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the12volt's install bay
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 Topic: bulb out indicator
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urhottboy85
Joined: July 25, 2011
Posts: 20 |
| Posted: November 16, 2011 at 9:37 PM - IP Logged |
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| I'm trying to find a diagram for wiring an led to my taillights so that I know when one of the bulbs burned out...I can't seen to find any anywhere but I know it had to do with resistors and relays...all the luxery cars come with this already but I have a 05 jeep and wanted to install this in..just need the diagram help...thanks |
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oldspark
Joined: November 03, 2008
Location:
Australia
Posts: 2,855 |
| Posted: November 17, 2011 at 1:13 AM - IP Logged |
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It's a current sensing circuit.
Traditionally done with a resistor in series with the filament(s) to create at least a 0.6V drop with normal current.
That 0.6-0.7V is used to turn on a transistor which STOPS the indicator (LED) from lighting.
If a filament blows, the less than 0.6V (eg, 0.3V or 0V) does not turn on the transistor and hence the LED is lit by the 12V applied to the filaments (whether parkers or brakes etc).
These days chips like Op-Amps or Comparators can be used to sense lower voltage drops - hence a smaller resistor without the large Wattage rating , ie, 0.6V @ 7W for a single tail filament means ~0.6V x ~ 0.5A => a 1./2W resistor, but 2 tails means a 2W resistor, and a brake filament (23W => ~1.75A = 1.1W, hence...) requires a 2W resistor, or 5W for two filaments.
The "logic" is done by extra components - eg, if +12V and NOT > 0.6V, turn the LED on, though the comparator (or op-amp) can do it in one go by an appropriate resistive voltage divider to each input (ie, if voltage on the non-inverting input exceeds the inverting input, then turn the output on). {hey - there's another PICAXE prohject!)
For LEDs, relays are not required as most chips etc can sink or source the ~20mA required for the LED.
And I'm hoping I got the above "designs" correct. It's been a while, and until knowing what current(s) and whether it's +12V or the rarer ground switching to turn on the lights, I'm not spurred enough to check my logic or search (google) for circuits.
But essentially it is a current sensing circuit that turn on a LED when the lamp is on and the current is BELOW the normal total circuit current. |
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urhottboy85
Joined: July 25, 2011
Posts: 20 |
| Posted: November 17, 2011 at 1:38 AM - IP Logged |
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| So on a comparator would all the legs be positive? And you would just line them up across from each other for which led goes to what bulb then ground each led? Radio shack has a 14 point comparator...so would that be good for 7 lights or would it depend on how its wired |
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oldspark
Joined: November 03, 2008
Location:
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Posts: 2,855 |
| Posted: November 17, 2011 at 2:09 AM - IP Logged |
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I'm just talking about 2-input comparators. They exist in single, dual or quad versions in chips (ICs, aka Integrated Circuits).
I'd prefer a system that tells me at least which side is faulty (because if it is the near-side, down here it does not make the car unroadworthy wrt on-the-spot fines or defect notices).
And if it's dual filaments per side or "central only" as on a motorbike, I'd like to know if one or both are out. (Same reason, plus safety.)
But some cars simply let you know that a tail (or brake) light is out, and some are happy enough with that.
I recall old Volvos with a single "bulb out" indicator which signal any number of running or park bulbs etc.
So the "resolution" is up to you.
And if that 14-point comparator can be configured for a single reference (ie, +12V to the bulbs) plus 1 to 13 sensors and indicate appropriately, then yes it would be suitable if that's what you want.
And maybe that comparator is a programmable PIC etc so you can mix stop and tail and running lights...
And yes, relative to 0V (ground), all the sensed voltages MUST be positive. The current sensing could output (say) from 0.2V to 0.6V and above - or 0.2V or 0.6V or more down from the +12V - depending on which side of the bulb(s) you put the sensing.
