555 and 4017 led control

[tim123]

hi guys

i want to make a small led brightness controller, the leds are already in strips on a seperate board, runs of 12v at the moment.

a friend recommended that i look into pwm brightness control. i have done a bit of reading, and looked into control with a pic as well. i will have to control 2 sets of leds seperately, so the blue can be brighter at night and the white brighter during the day.

i had setup something like this..

555 timer -> 4017 -> (pin1: 400ohm) -> 555 timer pin 6
-> (pin2: 300ohm) ->
-> (pin3: 200ohm) ->

i had hoped that the different resistors on the 4017 outputs would change the timing amount on the 2nd 555.. in crocodile clips i could not get it to work.

any ideas guys?

 

[tim123]

anyone?

also is pic goign to be easier.

tim.

 

[Roff]

anyone?

also is pic goign to be easier.

tim.

Your text-based schematic doesn't do it for me - and I suspect that's why you haven't gotten any responses. If you have a Crocodile Clips schematic, you should be able to post it.

 

[tim123]

hi,

i have been playing around with c.c again, but can't figure out how to get a resistor coming from a pin on the 4017 to vary the timing on the 555 timer.

basically i want to use pwm brightness control on the LED, by having the 2nd 555 timer oscilate fast enough so the led is brighter/dimmer but not flashing. I was trying to do this by somehow affecting the timer circuit through the 4017 outputs.

I plan to have 8 steps on the 4017, each step 3 hours.. so the LED will vary brightness every 3 hours, and continue this cycle every day.

I hope its not too bad a schematic.. but you will hopefully understand where im going wrong, or if im just doing something thats completely impossible.

thanks.

 

[audioguru]

An ordinary CD4017 Cmos stepper with a 5V supply has outputs that are about 1.2k ohm from either the positve supply (turned on) or from ground (turned off).
A 74HC4017 has outputs that might have a resistance that is about 100 ohms from either the positive supply or from ground.

The outputs of your CD4017 can supply base current to a transistor through a resistor and the transistor will have a very low resistance to ground when it is turned on.

 

[k7elp60]

I have had excellent results using CMOS analogue switches to change both
timing resistors and capacitors for 555 timers.

 

[tim123]

does the idea sound right though?

just trying to get out of buying all the pic equipment, and having to learn the programming.

thanks guys

 

[audioguru]

I have had excellent results using CMOS analogue switches to change both
timing resistors and capacitors for 555 timers.

Of course. An analog switch has a low resistance when turned on. A Cmos logic output isn't.

 

[audioguru]

does the idea sound right though?

I would use the high output resistance of a CD4017 to change the high resistance reference voltage of a triangle-wave-comparator PWM circuit, not the timing parts of a 555.

 

[Roff]

You won't be able to get a 3 hour period from an RC oscillator (555 or other), because of capacitor leakage. You will need a frequency divider or a microcontroller.

 

[tim123]

OK, i have started a circuit using the pic micro controller, just need to know whether its worth counting up to 3 hours within the pic, or use an external timing source (555 and 4017(s)) to count and provide an input to the pic..

i will let you all know how i get on.. any good tutorials you have seen regarding PWM from a pic?

thanks.

 

[William At MyBlueRoom]

Simple Timer & LED with ICD2 connector for debugging

Here's my quick and simple schematic for a timer with a few options.

I've selected the 12F675 as it's cheap and has an A/D converter.

The best tool for learning and programming at the same time IMHO is a ICD2 or clone

 

[tim123]

OK, so i maybe if i do the counting in the pic, using lots of loops etc. I understand that the 16f627a has one PWM output, i plan to hook up a bc548 or simliar off the output, to drive a mosfet, which all of the leds will drive from.

could someone write me a basic code example, of how the PWM actually works. Just to turn the led on at a certain time at a specific brightness.

btw is there a basic programmer for the pics, i have found many basic compilers, but nothing which I can write my basic code into. I am not good at all with asm as many of you would have noticed.

cheers all.

 

[William At MyBlueRoom]

No A/D on PIC16F628A

Try the 16F88 instead it has PWM and A/D

 

[tim123]

ok the 16f88 sounds good.

is there a basic simluator that will show me the pwm output of the port?
the reason i ask is that i am told that its more difficult to do "if.. then.. else" statements within asm.

for those of you wondering i have attached the xls file in which i was hoping to base my pwm calculations from, 255 being full on, to 0 completely off.

I've read a fair bit of the asm tutorial now, can get an led to flash on an off.. still got a fair way to go by the sounds of it. if anyone has a spare few moments and wants to write some code to count the 8 sections of 24 hours (3 hour sections) and operate the pwm on the port (single color, i'll use a pic each for white and blue) i'd be very happy to get some code from you and try to learn it.

 

[d_6789]

I need hepl 30 led of 4017