Flickering LED on PWM

Other Hardware LEDs and Multiplexing
1

I am using an 2N7000 MOSFET to drive a 12v 1W LED, I am using JLed library to fade the LED on, hold on, fade off then delay.

The problem I am getting is that on the fade down as it gets close to completely off there is a little flicker.

This is all new to me, so could be that I have to add a resistor somewhere, or the MOSFET could not be up to the job (eventually the LED will have to be held in the on position for 5mins).

Could anyone point me in the right direction ?

#include <jled.h>


auto led = JLed(6).Breathe(1000, 5000, 1000).DelayAfter(3000).Forever();

void setup() {
}

void loop() {
  led.Update();
}

20230501_183501
20230501_1835011920×1440 288 KB

2

This is not an appropriate MOSFET.

Google logic level MOSFET.

  • Confirm the blue wire is going the Arduino GND.

  • Does the transistor saturate ?

  • Add a gate 10k pull-down

  • Just in case, make pin 6 OUTPUT.

3

I'm not sure why it's flickering but.

Yes you need a series resistor or some kind of current limiting/control. With "high power" LEDs an active constant current (or controlled current) source is normally used, usually not a resistor but a resistor can work.

LEDs are "current controlled". You supply the correct current and the voltage "falls into place", basically the opposite of how everything else works...

And in any case you need "extra" voltage for the current source to work. With a resistor, 18V would be OK, with 6V dropped across the resistor and 12V across the LED.

With 6V across the resistor (and full power/brightness) the resistor will be dissipating 1/2W so you need a "power resistor" and it's good to use an over-rated resistor rather than pushing on to its limits.

4

Thank you for the quick reply.

Yes the blue wire goes to arduino ground.
Not sure how to check transistor saturation?
I added a pull down resistor, still got the same effect.

Would this MOSFET be suitable?

5

Write a test sketch that makes the Arduino output pin to the MOSFET go to HIGH, this turns on the MOSFET.

With the MOSFET turned ON, what is the voltage V(DS) .

See this PDF:

The MOSFET looks okay.

6

The voltage is 12.27V when measured without the LED.
When the LED is fading the voltage moves between 7.5V and 10.4V.

7

You need to write a test sketch that turns the MOSFET ON and keeps it on (NOT FADING).
The Arduino output pin will sit at 5v.

Measure the V(DS) of the MOSFET when the MOSFET is ON.

8

The gate of a transistor can have very low resistance. You need ~ a 1KOhm resistor in between the output pin of the board and the gate. You also need ~ a 10k from the gate to ground so that any charge built up in the gate can dissipate properly.

9

Which is completely normal.
Our eye's response to brightness is not linear, its logarithmic.
PWM steps near 255 PWM are invisible, PWM steps near 0 PWM are not.
12-bit PWM and a logarithmic PWM scale could minimise this problem,
or don't dim any lower than a PWM value of 10 or 20.
Leo..

10

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