OK then current will flow through the earth wire and thus illuminate the LED but if the filament is gone then current will not get as far as the earth cable and so the LED will not light.

Thus I would be able to tell at a glance that my bl**dy front offside sidelamp is on the blink again.

I also think that if I took my time it could look quite neat particularly if I used white amber and red LEDs.

So far as I can see the rear side/brake and the headlights share a common earth between both filaments so it would not be perfect but then again if side lights were not on and you braked the LEDs should light up. Similarly if switching from dipped to main the LEDs changed then you would know one filament had gone since it isn't always quite so obvious one is not working especially if it is half light.

Any thoughts? It has occurred to me it could completely mess up my earthing but hopefully if this is likely someone will point it out to me!

Thanks for looking

Malcy

[LandyManLuke]

You can't run the main and tell-tale lights in series, that won't work.

Also, you cant run anything off an 'earth' if there is still a direct path to earth, all the current will take the path of least resistance, and avoid the LEDs completely.

In theory, you could add a high power (rated to the lights), extremely low value resistor in series, and test for a PD across that. Or, add a light sensor to each lense and use that.

[FridgeFreezer]

Running current past the LED's they won't light unless you use a shunt resistance, which will seriously limit the brightness of your lights if it gets near enough resistance to make an LED light. Shunts are also extra complexity, and high-power ones are costly.

Running current through the LED's to ground (as your plan seems to suggest) will blow the LED's, tehy are only happy up to ~50mA, maybe a smidge more for the really bright types - but who wants a dashboard of blinding bling when you're trying to drive at night?

Why not just walk round the truck with the lights on when you get in or out of it, far cheaper and more reliable

All those squillions of cables will be adding resistance and extra points of failure too, I tend to join cables at convenient points and run just a few big earth wires back to the battery or earth point - for example, one for each wing (or light cluster) and one or two for the engine, fans, etc.

Keep it simple!

Oh and if a bulb fails short circuit the LED won't indicate a fault.

[TheBeastie]

OK Thanks for the input.

Not a good idea then - bang goes my chance of a fortune!

As you say walking around does sort but I never seem to remember to do so.

Back to the drawing board - yellow post-it on sun visor - "Have you checked your lamps" I guess!

A bit of me hates low tech!!!

[FridgeFreezer]

I believe the way BMW did it was to have a bank of lights (or LED's) wired to a switch, and then wired individually to each (body) light's wire AFTER any switches/relays/flashers - with the normal lighting switches turned off, sticking 12v onto the LED's supply would cause each one to ground through the relevant bulb (which don't light up with 20mA going through it) and giving an indication.

If you use LED's you need proper 12v ones or to add a 220R-1K resistor in series with each one (usually much cheaper).

[landmannnn]

A nice low tech solution is to use some fibre optic cable, some cars in the 80's used this method, I can't remember which though.

The fibre optic cable pointed at the light through a hole in the backing plate of the light unit. The cable was then routed to a little panel visible to the driver. If the light is on then you see a glow, if not then the bulb is not on. Only works at night of course.

If you know someone in IT you could get some fibre optic cable off cuts for free.

[FridgeFreezer]

A nice low tech solution is to use some fibre optic cable, some cars in the 80's used this method, I can't remember which though.

I can get fibre optic, we have the odd bit knocking about but frankly I can't see it working terribly well - a fibre is tiny and unless you cleave it neatly and terminate it properly it won't let in much light, and what comes out the other end will be a tiny pinprick.

Thick (~0.5mm) plastic fibres can work and are used on some stuff, IIRC there is a similar idea for motorbikes with the back light. However, that stuff is pretty poor and unlikely to be found in any IT or comms situation.

[TheBeastie]

OK

Following FridgeFreezers idea wouldn't the LED still light if there was a wiring fault - i.e. the current would go through the LED and then earth out BEFORE it hit the bulb if there was a fault, thus giving a false reading?? I do like though the idea that you could run through both filaments seperately on headlamps and side/stop bulbs which would be neat.

Perhaps the old fibre optic route would not be such a silly one after all though since the concept of false positives, or is it positive falses, is overcome even if the double filament bulb isn't.

Still need to sleep on it but my hopes have been restored! Thanks

[LandyManLuke]

If you test the electrical side of things, you've got to carefully consider how the bulb might fail, and the possibility of false readings. the only accurate solution would be some sort of optical sensor, set up to accomodate and discount daylight.

Ultimately, You've got to wonder why go to the hassle? A seperate test circuit would require you to remember to press the button, or yet more circuitry to test the lights automatically before letting you switch them on.

[FridgeFreezer]

Any test is only as good as the test system - if there's a fault with the test, or a failure that causes a false reading, then you're no better off. or to quote Red Dwarf "The damage report machine is broken"

I really doubt fibre optic is going to work easily for a variety of reasons.

[western]

much quicker & cheaper & more reliable to switch all the lights on & do a 360 walkround the vehicle.