using PWM to dim LED NEON STRIP

[MrDEB]

couldn't locate this thread but here is the issue
I breadboarded this circuit and it seems to work driving just an LED but I am driving a 12v led neon strip.
The circuit flashes the strip slowly to fast as per pot adjustment.
The 555 circuit works as planned with one LED (it is supposed to change the duty cycle
Thinking the two different supply voltages has something but what to do? Any suggestions?
NOTE THE 7555 IS POWERED BY THE pic
the MOSFET is logic level

 

[Nigel Goodwin]

The 'clock=' statement is in MHz.

The intosc.bas module should take care of setting the CONFIG and OSCCON registers to get 32MHz, although I doubt it's actually been tested with the ancient 18F2221.


Good question...

What is his source impedance feeding the inputs, and what (if anything) has he set the switching delay to?. The symptoms sound 100% the normal sample and hold charge/discharge issue.

 

[tumbleweed]

The schematic shows the pots as 10K, so that would be the max seen on the wiper.

The ADC.Read() function has a delay built-in after the channel is selected,
and the delay is set by the call to 'ADC.SetAcqTime(100)' to provide a 100us delay

 

[For The Popcorn]

The schematic shows the pots as 10K, so that would be the max seen on the wiper.

MrDEB's schematics should be regarded as notional at best, as many of us have found.

 

[Pommie]

A small capacitor (~10uF) from the wiper to ground should fix it. A 10uF cap charging a 120pF SAH cap should be almost instantaneous.

MrDEB, get the potentiometers working first as they will control the brightness of the LEDs. No amount of external circuitry (555s etc.) will fix this.

Mike.

 

[MrDEB]

With all the issues with dimming using the pic and the issue with the 555 timer being controlled using pin 4 to the pic (been there, done that and a need for more MOSFET outputs I redesigned the schematic. Maybe add two or three outputs that are just connected to the 12V ground so they are on all the time.
Using an NPN transistor on the 555 outputs, the Neon strips with dimming are enabled via the pic.
Waiting for parts to test the 555/transistor combo.

 

[MrDEB]

page 2 of schematic

 

[tumbleweed]

A small capacitor (~10uF) from the wiper to ground should fix it.

In situations like this I've had good results using a small value C at the ADC input, something like 0.1uF has worked too.

 

[Pommie]

I just figured a 10uF would charge almost instantaneously from the pot however the SAH cap is 120pf so 0.1uF is 800 times bigger so should be more than adequate. So I agree with your suggestion.

Mike.

 

[MrDEB]

will try the .1 cap