Help with IR LED!

[256mxr]

Hi!

This I am sure it is a newbie question but here it goes...

I am doing a project where I need to send an IR signal. This part is done and the receiver it is getting it right...

But the problem is that it only works at a very small distance like 20cm...

After some search I found out that typically IR leds need more current than normal Leds so I have no resistor between vcc and the positive side of the led...

Still no luck....it got better but I want just a little bit more...

Searched again and I think I have to use some kind of transistor to get more current trough the led...

My VCC is only 3V and the most transistor I found to do this are 5V

Can anyone help me with this??

Thanks

 

[Nigel Goodwin]

You MUST have a current limiter feeding the LED, and for any kind of range you need a transistor to drive the LED, usually pulsing it at about 1A of current. If you check my IR PIC tutorial then you will see how it's done, 3V is fine, transistors don't have 'voltages' as such, but for 3V use only one LED and adjust the resistor accordingly.

 

[256mxr]

You MUST have a current limiter feeding the LED, and for any kind of range you need a transistor to drive the LED, usually pulsing it at about 1A of current. If you check my IR PIC tutorial then you will see how it's done, 3V is fine, transistors don't have 'voltages' as such, but for 3V use only one LED and adjust the resistor accordingly.

So I have to use the transistor...

Are there alternatives to BC337 in case is not available in store???

I just hope this works... I have the positive, the output and a ground connector. And my led were connected to positive and negative only. The negative is my output that can be 3V or 0V. So this will be my "RB1" right???

 

[Nigel Goodwin]

The drive to the transistor is just a logic level output from a PIC, exactly what are you using to drive it? - I'm presuming you're using 38KHz (or so) modulation and an IR receiver IC?..

 

[256mxr]

I am using a GPIO from a microprocessor...

I am modulating 39Khz in my case...and communication is working! I just have the distance problem..

 

[MikeMl]

I am using a GPIO from a microprocessor...

I am modulating 39Khz in my case...and communication is working! I just have the distance problem..

What are you using for the receiver?

 

[256mxr]

A TSOP receiver to match the frequency...

 

[MikeMl]

I needed an IR beam-break detector. Using an simple IR led and TSOP receiver, I am able to get 3m in sunlight. I am pulsing the emitter with about 50mA.

 

[256mxr]

The thing is that I did not how much current it was goind through the led...started out with a 220ohm resistor...the I took that off and it got better...

So it seems I really have to have a transistor to have more current...

More alternatives to the transistor reference above please??

Does BC337 works with 3V Vcc??

 

[Nigel Goodwin]

Does BC337 works with 3V Vcc??

As I said before, it's not relevent to a transistor - all will work fine at 3V.

You just need a small general purpose NPN that will handle 1A or so.

 

[audioguru]

A TSOP receiver to match the frequency...

TSOP IR receivers have an automatic gain control that turns down the gain to avoid interference from compact fluorescent bulbs. If the 39kHz IR pulses are not in bursts like shown in the datasheet then the gain is very low so the range will be very low.

 

[audioguru]

I needed an IR beam-break detector. Using an simple IR led and TSOP receiver, I am able to get 3m in sunlight. I am pulsing the emitter with about 50mA.

As was said earlier, cheap remotes for TVs pulse their IR LEDs with 1A!

Don't you know that sunlight is full of IR?

 

[audioguru]

The thing is that I did not how much current it was going through the led...started out with a 220ohm resistor.

Ohm's Law simply calculates the current. The power supply voltage to the resistor is reduced by the voltage of the LED that is listed in its datasheet. The difference between the power supply voltage and the LED voltage is across the current-limiting resistor. The current is simply the voltage across the resistor divided by it value in ohms.

Is the simple arithmatic (grade 5) too difficult for you?

 

[256mxr]

Something is not working

I am using a BC547 instead of 337 and the Led does not blink...

I am connecting just like Nigel's link...

 

[MikeMl]

As was said earlier, cheap remotes for TVs pulse their IR LEDs with 1A!

Please explain how TV remotes get 1A out of two AAA batteries?

 

[Nigel Goodwin]

Please explain how TV remotes get 1A out of two AAA batteries?

I'm surprised you asked that?, and you obviously haven't read the link to my tutorial above which explains it - the current comes from a large electrolytic across the supply rail, in the case of my tutorial C2 470uF - a standard component in remote controls for this very reason.



Interesting that I explained that the transistor needs to be 1A, and the OP immediately uses a much lower current transistor (only 100mA).

256mxr - did you use one or two LED's?, two won't work off only 3V. Even though a BC547 is much too low rated, I wouldn't expect the transistor to die instantly, and it 'may' well work quite happily as the current is only in small pulses and not continuous.

 

[MikeMl]

I'm surprised you asked that?.

Nigel, Yes I read your stuff. My question stands!!!

Your circuit uses a low-impedance 5V power supply. The forward voltage drop of IR emitters used in remotes (e.g. **broken link removed**) ranges from 1.4V@100mA to 2.6V@1A. Since the Vcesat of your driver transistor is about 0.4V, the current during the pulse cannot be much over 250mA because the voltage drop across the 4.7Ω resistor would be 1.2V, leaving only 5-0.4-1.2 = 3.4V across two Leds, meaning 1.7V across each, which is consistent with the plot.

**broken link removed**

Have you ever ACTUALLY measured the Led current?

I still want to know how they get 1A through the LED in a handheld remote using two AAAs????

 

[256mxr]

I am using one led and I got it working...I have other Led cause the one I was using died

One meter is good enough for my project...

Thanks

 

[Nigel Goodwin]

I still want to know how they get 1A through the LED in a handheld remote using two AAAs????

Like I said, and my tutorial says, the current is supplied by the large electrolytic across the supply - remote controls don't work at all if it goes O/C (which does happen), or in rare cases will work, but only for a few days with brand new alkaline batteries.

The resistor used in my tutorial board was one that 'happened to be at hand' (which is why it doesn't give 1A), but still gives excellent range (further than I can measure indoors).

A 3V version that I made for multi-regioning Sony DVD players uses a single LED (obviously) with a 0.68 ohm current limiting resistor.

 

[magnatro]

this might help..