Simple Led PWM driver

Thread Starter

LostInSpace

Joined Mar 16, 2010
24
I look for a simple PWM LED driver

function

2 bit control = 1 BIT On/off 1 BIT for 25%/50% PWM

only 1 White LED so possible for use BC 547 as driver transistor

it shut work on 5V

Frequnce 10k+ Hz

My LED is work on 3.4V

my plan is to have 4.3v over the LED and the rest 0.7v over the BC 547

Do some have a idea for a simple setup

I shut build 20+ off the for garden light.

:rolleyes:
 

Adjuster

Joined Dec 26, 2010
2,148
It seems likely that you are going to apply too much voltage to the LED this way, possibly resulting in too much current passing and burning things out.

Could you please post a schematic?
 

Thread Starter

LostInSpace

Joined Mar 16, 2010
24
It seems likely that you are going to apply too much voltage to the LED this way, possibly resulting in too much current passing and burning things out.

Could you please post a schematic?
I know do more current in the LED it why i want to use PWM 25% & 50 %

I just how to design it simple with on/off so it can be controled by microcontroller

some say why not make the microcontroller do the PWM it have hard work by control network with the other microcontroller in the network
 

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thatoneguy

Joined Feb 19, 2009
6,359
Why 25% and 50%?

I believe 50% and 100% would be easier to apply, and save a bit of money on LEDs.

As for the method, an 8 piin microcontroller can do this, but if you don't know any programming languages. There is also a $50 "Startup Cost" for programmer and ICs.

A discrete solution would be a 555 timer, on off being power to the circuit, and 50% or 100% putting the 555 at 50% duty cycle into the output or out.

Trying for more levels would take a few more ICs.
 

Wendy

Joined Mar 24, 2008
23,415
I've helped several folks design PWM circuits from the ground up with fair success.

I've written an article on LEDs in general to get people started.

LEDs, 555s, Flashers, and Light Chasers

Chapter 5 - The 555 and PWM

What you want to do sounds pretty easy. How many LEDs and what kind of current (and power supply voltage) are you talking?

What I think your asking for is one 555 and a transistor, but I'll wait for more info before moving on.
 

Thread Starter

LostInSpace

Joined Mar 16, 2010
24
I've helped several folks design PWM circuits from the ground up with fair success.

I've written an article on LEDs in general to get people started.

LEDs, 555s, Flashers, and Light Chasers

Chapter 5 - The 555 and PWM

What you want to do sounds pretty easy. How many LEDs and what kind of current (and power supply voltage) are you talking?

What I think your asking for is one 555 and a transistor, but I'll wait for more info before moving on.

2 LED in par the transistor shut be BC547

Work on 5 Volt Max 50% Current The there will be 4.3 Volt over the LED in 50% off the time with freq on 10k+ the LED will not burn

25% option is for Safe power

1 PIN turn on/off from MCU
1 PIN to change betwin 25% and 50% PWM
:)
 

Wendy

Joined Mar 24, 2008
23,415
Here are my assumptions:

2 white LEDs, Vf =3.4V

Power supply is 5V.

Control input 0 = 25%, 1 = 50%

LED current is 20ma. (50% means 10ma average, and 25% is 5ma average).

NPN transistor (why? Little early to dictate parts.)

10KHz modulation frequency, this really does not make sense as it doesn't matter, but OK.

my plan is to have 4.3v over the LED and the rest 0.7v over the BC 547
This makes no sense what so ever. I suspect how you think these things work don't work that way.

Why not 100% of the LED?

Did you read the link I gave you? Especially chapters 1 and 2, the basics. The above quote suggests you are extremely new at this (which is not a problem).

To quote Mark Twain, "It ain't the things people know that gets them into trouble, but the things they know that ain't so.".

Be right back, drawing...
 
