LED questions

[GreekPIC]

Ok, guys, I made the attached circuit and is working.

I substituted the red LED with an IR one and it's still working. Should I change anything? Am I pushing the IR too much or on the contrary not getting all I can out of it?

If I want to use 4 LEDs I guess I should change the 1K resistor, right? To what?

Thanks,
Nikolas

View attachment 37309

 

[audioguru2]

Hi Nikolas,
Your circuit feeds the LED very little current. The 555 has an output voltage loss of about 1.4V and the IR LED has a forward voltage drop of about 1.2V, so the resulting voltage across the 1k current-limiting resistor is 3.4V and the current is 3.4mA with a 6V supply, and 9.4mA with a 12V supply.
You need to check its max continuous ratings, but most IR LEDs are rated for at least 30mA, so you can reduce the value of the 1k resistor to 120 ohms with a 6V supply, or 330 ohms with a 12V supply and the IR beam will have greater range.

Since the voltage for LEDs is limited to only 4.6V with a 6V supply, you can have only 3 IR LEDs in series and in series with a 33 ohm resistor, or 8 IR LEDs in series and in series with a 33 ohm resistor with a 12V supply. If you want only 4 LEDs in series when using a 12V supply, use a 200 ohm current-limiting resistor in series with them. ;D

 

[GreekPIC]

Thanks Audioguru. Oops! I forgot to say I use a 9V battery.
So for 4 leds (I want to place them at right angles so they're visible 360

 

[audioguru2]

Oops! I forgot to say I use a 9V battery.
So for 4 leds (I want to place them at right angles so they're visible 360View attachment 37313

 

[GreekPIC]

Sorry, I meant visible with night vision equipment, I want to use the IR LEDs.

I like the 2+2 alternating LEDs design, I think this is what I'll do.

Thanks,
Nikolas

 

[GreekPIC]

I've seen IR emmiters for remote controls, what's their difference from ordinary IR LEDs? Narrower beam angle?

 

[audioguru2]

The remote for my TV and digital video recorder works pretty far away no matter where I point it, even if I point it at the white closet doors behind me. So it must transmit IR in a fairly wide angle. The 3-pin IR receiver IC responds with great sensitivity to modulated IR and has automatic gain control. ;D

 

[GreekPIC]

So I can use an IR emitter instead of a IR LED?

 

[audioguru2]

Same thing.

 

[hamoodyjamal]

IR emitter = IR LED

The reason TV remotes have great range is because the signal (form the remote) is sent in bursts (i.e. 10 bursts a second) meaning you can allow more current to pass through the LED --> strong signal.

You see, LED's are rated i.e. 30mA max at continous current. If you look in the datasheet of most LED's you'll see that you can pass current greater than 30mA, say 70 or 80mA, if the current is in bursts. If you point a digital camera at the LED of a remote, and press any button, you'll see the IR LED flash and not continously ON. Which means greater current is passed at smaller intervals, thus more power = Greater range.

Something that also helps, like audioguru mentioned, the remote sends modulated light (along with data) which can be detected easier by the IR receiver than just plain IR light.

Even if you replace a remote's IR LED with another one, you'll most likely get the same range.

;D

 

[GreekPIC]

Thanks guys