How do I make an IR LED pulse at 500 Hz?

[gtfreeflyer]

I have an infrared LED that I need to pulse at 500 Hz with a 50% duty cycle. This is what the IR receiver is tuned to seeing. I believe this means a pulse of 1 millisecond on, 1 millisecond off continous square wave function. What components do I need in my circuit to get it to pulse like that? A schematic would be most excellent! The stuff I found through google didn't help me too much.

The IR LED operates at 1.3V DC (1.7V Max) and I want it to draw around 40-75 mA. I'd like to use 2 D batteries (3V source) to run the circuit. I have a slightly-more-than-basic knowledge of circuits, so your explanation doesn't need to be too elementary.

Thanks!

-Jonathan-

[psyrex]

An astable 555 circuit should give you the square wave generator you desire. A quick google seach for "555 square wave" should provide you with a small calculator to determine resistor and capacitor values.

[gtfreeflyer]

I see that the 555 can't really get lower than a 55% duty cycle. Other schematics I saw use more than 1 LED or a power source greater than 3V. Still looking for help, possibly a link to a schematic that shows what I'm trying to do. All the calculators I saw spit out the info I already have. Is there one where I can input 500 hz and 50% duty cylce and then it will spit out the resistor and capacitor values?

[moyg]

If you want a perfect 50% signal, you can start w the 555 generating 1KHz & divide it in half w a flip-flop.
Curious: How do you provide control w a constant frequency & duty cycle signal? Is it an on-off function?

[zik]

You can get a 50% duty cycle with a 555 timer by forward biasing a diode across pins 7 & 6. This changes the available duty cycle range to 0%-100%.

Output high time is T1 = 0.7RaC
Output low time is T2 = 0.7RbC

So for 50% duty cycle use the extra diode and identical values for Ra and Rb.

There's quite a handy page at:
http://www.williamson-labs.com/555-circuits.htm