Need help with LED dimmer circuit

christineee · 2013-06-28 02:11

I am trying to create a circuit that is able to adjust the brightness of the LED accordingly to different light level.
100% brightness: dark surrounding
70% brightness: dim surrounding
0%(off) brightness: lighted surrounding
So far i am only able to come out with a code that is able to turn on in dark surrounding and turn off in lighted surrounding.
I found a set of coding(see below) and i need someone to explain what the meaning(highlighted in red)and how it helps the circuit to operate.

' BS2 PWM - Demonstrate BS2 "PWM" Function Operation
'
' This Program dims then brightens an LED.
'
' myke predko
'
' 05.09.11
'{$STAMP BS2}
'{$PBASIC 2.5}
' I/O Pins
LED PIN 14
' Variables
i VAR Word
' Mainline
DO
FOR i = 0 TO 254 ' Dim LED with PWM from
PWM LED, i, 5 ' 100% PWM to 0.4%
NEXT
DO WHILE (i <> 1) ' Brighten LED with PWM
PWM LED, i, 4
i = i - 1
LOOP
LOOP

Also, how can i modify it to suit my project?
I have attach a copy of my schematic and code that i did.

Thanks in advance.

Duane Degn · 2013-06-29 12:04

christineee,

Do you still need help with this?

You might get better replies if you ask these type of questions in the Basic Stamp forum.

I haven't use a Basic Stamp for several years but I'll try to help a bit.

christineee wrote: »
I found a set of coding(see below) and i need someone to explain what the meaning(highlighted in red)and how it helps the circuit to operate.
        [COLOR=#FF0000]DO WHILE (i <> 1)  [/COLOR]     '  Brighten LED with PWM

The "<>" symbol means not equal. The DO/LOOP will continue while "i" doesn't equal one.

There are several different ways of adjust the PWM value based on the brightness. You could use an equation which computes a PWM based on the brightness value. You could use a series of "IF", "ELSEIF" statements to set the PWM based on a range of brightness levels. You could use the "SELECT" and "CASE" statements instead of using multiple "IF", "ELSEIF" statements.

It's easier to read code posted to the forum if you use code tags. Here's a link to Phil's code tag tutorial.

attachment.php?attachmentid=78421&d=1297987572

christineee · 2013-07-01 23:04

Thanks for your help! will post it in the basic stamp forum!

Chris Savage · 2013-07-02 14:07

I removed the duplicate post in the BASIC Stamp Forum and will move this thread there as duplicate threads are not allowed on the forums.

christineee · 2013-07-04 23:10

I decided to add in another LED for my circuit.
So when it is in :
bright surrounding, both pin 14 and 13 will be low.
dim surrounding, pin 13 will be high and pin 14 will be low.
dark surrounding, both pin 14 and 13 will be high.

My new code is as follow:

' What's a Microcontroller - testing circuit 7.bs2
' Change digitized analog phototransistor measurement to a binary result.
' Turn an LED on and off.
' Switching LEDs to operate on different light level.


' {$STAMP BS2}
' {$PBASIC 2.5}


valMax         CON        4000                  'variable declaration
valMin         CON        168


time           VAR        Word


PAUSE 1000


DO


  GOSUB Rc_Time
  GOSUB Display


Rc_Time:                                        'to charge the capacitor.


  HIGH 2
  PAUSE 100
  RCTIME 2, 1, time


Display:


  time = time MAX valMax MIN valMin


  DEBUG HOME, "time = ", DEC5 time
  IF time > (valMax - valMin) / 4 * 3 THEN
    DEBUG CR, "Turn LED 100% on"                  'when time = more then 3/4 of the "turning led on" value.
    HIGH 14
    LOW 13
  ELSEIF time > (valMax - valMin) / 4 * 2 THEN
    DEBUG CR, "Turn LED 70% onn"                  'when time = more than 2/4 of the "turning led on" value.
    PWM 13, 190, 5000
    LOW 14
  ELSEIF time < (valMax - valMin) / 5 THEN
    DEBUG CR, "Turn LED 0% offf"                  'when time = less than 1/4 of the "turning led on" value.
    LOW 13
    LOW 14
  ENDIF


LOOP

However, now the problem i encounter is that the pin 13 when activated, it keeps flashing/flicker. Which part of my coding did i make a mistake? For the pin 14, there's no such problem.
Also, by adding that PWM command in my coding, does that make my led in pin 13 to light up to 75% brightness?

Thanks all in advance.

NWCCTV · 2013-07-05 00:02

I would suggest trying to get this to work the first way you had it before adding anything else. Maybe try changing the PWM command on pin 13 to HIGH and see if it still flickers.

christineee · 2013-07-05 00:55

NWCCTV wrote: »

I would suggest trying to get this to work the first way you had it before adding anything else. Maybe try changing the PWM command on pin 13 to HIGH and see if it still flickers.

