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# Circuit Reverse Engineering - Different LED Resistor Values

#### dvm

##### New Member
I've got a simple circuit I am looking to reverse engineer to suit a slightly modified use case for my car steering wheel controls. I am only working on the 4 LED (assumed) lights at present as this is basically a separate circuit on the board to the main functionality.

The blue markup in the images is ground, the red is 12v from the car electrical system.

The measured resistor values are as below:
202 = 1.99k
272 = 2.705k
242 = 2.396k
182 = 1.806k

Question:
What I do not understand is why the resistor values differ for each LED ? I will be removing these LED's to use on my own custom made board.

It's because the voltages and current before each LED are different.

It's because the voltages and current before each LED are different.
Thanks for the fast reply !! So I'm struggling to understand why this would be in that case ... I assume if I place the same values in front of each of them in the new design I may get differing brightness ?

You can check the forward voltages with a DMM, but I see no reson for them to use different LED's in one design unless the usage changes.

They may have desired couple LEDs to be dimmer, either because of the function or location.

I've just put a 13.8V supply onto the board and all of the LED's (by eye) look to be of the same brightness. Measured the voltages each side of the resistor connected to each LED and noted the below ... hopefully this is helpful.

Determine the specifications of each LED, such as forward voltage (Vf) and forward current (If). This information is typically available from the datasheet or can be measured using a multimeter.

Calculate Resistor Value for Each LED, To calculate the resistor value for each LED, you can use Ohm's law (V = I * R). The formula for calculating the resistor value is:
Resistor Value (R) = (Supply Voltage - LED Forward Voltage) / LED Forward Current
The supply voltage is the voltage across the LED circuit (usually the voltage of the power source, like a battery or power supply).

The difference between the resistors is small. The LED with the most current will be taking about 1.5 times as much current as the one that takes the least current. All the LEDs illuminate at the same time. I think that they resistor values have been adjusted to make the illumination level as even as possible.

I'd guess compensation for slightly different optics, window thickness or angle etc?
As others say, It's probably not enough variation to notice in use, without careful examination.

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