I currently drive a 2013 Avalon Limited and have installed 4 LED strips to act as additional mood lighting. Two are in the footwell and 2 are in between trim pieces where the later models have stock LED mood lighting. The whole circuit is controlled by a PWM dimmer button in the map light area. It toggles between High, Low, and Off for the stock mood lighting.

My issue is that the strips I have installed (which already have rubbery light diffusers on them) are far too bright even on the Low setting. Is there an electrical solution to this problem or am I stuck with trying to slide some ND filter gel sheets in front of the LED strips in order to reduce their brightness?

I doubt I can add another PWM controller dial after the stock one, and potentiometers don't seem like a good fit for LED strips from what I've read. It is difficult to add anything upstream of the factory installed PWM controller button due to where it is. My LED strips are simply spliced into one of the wires leading to the stock cup holder mood light, which is very easy to access.

Any suggestions?

 

If we ignore standards, car regulations and warranty, what are your skills with a multicet, solderer and electronic circuits?

 

I'm pretty comfortable soldering and working with automotive circuits. I do all my own work with car audio and multiple types of interior/exterior lighting. I just don't have much experience with circuit boards and more in-depth circuitry (resistors, capacitors, diodes, relays, etc.) I can install simple switches and diodes individually but I am lost when it comes to making a circuit that includes multiple components working together.

 

Do we know how the existing dimmer works?
What voltage is supplied to your strips when on HIGH?
What is the total current draw by the strips?
Do you want to ADD a new dimmer, or just reduce the brightness from the existing dimmer switch?

With this information you may be able to limit supply current to the LEDs with a resistor in series with the power leads of the LEDs.

You may need to do a bit of testing, I have LED strips that each draws 16mA, a six inch strip draws 150mA. Let’s drop this to 100mA. At 12V, ohms law states I need 120Ω. You can calculate the resistor rating from this info.

 

You could use a zener diode in series with the added LED strips to drop the voltage and decrease their brightness to an acceptable level. What is the current draw of the LED strips?

 

Here are the provided specs on a single strip.

Op. Voltage: 9~14.2 VDC
Current Draw @ Op. Voltage: 300mA
Power Consumption: 3.6W

There is no specified minimum current.

I will measure the voltage coming out of the stock PWM switch (on High & Low) when I get home shortly.

 

You lack the basics. You need to change the PWM duty cycle or to reduce the current. I am not certain that these lights are PWM controlled.

Can you put a resistor (5Ohm) right before the strip?

 

Here are the readings from the measurements I took.
High: 13.25V
Low: 6V
Off: 2.2V

I could always try a resistor, but I'd like to have the option of having the strips at full brightness. I just had an idea. What if I simply left the stock button toggled to full brightness @ 13.25V and added a 12V relay for only my LED strips? The relay would receive the PWM signal and see 12V as a trigger, but would provide a non-pulsed 12V to the LED strips. I could then buy a simple PWM dial and add that in between the relay and the strips. Then I can control the brightness of my strips with the aftermarket PWM dial, and turn everything off with the stock button (because the relay won't trigger with the 2.2V OFF toggled setting). Does this sound alright?

 

Here are the provided specs on a single strip.

Op. Voltage: 9~14.2 VDC
Current Draw @ Op. Voltage: 300mA
Power Consumption: 3.6W

There is no specified minimum current.

I will measure the voltage coming out of the stock PWM switch (on High & Low) when I get home shortly.

How many LEDs are in each strip?
LED strip part number?

 

How many LEDs are in each strip?
LED strip part number?

https://www.superbrightleds.com/mor...e-lights-super-flexible-led-rope-lights/2959/

The cool white option, 12 inches.

 

Here are the readings from the measurements I took.
High: 13.25V
Low: 6V
Off: 2.2V

I could always try a resistor, but I'd like to have the option of having the strips at full brightness. I just had an idea. What if I simply left the stock button toggled to full brightness @ 13.25V and added a 12V relay for only my LED strips? The relay would receive the PWM signal and see 12V as a trigger, but would provide a non-pulsed 12V to the LED strips. I could then buy a simple PWM dial and add that in between the relay and the strips. Then I can control the brightness of my strips with the aftermarket PWM dial, and turn everything off with the stock button (because the relay won't trigger with the 2.2V OFF toggled setting). Does this sound alright?

Hmmm...
Readings look a little strange for a PWM. A PWM controls current by varying the duty cycle of the output pulses driving the LEDs. A shorter pulse width decreases the time the LED is on, lowering the visible intensity of the LED. But the voltage should stay relatively constant. Not saying it’s wrong, just a little odd...

 

Do you have the option of testing one strip, by connecting it to a battery(even your car battery will do, but the connection will be directly to the battery)?

 

Hmmm...
Readings look a little strange for a PWM. A PWM controls current by varying the duty cycle of the output pulses driving the LEDs. A shorter pulse width decreases the time the LED is on, lowering the visible intensity of the LED. But the voltage should stay relatively constant. Not saying it’s wrong, just a little odd...