Below is the code whereby i change the PWM command to pin 13 HIGH.
Led in pin 13 does not flicker anymore. So i guess its the PWM command that has error?
How do i modify it then? Because so far i seen PWM command are to be written in this format: PWM led pin, duty cycle, duration
However, the led in pin 13 does not dim as i want it too.

' What's a Microcontroller - testing circuit 7.bs2
' Change digitized analog phototransistor measurement to a binary result.
' Turn an LED on and off.
' Switching LEDs to operate on different light level.


' {$STAMP BS2}
' {$PBASIC 2.5}


valMax         CON        4000                  'variable declaration
valMin         CON        168


time           VAR        Word


PAUSE 1000


DO


  GOSUB Rc_Time
  GOSUB Display


Rc_Time:                                        'to charge the capacitor.


  HIGH 2
  PAUSE 100
  RCTIME 2, 1, time


Display:


  time = time MAX valMax MIN valMin


  DEBUG HOME, "time = ", DEC5 time
  IF time > (valMax - valMin) / 4 * 3 THEN
    DEBUG CR, "Turn LED 100% on"                  'when time = more then 3/4 of the "turning led on" value.
    HIGH 14
    LOW 13
  ELSEIF time > (valMax - valMin) / 4 * 2 THEN
    DEBUG CR, "Turn LED 70% onn"                  'when time = more than 2/4 of the "turning led on" value.
    HIGH 13
    LOW 14
  ELSEIF time < (valMax - valMin) / 4 THEN
    DEBUG CR, "Turn LED 0% offf"                  'when time = less than 1/4 of the "turning led on" value.
    LOW 13
    LOW 14
  ENDIF


LOOP

PJAllen · 2013-07-05 06:03

PWM goes only for a given period.
The PWM pin is inactive during the time that it takes to RCTIME and getting the IF..THEN..ELSE sorted out.
That's why there is flickering.

Additionally, I think the structure of your programme needs help.
I think that it ought to be:

DO
  GOSUB Rc_Time
  GOSUB Display
LOOP


Rc_Time:         
  HIGH 2
  PAUSE 100
  RCTIME 2, 1, time
  RETURN

Display:
  time = time MAX valMax MIN valMin
  DEBUG HOME, "time = ", DEC5 time
  IF time > (valMax - valMin) / 4 * 3 THEN
    DEBUG CR, "Turn LED 100% on"               
    HIGH 14
    LOW 13
  ELSEIF time > (valMax - valMin) / 4 * 2 THEN
    DEBUG CR, "Turn LED 70% on"              
    HIGH 13
    LOW 14
  ELSEIF time < (valMax - valMin) / 4 THEN
    DEBUG CR, "Turn LED 0% on"                
    LOW 13
    LOW 14
  ENDIF
  RETURN

christineee · 2013-07-07 20:37

PJ Allen wrote: »

PWM goes only for a given period.
The PWM pin is inactive during the time that it takes to RCTIME and getting the IF..THEN..ELSE sorted out.
That's why there is flickering.

I dont get you at this part. Do you mind explaining this to me in details.
I am not very good in understanding pbasic programming as this is my first time using it.
Thanks.

Mike Green · 2013-07-07 21:11

Read the description again of the PWM statement in the Basic Stamp Syntax and Reference Manual. The 3rd parameter to the PWM statement is the amount of time the Stamp is to produce the PWM signal. When that time is over, the Stamp stops producing the signal and it continues on to the next statement (the one after the PWM statement). While the Stamp is executing other statements (like the RCTIME and IF..THEN..ELSE), it's not generating the PWM signal any more and the LED will remain off (unless your program turns it on or off with a HIGH or LOW statement (or vice versa depending on how the LED is wired).

Remember that the DEBUG statement takes some time to execute and that will slow down your program. Each character sent by the DEBUG statement takes about 1ms to send. "Turn LED 0% on" takes 14 characters, thus a little over 14ms to execute. The other DEBUG statement (after the Display: label) takes another 13-14ms to execute. The Display: subroutine probably takes 35ms or so to execute

christineee · 2013-07-08 01:12

Mike Green wrote: »

Read the description again of the PWM statement in the Basic Stamp Syntax and Reference Manual. The 3rd parameter to the PWM statement is the amount of time the Stamp is to produce the PWM signal. When that time is over, the Stamp stops producing the signal and it continues on to the next statement (the one after the PWM statement). While the Stamp is executing other statements (like the RCTIME and IF..THEN..ELSE), it's not generating the PWM signal any more and the LED will remain off (unless your program turns it on or off with a HIGH or LOW statement (or vice versa depending on how the LED is wired).

