Shop OBEX P1 Docs P2 Docs Learn Events
maximum effect from IR LED:s — Parallax Forums

maximum effect from IR LED:s

HoffaHoffa Posts: 46
edited 2006-09-07 17:59 in BASIC Stamp
I have 2 IR LED:s (350-00003) and 2 IR detectors (350-00014). How do I do to detect things at the longest distance. As I have understand it 37500 is the best frequense, but which resistor should I use?

Comments

  • Mike GreenMike Green Posts: 23,101
    edited 2006-09-07 17:59
    The LED is rated for a maximum of 50ma continuous current and most of the specs are at a current of 100ma (less than 50% duty cycle). If you're driving the LED directly from a Stamp pin as is usual, you want to limit the current to 20ma. From the datasheet, forward voltage at 20ma is about 1.3V. Voltage drop through an I/O pin is about 0.6-0.7V. That means the resistor has to drop 5V-2V = 3V at 20ma. Plugging into Ohms Law: R = V / A = 3.000V/0.020A = 150 Ohms. The Stamp chip is rated for a maximum current on any one I/O pin at 45ma. You could drive the LED at 40ma and have a little margin by using a 75 ohm resistor. Theoretically, you could use more than one I/O pin and drive them simultaneously for more current, but it would be safer to use some kind of external buffer like a transistor.
Sign In or Register to comment.