Some Questions Which Confused Me =/ - Electronic Circuits Projects Diagrams Free

westkid85 · 19th March 2008, 01:39 AM

Hi everyone, i was asked by few questions which confused me for quite some time,

We all know that in order to control the LED brightness, we can use a resistor in order to control the brightness,
but i was told that this isnt a good idea because it dissipate some power.
and i was told there is a better way which is to use the motor to control the LED brightness and it is perfered because it doesnt dissipate power.
If anyone know know what it the method and how is it be done?

my 2nd question is in order to use PIC to detect light source, i planning to use a LDR(Light Dependant Resistor) on it. But the problem is LDR detect light by varying the resistance and PIC detect the signal by voltage, the problem is how do i convert the change in resistance to voltage signal in order to be detected by the PIC?

**3v0** · 19th March 2008, 02:39 AM

Quote:

Originally Posted by westkid85

Hi everyone, i was asked by few questions which confused me for quite some time,

We all know that in order to control the LED brightness, we can use a resistor in order to control the brightness,
but i was told that this isnt a good idea because it dissipate some power.

Only if you need to save power, as in battery operated.

and i was told there is a better way which is to use the motor to control the LED brightness and it is perfered because it doesnt dissipate power.
If anyone know know what it the method and how is it be done?

It is called PWM or Pulse Width Modulation. You generate a square wave and use it to control the power to the LED. As the ratio of hi to low increases the LED gets brighter. Wikipedia will have good info on this.

my 2nd question is in order to use PIC to detect light source, i planning to use a LDR(Light Dependant Resistor) on it. But the problem is LDR detect light by varying the resistance and PIC detect the signal by voltage, the problem is how do i convert the change in resistance to voltage signal in order to be detected by the PIC?

Use the LDR as one half of a voltage divider.

.....................

3iMaJ · 19th March 2008, 03:44 AM

Quote:

Originally Posted by westkid85

We all know that in order to control the LED brightness, we can use a resistor in order to control the brightness,
but i was told that this isnt a good idea because it dissipate some power.
and i was told there is a better way which is to use the motor to control the LED brightness and it is perfered because it doesnt dissipate power.
If anyone know know what it the method and how is it be done?

:

I wouldn't say that it doesn't dissipate power. The device that you're using to modulate the pulse (in this case a motor) is dissipating the power instead of the resistor. Theres no free lunch, energy is still conserved. However, it may be more efficient, but in the end we're talking about the power dissipated by an LED. I think using a PWM to control the brightness of a LED is like using a bazooka where a BB gun will do. Unless of course you're already using the PWM for something else and have the signal available.

westkid85 · 20th March 2008, 03:30 AM

hmmm...anyone know how to solve my 2nd problem?
what is the meaning of one half of a voltage devider and how could this be done?

Pommie · 20th March 2008, 03:34 AM

If you have your LDR from the pic pin to 5V and a 20k resistor from the same pin to ground then when the LDR is illuminated the pic will see a high on it's input.

Mike.

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19th March 2008, 01:39 AM	(permalink)
westkid85	Some Questions Which Confused Me =/ Hi everyone, i was asked by few questions which confused me for quite some time, We all know that in order to control the LED brightness, we can use a resistor in order to control the brightness, but i was told that this isnt a good idea because it dissipate some power. and i was told there is a better way which is to use the motor to control the LED brightness and it is perfered because it doesnt dissipate power. If anyone know know what it the method and how is it be done? my 2nd question is in order to use PIC to detect light source, i planning to use a LDR(Light Dependant Resistor) on it. But the problem is LDR detect light by varying the resistance and PIC detect the signal by voltage, the problem is how do i convert the change in resistance to voltage signal in order to be detected by the PIC?

20th March 2008, 03:30 AM	(permalink)
westkid85	hmmm...anyone know how to solve my 2nd problem? what is the meaning of one half of a voltage devider and how could this be done?

20th March 2008, 03:34 AM	(permalink)
Pommie	If you have your LDR from the pic pin to 5V and a 20k resistor from the same pin to ground then when the LDR is illuminated the pic will see a high on it's input. Mike.