Thanks! Now i understand how it works.

Remember that the DEBUG statement takes some time to execute and that will slow down your program. Each character sent by the DEBUG statement takes about 1ms to send. "Turn LED 0% on" takes 14 characters, thus a little over 14ms to execute. The other DEBUG statement (after the Display: label) takes another 13-14ms to execute. The Display: subroutine probably takes 35ms or so to execute

So how do i make sure that the LED will not flicker? Should i move the PWM command up to the top/move it down the code? Or is there no way i can control the LED due to the time taken for the program to execute the other statements?

Mike Green · 2013-07-08 05:21

There will always be some flicker, but it may be short enough so you won't notice it. For example, you don't notice flicker when you're watching a movie where there's flicker on the order of 30ms or less. Try commenting out the DEBUG statements and see what happens. When you choose the resistor and capacitor values for RCTIME, keep the charge and discharge time under 15ms (so the total is less than 30ms). Alternatively, use an analog to digital converter like the ADC0831 to read the pot value. That's faster than using RCTIME.

Beau Schwabe · 2013-07-08 12:56

The reason for the flicker as it has been hinted to is the fact that the Basic Stamp II is a single threaded processor, which means that it can only perform one task at a time. While you are doing PWM the LED comes 'on' at the desired brightness, however when the program reaches the RCTIME command, the LED goes out or is perceived to 'flicker", because just as soon as it goes out the program comes around again and supplies the PWM signal to the LED. When you specify the LED to be solid 'on' or 'off' (HIGH or LOW), the LED does not flicker because there is no processing required to 'hold' the 'on' or 'off' position.

What to do?

There is a solution for your particular application that will allow you to keep the LED 'on' flicker free at a desired brightness. Keep in mind there are many ways to solve this, this is just a suggestion. By slightly modifying a basic LED current regulator circuit we can apply PWM to 'hold' the current at a particular value for short periods of time. Enough time that it allows the Basic Stamp II to process other things without flicker.

In the above image note that the input is inverted. This means that if you want the LED to turn 'on' you need to supply a LOW to the input of this circuit. Likewise for PWM ... A duty cycle of 255 will turn the LED 'off' while a duty cycle of 0 will turn it 'on' ... in practice the duty cycle ranged from 200 to 230 with 200 being full 'on' and 230 being full 'off'.

LED current regulator reference:
http://electronicdesign.com/energy/low-dropout-current-regulator-improves-led-driver-efficiency

Tracy Allen · 2013-07-08 19:39

christineee,
In your original post you mentioned only 3 intensities, 100%, 70% and 0%. You don't need PWM for that. It can be done with two Stamp pins and two resistors. No flickering.
For example, here is a scheme.
...

If p13 is an input, and p14 is a low output, then call that 100% intensity. The LED is in series with 330Ω and the current is about 11.2mA. Then make pin p14 an input, and make p13 a low output. Now the LED is in series with 470Ω and the current is about 7.9 mA, which is ~70% of the first current. Make both pins high outputs (or both inputs) to get 0%. You get a bonus 170% current if you make both p13 and p14 low. Intensity may be a subjective thing, so you can adjust the resistor values to suit.

Beau Schwabe · 2013-07-08 20:02

christineee,

See, another way to solve the problem. Tracy Allen's approach is very simple, and all you might need to do is a little experimentation on which resistor values are best suited for your needs.

Thanks Tracy :-)

christineee · 2013-07-09 19:02

Thanks guys for your help but i would like to try out Tracy Allen's method to see if it fits my needs for the project.

Thanks guys for teaching me.

Tracy Allen wrote: »

The LED is in series with 330Ω and the current is about 11.2mA. Then make pin p14 an input, and make p13 a low output. Now the LED is in series with 470Ω and the current is about 7.9 mA, which is ~70% of the first current. Make both pins high outputs (or both inputs) to get 0%. You get a bonus 170% current if you make both p13 and p14 low. Intensity may be a subjective thing, so you can adjust the resistor values to suit.

Hi Tracy Allen,
May i know how do you get the value 11.2mA & 7.9mA?
Is it by applying the ohms law?

Cause these are the values i get..
V=IR ;
9V(power supply) ;
1.8V (voltage drop for RED LED)

100% brightness:
9V-1.8V=I*(330ohm)
I = 21.8mA

70% brightness:
21.8mA * 70% = 15.27mA
9V-1.8V=15.27mA*R
R = 471.51ohm ~ 470ohm

Thanks.

Need help with LED dimmer circuit

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