Author Topic: 555 LED PWM PCB layout (Read 9558 times)

Grayfox · « **on:** July 30, 2013, 11:17:40 am »

I am trying to make a LED PWM circuit but i am having trouble getting this schematic designed on a single sided board I really want to avoid using 0-ohm links or wire bridges

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Tita · « **Reply #1 on:** July 30, 2013, 11:27:33 am »

Google?

I only look one minute on google and i found a pcb ! O0
http://xtronic.org/circuit/fake-car-alarm-circuit-555-led-flash/

Rerouter · « **Reply #2 on:** July 30, 2013, 11:36:49 am »

as i went to the effort i'll post it anyway,

Grayfox · « **Reply #3 on:** July 30, 2013, 02:16:38 pm »

Thanks for the help.

I take it with the caps and resistors i will be using will get these
Frequency:    128.267    KHz    Cycle Time:    7.796    uS
Duty Cycle:    50.22    %
Time High:    3.915    uS    Time Low:    3.881    uS

Grayfox · « **Reply #4 on:** August 01, 2013, 02:05:35 pm »

I had to remake the pcb design due to my luck screwing me over yet again
I added a SPDT relay since this will be a DRL controller and would like to know if there is any issues with it.

During the day DRLs will work at full brightness, but at night the DRLs will work at 50% brightness.

JoeO · « **Reply #5 on:** August 01, 2013, 07:04:38 pm »

No one knows since you haven't included a new schematic.

Dave · « **Reply #6 on:** August 01, 2013, 08:25:17 pm »

(14V)^2 / 5 ohm = 39W

You really think that resistor is going to survive that?

JoeO · « **Reply #7 on:** August 01, 2013, 09:37:22 pm »

I think you may be missing a diode across the coils.

EpicIntelGamer · « **Reply #8 on:** August 02, 2013, 01:14:43 am »

Not to hijack the thread but is there an adjustable PWM output 555 based circuit that doesn't require any weird value caps (pF and nF)?

Rerouter · « **Reply #9 on:** August 02, 2013, 02:16:09 am »

a 555 timer relies on a time constant to work, if you want to use bigger caps, you lower the resistance, if the resistance gets too low, you buffer the 555 discharge pin, etc until you get a working solution, pF and nF are not weird in fact in most cases they are conveniently tiny and in more reliable constructions,

amyk · « **Reply #10 on:** August 02, 2013, 02:44:48 am »

@Rerouter: Your name is most appropriate for this thread.

Grayfox · « **Reply #11 on:** August 02, 2013, 03:41:43 am »

Quote from: Dave on August 01, 2013, 08:25:17 pm

(14V)^2 / 5 ohm = 39W

You really think that resistor is going to survive that?

It is based on routers design.

The resistors are for the cycle speed.

Rerouter · « **Reply #12 on:** August 02, 2013, 07:24:06 am »

yes its understandable that they are for cycle speed, but why not swap to smaller capacitances so you can use higher value resistors... my design was a quickly tossed together thing, if the package size doesn't match, just upsize.

Grayfox · « **Reply #13 on:** August 02, 2013, 08:52:05 am »

What is the best frequency to use for LED?

I heard that you can get squeaking below 20Khz

Rerouter · « **Reply #14 on:** August 02, 2013, 10:45:26 am »

your driving an relay on your circuit... which has a max speed of probably 5Hz,

as for an led, anything over 200Hz is not visible, and unless you have a multi layer ceramic or metal core inductor on the output your circuit should not make noise (you need a speaker either magnetic or electrostatic to make noise) the higher frequencies are just more ideal for low duty cycles, as you notice less and brightness is more linear at the bottom end,

Grayfox · « **Reply #15 on:** August 02, 2013, 12:08:43 pm »

The relay is just to switch from full brightness during the day to the nightmode where they will be dimmed.

Going for the audi styled look where there DRLS become sidelights.

With a 5nF cap and using a 100ohm and 120Kohm 1 watt resistors I found laying about.

I should get this, if the calc i'm using is correct.
or is R1 still too low?
Frequency:    1.202    KHz    Cycle Time:    831.947    us
Duty Cycle:    50.02    %
Time High:    416.146    us    Time Low:    415.800    us

Rerouter · « **Reply #16 on:** August 02, 2013, 02:06:45 pm »

comes to ~2W, so still a bit hot...

Grayfox · « **Reply #17 on:** August 02, 2013, 02:21:52 pm »

Could i use a picofarad cap?

with a 1nF cap and a 400ohm R1 and a 480Kohm R2 i still get the same values which is 1/2w heat dissipation could even use 800ohm R1 and 1Mohm if i can use a 500pF cap.

TerminalJack505 · « **Reply #18 on:** August 02, 2013, 06:10:25 pm »

You might want to change your schematic. There are a couple of better ways to get a 50% duty cycle with a 555.

One is shown in this datasheet on page 12.

Another way is to use a couple of diodes to change the charge/discharge paths. This circuit shows the basic concept.


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Author Topic: 555 LED PWM PCB layout (Read 9558 times)

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