Making 12 V lights fade in using Raspberry Pi Pico PWM

Question

I have 4 small LED lights (raptor lights for a RAV4 grille) for my vehicle and I thought it would be cool if when I start my car, the outer lights would fade in, and then the inner lights would fade in, just for a cool effect when I first turn my car on. I've learned so much so far trying to make this idea happen, but I am at a roadblock and now I have reached out here to see if anyone else would know how I can achieve this.

I have the Raspberry Pi Pico code all set and I have demo code stepping the voltage up to 3.3 V and then back down to 0 V. Using a multimeter, the voltage is stepping how it should.

To power the 12 V source for the lights, I was told that maybe a MOSFET would be best and I hooked up a IRF520 drive module (Link to product) yesterday for the first time. But while the code is executing, the 12 V is not stepping from 0 V to 12 V and then 12 V to 0 V respectively.

I am guessing (and where I need some expertise on) a MOSFET is either on or off based on the signal voltage. Am I correct?

I have some other lights that will be controlled from a timer that I purchased from timers.shop. This timer kind of has this feature, but I want to be able to control the amount of time the fade starts and ends, which the timer is pre-set with a fade value for that.

If anyone could point me in a good direction that would be great. As I am out of ideas currently, as I am a novice to circuit building.

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I wanted to post an update on this, and I definitely feel like a newb now. I just received my IRL540N MOSFETs today in the mail and ended up hooking it up and testing the voltage. To my dismay, the voltage was not changing from 0 to 12v and from 12v back down to 0v using the Raspberry Pi Pico PWM pin. I was really confused why any of the 3 types of MOSFETs were not doing what I needed, so instead of using a voltage meter to check the output voltage, I went ahead and just hooked up the LEDs to the output. To my surprise, my LEDs were fading in and out just how I needed them to! I went ahead and tested some of the other MOSFETs I bought, and sure enough they were doing the same thing.

To sum this all up, it was definitely a learning experience and I should have just checked the lights before but thought checking the voltage would be enough. I am still unsure how the lights are fading in and out if the voltage is not going down to 0 from 12 and back up again. So if someone could explain that, that would be great! I'm here to learn!

Answer 1

To power the 12 V source for the lights, I was told that maybe a MOSFET would be best and I hooked up a IRF520 drive module (Link to product) yesterday for the first time. But while the code is executing, the 12 V is not stepping from 0 V to 12 V and then 12 V to 0 V respectively.

The product description says:-

• Voltage: 3.3V, 5V; Output load voltage :0-24V; Output load current: <5A (1A above need to add heat sink)
• Ports: Digital Level;Platform: Arduino, MCU, ARM, Raspberry Pi.

This is untrue. The IRF520 has a Gate threshold voltage (voltage at which it just starts to turn on) of between 2.0 V and 4.0 V, so at 3.3 V it may not turn on at all.

Many online sellers are promulgating this error (and probably getting a lot of bad customer feedback as a result) but a few do get it right - it only works with 5 V logic and higher (eg. 5 V Arduino). Here's an explanation from one vendor who bothered to test the product they sell:-

KEY FEATURES OF IRF520 N-CH MOSFET MODULE:

Switch loads up to 24V @ 5A 5V compatible (actually semi-compatible. See notes below)

The IRF520 is not fully 5V logic compatible but it will work fine for many applications if it is derated to the specs we show here...

The IRF520 MOSFET device is actually rated for up to 100V @ 9.7A. That rating assumes that it is being driven by a gate voltage of 10V to fully turn the device on with good heat sinking, etc.

Since this module is mainly designed to be used with an Arduino or similar MCU that can only drive 5V, the IRF520 will not be driven to full saturation and therefore will dissipate more heat. For that reason it needs to be derated from the theoretical maximum specs which is how we end up with our 24V @ 5A rating.

In our load testing, driving it with a 5V logic signal and pulling 3A continuous resulted in 56°C on the tab and 3.5A resulted in 71°C

MOSFET Gate threshold voltage is difficult to tightly control, so manufacturers specify it loosely so they can sell more devices. Some purchasers of this FET module may be lucky enough to get one with a low threshold voltage that works 'OK' for them on 3.3 V, while others (like you) get one with a higher threshold that doesn't work at all.

To work reliably with 3.3  logic levels you need a FET which is specified for operation with 3.3  Gate drive or lower (eg. 2.5 V).

Answer 2

Basic concept is that LED brightness depends on the current that flows from it. So, I will suggest you convert that PWM voltage variations to the current variations with the help of a resistor and then feed it to a BJT base instead of a MOSFET. BJT will be able to control the current better with respect to it's base current. example - you can think of as base current varying from 100-200nA which varies collector current 100-500uA.