I'm looking for a very simple circuit to drive two IR LEDs at 38kHz. I'm pretty much a beginner at electronics, so the easier to understand and the fewer components the better. I suppose it would be helpful if I could power it from 4 AA batteries or a similar portable source. I don't have any particular LEDs in mind. Could I source them from a remote control?


Thanks,

Scott

 

A 555 IC makes an oscillator that can operate at 38kHz and can drive two LEDs in series with a current-limiting resistor. When the battery voltage drops below 4.5V then the 555 might stop.

 

There are CMOS versions of the 555 timer that will run at even lower voltages, and use less power. The ICM7555 is one such timer. Radio Shack carries the TLC555 timer, also CMOS.
http://www.radioshack.com/product/index.jsp?productId=2062595
Radio Shack is an expensive place to buy such items, but if you're in a hurry...

Radio Shack also carries IR LEDs, but the documentation provided on the package is in error; they will NOT take 200mA. It should've said 20mA.

Digikey is a good place for hobbyists to buy components. They'll ship 1st class mail for small orders, which will save you money.

Whichever 555 timer you use, you should use the output of the timer to sink current from the LED's cathode rather than source current to the anode.

The CMOS version can sink 10x as much current as it can source.
The bjt (bipolar junction transistor) version's output goes nearly to ground, but when high loses 1.7v from Vcc.

 

The OP is using 4xAA batteries so its about 6V supply voltage.

The CMOS version of 555 spec its output current IOL @8mA at 5V so it seems not much current is available to drive an IR LED even at 6V supply voltage.

I suspect it has to do with the output stage as this current value increased to 100mA if the 555 is working at 15V.

 

Here is a schematic I found in a robot kit assembly manual. Can anyone see anything here that they would change for practical reasons? This just uses one IR LED, I'm okay with that. It uses the same IC that you've all mentioned, so I'm hoping this is might work.


Thanks for all of your input. I've been reading up on some of the terms you've used in your posts. I think I've learned a lot about electronics just deciphering your input.

Thanks so much,

Scott

 

The output high voltage of an ordinary 555 with a 6V supply is 4.7V. The IR LED is about 1.3V so 3.4V is across the 470 ohm current-limiting resistor. Then the current in the LED is only 7.2mA which is pretty dim and it won't go far.

A TV remote uses pulses of up to 1A.

 

If I substitute a 47 Ohm resistor that would put my current at 72.3 mA, I think. (Ohm's law is both new and beautiful to me.) I don't have any real understanding of what kind of current I can put through the LED, but I'd be thrilled with a distance of 1 meter.

Any IR LED I purchase would provide me with a current rating, but if I scavenge an LED, and burn it up, I guess I'm not out too much money.

 

I already suggested to use the 555 to sink current rather than to source it.

I guess I'm just talking to myself here.

 

Thanks! I'm still learning. By sink do you mean change the polarity of the LED?