Electronics > Beginners

Battery powered LED PWM dimmer circuit

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sokoloff:
Do these LED modules have their own driver circuitry already? If you can hook them up to a widely varying DC voltage and get a constant amount of light out of them, it seems likely that they do.

What you might need/want to do is open the module, figure out what driver it's using and see if there's a PWM pin on the driver IC already and send a square wave (with the right duty cycle) for the half-bright setting and send an always-on for the full bright setting. If there is a driver IC module and some amount of local capacitance on the module, you might not be able to effectively dim it by just feeding the power input with a PWM signal.

tooki:
As someone who is sensitive to flicker and finds many PWM-dimmed tail lights to be extremely distracting when driving, I implore you to not use anything with a low PWM frequency (like under 10 KHz).

IMHO, the easiest way (which I wish more auto makers would use) would simply be to have the LEDs electrically divided into two banks (e.g. 1/3 and 2/3) that you can switch on and off independently, giving you a few discrete total brightness levels while requiring no active circuitry whatsoever. You would just distribute the LEDs of the two banks evenly within your LED array.

doublec4:
Just to follow up on this thread. I took apart the LED module and it appears to have 3 main components responsible for operation of the LEDs.

1. There is a 5V regulator transistor https://www.diodes.com/assets/Datasheets/ZXTR2005Z.pdf
2. The regular transistor (because of the low current) is likely powering the PIC chip that I identified as: http://ww1.microchip.com/downloads/en/DeviceDoc/41211D_.pdf
    This chip is responsible for the pre-programmed flash patterns (single brightness, none of which are dim) that are selected by pulsing 12V momentarily to a third wire coming out of the module.
3. There are two MOSFETS driving the LEDs identified as https://www.components-mart.com/datasheets/82/DMT6008LFG-7.pdf

So when I hook up the module to the PWM circuit, nothing happens as I'm probably just turning the PIC chip on and off really fast
But when I connect the LED module to my power supply and vary the voltage, the unit starts to dim noticeably around 9V and by 8V it appears ~50% brightness.

Using the multimeter in series to measure current draw, the when the module first turns on there is a spike up to 400mA then it quickly settles around 190mA draw at 12V
Dropping the voltage, the current decreases with voltage drop. At 9V it reduces to about 100mA, then at 8V it is only ~25mA

So in this case PWM is not working and varying voltage does, but I imagine I am stressing the LEDs this way?

doublec4:

--- Quote from: Zero999 on May 23, 2019, 10:20:01 am ---A simple 555 timer circuit will do the job. You could use a potentiometer with a switch in series with the whole circuit to make sure it turns fully off, drawing no power.


--- End quote ---

Also, I tried this circuit for fun with some regular 5mm LEDs that I had on hand... I could only get them to very faintly light and then ever so slightly dim. It seemed as if the adjustment range of the pot was extremely narrow.

Audioguru:
Which circuit is correct?
Do these circuits both work?

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