Help with Relay, LED, Components....

[Dieter2]

Hello!

Forgive my ignorance up front with components and terms, no real electronics background but I have a problem that I'd like to solve, any help greatly appreciated.

System: Automotive 12VDC

I have a voltmeter to do the obvious. However, the voltmeter has an integrated, red LED warning light. I assume the voltmeter has the built-in resistor for the LED already as on the bench the LED works fine through the expected range of voltages. The problem is, the voltmeter does not talk to the LED itself. The LED just has a + and - wire independent of the voltmeter. Kinda silly that the voltmeter wouldn't somehow take advantage of the LED to indicate a low voltage/charging issue. The factory offers no solution, just that "most people don't use it". (Then why put it in there?)

So, I need a way to tell the LED to illuminate under a low voltage/poor charge condition. Car running/charging voltage is about 13.8V. I guess it's somewhat subjective as to low voltage - but let's just say 13V?

My approach was to use a typical 5-pin automotive relay with the NC contact connected to the LED + wire. The NC circuit would be fed by +12V on a wire that only sees power when the ignition is key is turned to the accessory position/car running. Easy enough.

In triggering the relay, I'd use another wire that sees whatever voltage the system is carrying (also key on condition) that would activate the relay and switch to NO, removing power from NC, disabling the LED. The trick is to trigger the relay when the voltage on that wire has achieved greater than that low voltage, 13V. So how to do this, or is this simply a poor approach?

Thanks for any help/suggestions.

 

[Harald Kapp]

My approach was to use a typical 5-pin automotive relay with the NC contact connected to the LED + wire. The NC circuit would be fed by +12V on a wire that only sees power when the ignition is key is turned to the accessory position/car running. Easy enough.

The relay will not turn off until the supply voltage is way below the 13 V limit you want to achieve. You need an active voltage monitoring circuit. You could build such a circuit yourself, but I guess an off the shelf module like e.g. this one is more to your taste.

I guess it's somewhat subjective as to low voltage - but let's just say 13V?

Too high. Set the limit more like 12 V, see the discussion here.

 

[Dieter2]

Thank you Harald. A very good point, you are totally right, I didn't even think about that - the relay would indeed stay engaged even after the voltage failed. I think it only needs around 6V to actuate. So flawed approach on my end! That's why I'm here, to see what I can't see courtesy of people that actually know what they are doing.

Sure, I can set the limit to 12V and I will take a very close look at that off the shelf module. Though I do remain interested in building my own - just no idea what components to use!

 

[Harald Kapp]

Though I do remain interested in building my own - just no idea what components to use!

The circuit you're looking for is a "voltage comparator". You'll find lots of variants for this kind of circuit with different performance chracteristics.

 

[Dieter2]

Thanks. I'll check into circuits and components that might work for this. But in the meantime, given how cheap it is, I think I might just try out one of the pre-built ones, found a few more possibilities too on eBay. I do like the path of least resistance (and low price), but the only way I'm going to learn about this is to actually dive in, so easy solution first, but in semi-parallel, build my own.

And how do you quote people on this forum? Is it a function of number of posts? I don't seem to be able to do that just yet....

 

[Harald Kapp]

And how do you quote people on this forum?

Just like this, mark the text to be quoted, then klick on the "Reply" button:

 

[Dieter2]

Just like this, mark the text to be quoted, then klick on the "Reply" button:

Clever!! Thanks!!

 

[Dieter2]

You could build such a circuit yourself, but I guess an off the shelf module like e.g. this one is more to your taste.

Ended buying that one you recommended, and it works nicely. Came pretty quickly, board seems well-built, the case arrived pre-broken, but nothing that can't be overcome. Testing on the bench, does exactly what I want it to do. Problem solved, thanks Harald!

 

[Kiwi]

What brand of gauge?

I would guess that the LED is an alternator warning light.
Probably also has a resistor in parallel to increase the current to simulate a 2W bulb to excite the alternator.

 

[Dieter2]

What brand of gauge?

It's still badged as VDO, it's a marine line of gauge. But the line has actually been taken over by Veratron of Switzerland.

https://www.veratron.com

And indeed, this gauge was made in Switzerland, but still has VDO printed on it, and packaging is VDO as well....funny, but you still see the instrumentation (with outdated technical info) on the VDO website, so not sure what the deal is there. A very helpful fellow at a marine outfitter in Florida actually turned me on to this change, and contacting Veratron was the only way to get any really updated info on this line of gauge. VDO couldn't tell me how to wire it, but they did fill in a few blanks verbally that was missing in the online literature. Surprise even them. But "most people don't use it" was the response to wiring the indicator bulb.

I would guess that the LED is an alternator warning light.
Probably also has a resistor in parallel to increase the current to simulate a 2W bulb to excite the alternator.

It is indeed a warning light, and it may at one time have been able to excite the alternator, but it doesn't work that way in this current generation (I tried different ways of doing this without success before contacting Veratron). It is just a LED that wants + and -, user figures out the trigger. Veratron verified that there used to be a "sender" of sorts that handled this, but they no longer make it (or VDO doesn't make it, and Veratron didn't continue it). That's what brought me here.

None of this was even necessary as the car's original incandescent warning bulb in the dash works like a charm, but since the LED warning light is there in the new instrument, well, why not use it? It needs to fulfill it's purpose. Just simple on/off function though, unlike the incandescent bulb that will glow at varying intensities as the charging system begins to fail, which is kinda nice.

All of the instruments in this line have LED warning lights, but apparently the installer needs to get creative with how to trigger them. My next challenge will be how to figure out the warning lights in the oil pressure and temperature gauges that I'd like to add, instead of just relying on the idiot light.