Voltage monitor lighting two led's depending on voltage

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dynamis_dk
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Voltage monitor lighting two led's depending on voltage

Postby dynamis_dk » Fri Sep 27, 2013 3:19 pm

I've got a little project for the car I'm looking into so wanted some general advice.

I want to be able to take a feed from the dials and monitor the voltage so when the voltage hits a pre-determined 'low' value a red LED will light, and any value above this set voltage a blue LED will light.

I'm not really sure where to start so if anyone has suggestions for a suitable circuit or a direction of research I can start at, advise happily received.

phenoptix
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Re: Voltage monitor lighting two led's depending on voltage

Postby phenoptix » Mon Sep 30, 2013 12:47 pm

I'm sure this is something we could help with. We could base the solution on something like this https://www.sparkfun.com/products/11087 from Sparkfun. I'm happy to make up the board and have a play so you can have a tidy two light solution. There are probably analogue solutions too but I like microcontrollers.

dynamis_dk
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Re: Voltage monitor lighting two led's depending on voltage

Postby dynamis_dk » Mon Sep 30, 2013 2:44 pm

That would be great if you can come up with a solution. Its for the car so I expect the voltage range could be 12v downwards but I'll get a multimeter on it just in case this helps with the design.

A bit of googling lead me to think a battery tester might be a good base but I didn't understand what I'd need to alter to get my required end product.

Ideally SMD parts too would help too (obviously breadboard anything will do as I'll be making a pcb for the final item.

phenoptix
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Re: Voltage monitor lighting two led's depending on voltage

Postby phenoptix » Mon Sep 30, 2013 2:57 pm

With cars you can get up to 16V on full charge, so we'll watch for that. When would you want the light to come on?

dynamis_dk
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Re: Voltage monitor lighting two led's depending on voltage

Postby dynamis_dk » Thu Oct 03, 2013 3:55 pm

It's going to be taking the power which feeds the fuel gauge, am hoping to have the blue led state to be all the time and the red when the voltage hits the low value where the car would normally display the fuel warning light.

I may need to have the circuit powered froma 12v source (as this would allow it to be hooked into the lighting which is toggled by the car mainline being on) and have the circuit monitor the feed to the gauge to get the voltages to say when a low points found.

Basically the project is part of my changing the whole dash backlighting to blue but I thought it would be cool that the whole file gauge lights change from blue to red on the low fuel warning. If needed I could always tap into the led which lights on low fuel but I thought it would be a cool project to play with :)

phenoptix
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Re: Voltage monitor lighting two led's depending on voltage

Postby phenoptix » Thu Oct 03, 2013 9:40 pm

Sounds like quite the project! Do you have size constraints?

dynamis_dk
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Re: Voltage monitor lighting two led's depending on voltage

Postby dynamis_dk » Thu Oct 03, 2013 11:38 pm

Yes and no. The LED's will be SMD and placed on a arch shape around 15mm thick around maybe 80mm length. This will sit directly behind the dials and be the lighting for the dials.

I can always make a mainboard with the works on and just have the above LED board separate to allow as much room as required.

I thought this maybe a simplier way to do the same effect without monitoring the voltages etc.

Image

The 12v feed is what is turned on whenever you turn the car lights on so normally this would be off until it dark, then this in turn sets the dial lights on.

I would like it to work like, the relay allows the 12v feed to power the BLUE LED's whenever the car lights (and dial backlighting) is on, UNLESS the 3.3v (not confirmed but think it is lol) feed is on as the cars own fuel low gauge is lit this will turn off the power to the BLUE LED's (via the relay) and the feed from the 3.3v will power the RED LEDS.

make sense lol

phenoptix
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Re: Voltage monitor lighting two led's depending on voltage

Postby phenoptix » Sat Oct 05, 2013 10:26 am

Makes sense, however rather than a relay have a look at using a transistor. You have a switching voltage already. Should be cheap and quick to do.

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Re: Voltage monitor lighting two led's depending on voltage

Postby dynamis_dk » Sat Oct 05, 2013 12:03 pm

Cheers I'll look I to that. Never had use for a transistor in the past so I'll do a bit of reading :)

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Re: Voltage monitor lighting two led's depending on voltage

Postby dynamis_dk » Sat Oct 05, 2013 1:41 pm

So done a little reading and it looks like I understand very little of it lol. I'm guessing that I would need to update the sketch above like this:

Image

Is there a simple way to select a transistor which is right for this application?

From what I can tell from a quick read I would need to put a resistor inline with the base to get the correct 'input' to the transistor (is it the Ma value which is important here?), this would act like the 'on' switch when the 3.3v from the fuel out LED is activated. This would then allow my 12v to the resistor to drop it to the right level for the BLUE LED's


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