With a smart system, you could even use one current sensor for multiple bulbs. |
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urhottboy85
Joined: July 25, 2011
Posts: 20 |
| Posted: November 17, 2011 at 4:04 AM - IP Logged |
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| So basically I couldn't use just a comparator them since they are already built into circuits? I can't take just a comparator by itself and plug in the positives to one side and the positive output to the opposite right across from the input?...so basically the comparators are useless unless you have a circuit they are built on to noticed the difference |
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oldspark
Joined: November 03, 2008
Location:
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Posts: 2,855 |
| Posted: November 17, 2011 at 8:00 AM - IP Logged |
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Yes you can.
A "comparator" can be a basic building block - eg, an IC.
But you can't just connect it to 2 voltages and expect it to know what to do.
You must set up appropriate circuitry (eg, 4 resistors) to convert your two "sampled" or compared voltages so that when one exceeds the other, the comparator switches (the LED on) to let you know there is a fault. |
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urhottboy85
Joined: July 25, 2011
Posts: 20 |
| Posted: November 17, 2011 at 8:37 PM - IP Logged |
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| Can you put together a diagram or something for me to go off of?
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oldspark
Joined: November 03, 2008
Location:
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Posts: 2,855 |
| Posted: November 18, 2011 at 6:26 PM - IP Logged |
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Alas I haven't yet recovered my recently lost data...
But from Linear Technology's Application Note AN-105 - Current Sense Circuit Collection (December 2005), under "Fault Sensing-2" on page 64, "Lamp Outage Detector":
(No Copyright, but thanks to Linear Technology & Tim Regan. IMO a great publication!)
That's just an example using a "high spec" comparator, but it is unusual in that is does includes the switching logic.
Usually it is the lamp switch that supplies the power to the lamp AND the detection circuit. Hence the detection is only active when the lamp should be on - ie, the bad brake-light LED(s) only light when the brake is applied (assuming the brake switch works LOL).
Refer also to Bristolwatch's Voltage Comparator Information And Circuits which is credited to Rob Paisley's Model Railroad & Misc. Electronics page. (Now that's a source I have used before...!  )
In particular see the "Comparator Operation" section a few screens down. That summarises the input-output relationship, noting that the leftside RV variable resistor (aka pot) is equivalent to an upper and lower resistor as shown for the rightside V-Reference "voltage divider". That was what I meant by "scaling" using 4 resistors - 2 for each input, though one would include a trimpot (variable resistor) for calibration.
Also Rob Paisley is referring to the LM339 and similar Comparators only.
The LM339 is very common and is liked because it uses a single supply, comes in a quad package (4 comparators in one $2 14-pin package), and is an Open-Collector output.
That means you don't have to mess with dual or balanced supplies (ie, +6V, 0V & -6V), and outputs can be directly tied together to make an OR circuit (ie, output LED is on if stop-left is bad OR stop-right is bad OR park-left is bad Or ... etc etc.) - often called Wired-OR logic or circuitry.
[ Open Collector outputs - often called ground switching outputs - are the normal inter-connection technique for digital/computer components, alarm sensors or outputs, ignition systems (eg, points and ignitors), and where systems with different supply voltages require interconnecting. ]
Maybe the type of circuit shown under Bristolwatch/Paisley's "Voltage Window Detector Circuit" (nearly halfway thru the html page) is what you want?
BTW - OpAmps can be used. Comparators are merely OpAmps that are preconfigured as comparators, hence avoiding extra circuitry - and often the requirement or confusion with dual-supply OpAmps.
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oldspark
Joined: November 03, 2008
Location:
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Posts: 2,855 |
| Posted: November 21, 2011 at 12:56 AM - IP Logged |
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Last night I played around with some rough designs.
Figured a 0.47 Ohm 1/2W resistor in series with a 7W bulb, and a 10k pot - and was it a 27k(?) resistor.
23W would be a ~.15 Ohm 2W resistor in series with the bulb.
A single LM339 quad-comparator package could hence monitor 4 bulbs.
What size and number are you looking at?
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urhottboy85
Joined: July 25, 2011
Posts: 20 |
| Posted: November 22, 2011 at 1:19 AM - IP Logged |
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I was thinking of all the back lights except for the third brake seeing as how t
hats not needed...so 6?..well d should be good since reverse lights are not used as much |
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