'Ramping' on an LED or bulb

[NewcastleSAR]

Hi Folks.

Following on from my discussion on the lighthouse circuit using either a PIC or 555 Timer, one other query I have is the ability to 'ramp' on and off the LED or Bulb.

What I means by 'ramping' is, rather than the LEd or Bulb being switched straight on or off, is it possible to switch it on gradually until it reaches its peak then gradually tail off. This would help in the lighthouse circuit to give a 'softer' flash as if the light beam was being rotated in the lantern.

Still working on the pros and cons of using either a PIC or 555 Timer.

Regards

Declan

 

[audioguru]

Just connect an electrolytic capacitor across the LED. It will slow down its turn-on time and make it dim slowly when it turns off. Try different capacitor values.

 

[NewcastleSAR]

Just connect an electrolytic capacitor across the LED. It will slow down its turn-on time and make it dim slowly when it turns off. Try different capacitor values.

Cheers for that.

Many thanks. What would you recommend as a starting value?

Declan

 

[Grant Fleming]

Hi Folks.

Following on from my discussion on the lighthouse circuit using either a PIC or 555 Timer, one other query I have is the ability to 'ramp' on and off the LED or Bulb.

What I means by 'ramping' is, rather than the LEd or Bulb being switched straight on or off, is it possible to switch it on gradually until it reaches its peak then gradually tail off. This would help in the lighthouse circuit to give a 'softer' flash as if the light beam was being rotated in the lantern.

Still working on the pros and cons of using either a PIC or 555 Timer.

Regards

Declan

Hi Declan,
Yes, you could use a PIC and programme it using PWM to "ramp-up/down" the LED. You could write a few different short, simple bits of code for different ramping rates so you could change them easily by downloading the one you want for a certain light at any time.
Cheers,
Grant

 

[HiTech]

If you're looking to have the LED turn on and off more slowly then try this simple circuit:

**broken link removed**

 

[NewcastleSAR]

Hi Declan,
Yes, you could use a PIC and programme it using PWM to "ramp-up/down" the LED. You could write a few different short, simple bits of code for different ramping rates so you could change them easily by downloading the one you want for a certain light at any time.
Cheers,
Grant

Hi Grant..

Would you have an example location of a tutorial that shows this in action?

Declan

 

[Grant Fleming]

Hi Grant..

Would you have an example location of a tutorial that shows this in action?

Declan

Here is an attachment of an LED circuit for a PIC.

Cheers,
Grant

 

[Grant Fleming]

What type of PIC will you be using? Have you used one before?

Cheers,
Grant

 

[justDIY]

what is the purpose of the 10k resistor ... it looks like a pull-down, but isn't the LED a good enough pull-down by itself?

 

[Nigel Goodwin]

what is the purpose of the 10k resistor ... it looks like a pull-down, but isn't the LED a good enough pull-down by itself?

Don't see any reason for it at all?, just the LED and series resistor are all that's required.

 

[stevez]

If you decide to use the capacitor method suggested and it turns on a little too quickly you might put a resistor ahead of the supply - to slow down the charge rate of the capacitor.

 

[audioguru]

If you decide to use the capacitor method suggested and it turns on a little too quickly you might put a resistor ahead of the supply - to slow down the charge rate of the capacitor.

Another resistor is not required.
The current-limiting resistor for the LED slows down the charge rate of the capacitor.

 

[Grant Fleming]

what is the purpose of the 10k resistor ... it looks like a pull-down, but isn't the LED a good enough pull-down by itself?

OK...OK guys!

I have required pull-down resistors for many projects I have made lately and just threw it in, i agree, no need to use it- get over it!
Point taken, thanks.

justDIY....
Thought you might have been happy and chosen something more positive to say, eg; about me having the resistor on the anode of the LED for thermal reasons!!

Cheers,
Grant

 

[audioguru]

Thought you might have been happy and chosen something more positive to say, eg; about me having the resistor on the anode of the LED for thermal reasons!!

Is it because the cathode of an LED is mounted on one pin and has good thermal conductivity to the copper on a pcb but the anode has just a thin bonding wire that cannot conduct much heat from the resistor?
Clever you!

 

[justDIY]

lol, actually my question wasn't intended to be rhetorical, although I see that it was ... I'm in the same boat and often use pull-down (or up) resistors, probably many times when they're superfluous