I will admit that the Multimeter I'm using isn't terribly good, but testing other small batteries it seems accurate enough. I tested the car circuit 3 times and got the same results.
I do have 3 reasons to suspect it's PWM. 1, it's a 2013 Toyota and automakers seem to be using PWM to control plenty of things from interior lighting to engine sensors to running lamps. 2, if the stock button was really just a voltage regulator then the LED's shouldn't light at all at 6V as far as I know. 6V is below the operating voltage for the strips. But with PWM, the meter would be receiving 12V but in longer increments, and would probably just average the voltage to get 6V. And 3, waving my hand in front of the strip shows visible trails from my fingers, leading me to believe the light is being pulsed.

 

Do you have the option of testing one strip, by connecting it to a battery(even your car battery will do, but the connection will be directly to the battery)?

I could try this, what am I looking for?

 

You just need to connect the strip directly to the battery, disconnect 1 strip without chips, electronics and connect the 2 power supply wires directly to the car battery(be careful about the + and -). If you have a picture of the strip it would help.

 

A meter will display the average voltage which is simply the supply voltage times the duty cycle, so the voltages measured are as would be expected.

"Ordinary" white LED strips have three LEDs in series with a series resistor, normally with the resistor value chosen for nominal operating current at 12 volts. This puts about 3 V across the resistor and makes the current quite sensitive to supply voltage. At 9 V the LEDs would be nearly fully off and at 15 V the current would be about twice as high as normal (the forward voltage of the LEDs at the higher current would be a little higher so not all of the "extra" 3 V would appear across the resistor). This configuration could be quite easily confirmed by making measurements with a variable DC power supply, but there is no way to do it with just a meter or a PWM supply.

 

So it seems the circuit is definitely PWM, else the LED wouldn't light at all with that 6V. Any input on the relay idea? It seems like such an easy solution.

 

A meter will display the average voltage which is simply the supply voltage times the duty cycle, so the voltages measured are as would be expected.

"Ordinary" white LED strips have three LEDs in series with a series resistor, normally with the resistor value chosen for nominal operating current at 12 volts. This puts about 3 V across the resistor and makes the current quite sensitive to supply voltage. At 9 V the LEDs would be nearly fully off and at 15 V the current would be about twice as high as normal (the forward voltage of the LEDs at the higher current would be a little higher so not all of the "extra" 3 V would appear across the resistor). This configuration could be quite easily confirmed by making measurements with a variable DC power supply, but there is no way to do it with just a meter or a PWM supply.

The LEDs in these strips are connected differently than you've described. The strips are made up of "segments" and each segment has 2 groups of LEDs with each group made up of 3 LED's connected in parallel and one series resistor. The LED groups are then connected together in parallel to make up one 6 LED "segment". There are 12 segments connected together in parallel to form one 12" "rope". This means a total of 72 LEDs make up the "rope". When powered, the parallel connections provide for only a faulty section of "rope" to go dark. The spec says each rope draws 300ma@12vdc for a total of 3.2W (I didn't do the math, just quoting the spec). Since the resistors are "built in" the TS can't measure the voltage drop across the internal resistors without tearing it apart. Its a good idea to measure the LED "rope" directly connected to 12V just to confirm the 300mA current draw. See schematic below. R1 is for experimentation only but can also be a rheostat.

BTW, I talked to the MFG about the rope design.

eT

 

So it seems the circuit is definitely PWM, else the LED wouldn't light at all with that 6V. Any input on the relay idea? It seems like such an easy solution.

Each LED is rated approx. 2-3v@ 20mA, so it will still light. The LED's require voltage but are more dependent on current to be visible.
I think the only way to really verify output is PWM is with a scope.
How do you know the "PWM" output isn't being overloaded by the "rope"? Be sure to check this because if an overload occurs a fire can result.

 

The strip is exactly as I expected, but I did omit mention of the fact there are multiple sections in parallel. Functionally, it makes no difference when the strip is viewed as a two-terminal circuit. It could be three big LEDs and one big resistor or a whole bunch of paralleled segments.

6 volts across 3 white LEDs simply isn't going to produce enough current hence light to be seen in anything other than pretty much total darkness. You might get a few photons per fortnight. If 6 volts produces dimmed but significant light, the controller is PWM.

Simple ON-OFF PWM control is the only method that is broadly applicable and will perform in a predictable fashion without knowing the characteristics of the connected LEDs. It will work with white LEDs in triples with a resistor per triple or red LEDs with one resistor per LED or anything in between - as long as they are "dumb" LEDs. It will work for a current requirement of 10 A (if it could handle that) or 10 mA. It can work open-loop, though it could be made to limit the maximum average output to 12 volts, which would have some advantage for automotive use. Where it won't work is downstream of a switchmode constant current driver - the PWM would work but the CC driver would lose its mind.

cgo5023: If your meter measures zero volts if you set it to AC and connect it to a DC source (any convenient battery), allowing a few seconds for stabilization, you can add more confirmation to PWM control. With the meter set to AC, you will get a significant voltage reading with the controller set to LOW. If it actually put out 6 volts of steady DC, the reading would be zero (perhaps a little above zero if there is some electrical "noise"). If the meter doesn't measure zero on AC ranges with a DC source, a DC blocking capacitor could be used in series with the meter, if you have some caps available.