Powering leds from low power source

[diogoribeiro]

Hello,
I'm developing a project to use on my daily drive car and I came up with a problem concerning the use of some LEDs. Thing is, I have a LPG switch with embedded lights to inform me about the amount of gas remaining. I'm thinking of adding a couple of led (3 green LEDs and 1 RGB) to show me that levels on the odometer. The original switch already provides those LEDs but they are SMD and I can't use them on the odometer. What I'm thinking off is soldering two wires in parallel with the LEDs ( on the switch ) and connect them to an auxiliary circuit that helps me powering up 3 or 5mm LEDs glued to the odometer . My idea is to use the standard 12v used on almost all car circuits and use a voltage regulator (to 5v maybe) and a couple of relays . This way, when the LEDs on the switch light up, i'll have a high level voltage on the relay and it will provide a path for the 5v to light up the 3 or 5mm LEDs . Is this a good apProach or can i use another one?
Thanks,
Diogo Ribeiro

 

[audioguru]

My barbeque burns expensive propane gas (LPG?) and my car runs on inexpensive and readily available gasoline.
My government doesn't tax gasoline to death (yet) like in many countries.

 

[MikeMl]

Diogo,

When you look inside the LPG controller, can you see a resistor (likely also a smd chip resistor) in-series with each smd LEDs?

If so, then the solution is very simple. Write back with the answer to my question, and I will suggest how to add external LEDs.

 

[diogoribeiro]

My barbeque burns expensive propane gas (LPG?) and my car runs on inexpensive and readily available gasoline.
My government doesn't tax gasoline to death (yet) like in many countries.

Where i live LPG costs nearly a half of the gasoline. If there wasnt this "small" inconvenient i would run the car on gasoline.

Diogo,

When you look inside the LPG controller, can you see a resistor (likely also a smd chip resistor) in-series with each smd LEDs?

If so, then the solution is very simple. Write back with the answer to my question, and I will suggest how to add external LEDs.

I looked at the LPG controller and i can confirm that exists a resistor for each led. I have 4 normal leds and one RGB led. I did a conductivity test and measured the resistors and i have 3 leds + the rgb led with 561 ohms resistors and one led with 331 ohms. I guess the last one uses a lower value because when the gas is on reserve the led blinks. Btw, i've attached a picture of the controller in case it might help.
View attachment 66904

 

[MikeMl]

Good data.

Look at the attachment for the basic idea, which is to parallel an external current limiting resistor/LED combination with the internal one. The order of resistor/LED or LED/resistor doesn't matter. Note that by connecting the wires across the internal resistor/LED, it is possible to connect an external resistor/LED as shown. I would use ultra-bright LEDs, and pick the external resistor so the external LED current is 2 to 5mA. I have found that ultra-bright LEDs are bright enough to use as indicators with just a mA or two. I'm sure that the internal circuitry can drive a few additional mA.

Another refinement is to minimize the number of wires going forward by studying the valve circuit to determine if the LEDS are being driven common-anode or common-cathode. Likely they are being driven by a chip which either sinks current (common anode) or sources current (common cathode). If you can figure out which, that will reduce the number of wires by almost half...

 

[diogoribeiro]

Hi!
Thanks for your solution! I did a conductivity test and yes, the leds are connected to the same ground !
To calculate the resistor to use it would be better to measure the voltage and calculate it based on the readings, right?
Thanks, Diogo Ribeiro

 

[MikeMl]

Yes. You could also just make the external resistor equal to or a bit higher than the corresponding internal one...

 

[diogoribeiro]

Yes. You could also just make the external resistor equal to or a bit higher than the corresponding internal one...

I'll try that solution asap and i'll give you some feedback.
Thank you very much for your solution.