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thatoneguy

Joined Feb 19, 2009
6,359
2 LED in par the transistor shut be BC547

Work on 5 Volt Max 50% Current The there will be 4.3 Volt over the LED in 50% off the time with freq on 10k+ the LED will not burn

25% option is for Safe power

1 PIN turn on/off from MCU
1 PIN to change betwin 25% and 50% PWM
:)
If you are using an MCU, it could do the entire project with one output pin and a transistor. :confused:
 

Wendy

Joined Mar 24, 2008
23,415
OK, first effort. I'm basically rejecting this one because of parts count, it can be done simpler.



The more I think about it the more I think 50%/100% is a better way to go. What do you think the advantage is in throwing away ½ the LEDs brightness? It can be adjusted down to any current via the resistor, and makes the design much simpler overall.

If you are using an MCU, it could do the entire project with one output pin and a transistor. :confused:
I agree.
 

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Thread Starter

LostInSpace

Joined Mar 16, 2010
24
I work on a MCU network base on PICAXE

all a run on 3 wire 1 base (master) and x slave

all slave main option is to listening for main and do what it wants and don't have time for this type off function

basic slave is remote I/O board on 200 meter netvork

your circuit give me some idea that i will try :)
 

Wendy

Joined Mar 24, 2008
23,415
I'll come up with a simple 555 circuit. It really will be easier to do 50%/100%, so I'll start with it. Are you trying to keep the modulation for the LED for some reason?
 

Thread Starter

LostInSpace

Joined Mar 16, 2010
24
Yes all system run on battery, power by solar cell, the master know from 1 of the slave how much power back in the system,

system will have 15+ MCU when it finish
 

Wendy

Joined Mar 24, 2008
23,415
This will do 100%/50%. You can adjust the max LED brightness using the LED resistors. The 50%/25% will be more complex, but not as bad as the first circuit I drew.

 

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Thread Starter

LostInSpace

Joined Mar 16, 2010
24
I change it a little bit

Key A Switch come from MCU PIN1 with Power (+5v)
Key B Switch Come from MCU PIN2 change 50%/100% PWM

and move R8 from Vcc to chip source ( it work fine)

add the R9 so Q1 close when Power remove from 555

I want to see if it possible to remove R4 & R5
 

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Wendy

Joined Mar 24, 2008
23,415
Actually it doesn't work fine. You are using a switch to turn the circuit off. In mine it was a true digital input, the 555 goes into standby. There is a substantial difference there, as I was designing for digital controls.

However, if it works...

Do you still want the 25%/50% circuit?
 

Thread Starter

LostInSpace

Joined Mar 16, 2010
24
The 2 On/off key only simulate input from a MCU


Yes 25% / 50% is next

in 25% at 10khz you can put more volt over the LED and it have 75% time to cool down

and in 50% it have 50% to cool down


I remember for old time where B&O remote control on the IR LED use 10% PWM and it have higher volt over the LED as normally remote control



if there is 4volt over the LED with 25% PWM there current will be higher than normally
 

Wendy

Joined Mar 24, 2008
23,415
I don't know where you got your information, but it is flat wrong. LEDs are not voltage controlled devices, ever. They are current controlled, with a constant voltage drop that does not change. Different color LEDs have different voltage drops. If you overdrive them you shorten the life of the LED, period. The flip side is they have such a long life that even halving their lifespan is no big deal.

There is no circumstance where LED dropping voltage changes, only the driving current.

You really need to read this...

LEDs, 555s, Flashers, and Light Chasers

PWM will not extend the life of a LED. It will allow you to vary the brightness of a LED linearly. That is all it is for.
 

Wendy

Joined Mar 24, 2008
23,415
OK last schematic. CR1 really screws with the math, so some fine tuning would be required. Making CR1 a germanium or Schottky diode would help though...



All of these are based from My Cookbook I have started.
 

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SgtWookie

Joined Jul 17, 2007
22,230
Wait a minute - each light will have two LEDs and a PICAxe processor in it?

Which PICAxe model are you going to be using in the lights, or am I missing something?
 
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