IR LED/detector circuit

Rsadeika

I am looking for some explanation on the IR LED/detector circuit I put together on my breadboard. The IR receiver module is a Radioshack part rated at 38khz center carrier frequency, and infrared passband 940nm +- 50nm with an elliptical lens. The IR emitter is also a Radioshck part, rated at 950 nm at peak emission. I have a 220 ohm resistor between the pin and the IR emitter.
The only time this circuit works correctly or at all·is when I give it a freqout x, 1, 36000. I was ecpecting it to work properly at freqout x, 1, 38000, since that is what the detector is rated at.
I guess the question that I have is, does anyone have a quick conversion for nm -> khz. The other question is, what does it really mean when it is stated "... at peak emission". Now, for the emitter it also states continuous forward current 150mA. Does this mean that to get to 940 nm +- 50 I need to have the BS2 pin output 150mA.
For what I thought would be a simple and straight forward circuit, this is getting to be quite complicated.

Thanks in advance for any input

ps I googled for this info, but I need this presented in lay terms.

Yanroy

nanometers (nm) doesn't convert to kilohertz (kHz).· Well, I guess if you're feeling really sadistic about what units of measure you're using, you can do the conversion, but in this case, they're talking about different things.· The nm rating is the wavelength of light being emitted or detected.· As I recall, red is about 750nm, so anything much above that, like 940nm, would be considered "infrared".· The 38kHz is the modulation frequency.· This means the light is being turned on and off 38000 times per second.· Peak emission will be the frequency of light your LED is "brightest" at.· For the receiver, it would probably be called peak sensitivity, which is the frequency at which the light can be "dimmest" and the sensor will still respond.· I can't really venture a guess as to why you're getting better results at 36kHz.

As for getting 150mA, you'll want to use a transistor, probably a 2n2222a or a 2n3904 (my personal favorite).· Connect the IR LED to +5 and the collector, connect the emitter to ground, and the base to a pin on your stamp via a resistor - probably something in the·1.5k ohm range will suffice.· I believe 3904's have a gain of about 50, so with a·1.5k ohm resistor you'll get ~3mA coming from the stamp pin, times 50 will yield a max current through the 3904 of 150mA.· That's only the roughest calculation... I kinda fudged the diode drop from base to emitter and other fun stuff you don't really need to worry about for this application.

SPENCE

I AM NOT SHOUTING. I CAN READ AND TYPE ALL CAPS MUCH EASIER.

YANROY,

I THINK IF YOU GO BACK TO BASICS YOU WILL FIND THAT WAVELENGTH IS THE RECIPRICAL OF FREQUENCY.

IE IN RADIO A HALFWAVE(IN FEET) = 468/FERQ MHZ. OR 936/3.9 MHZ=240 FEET WAVELENGTH. THIS IS USED TO DERIVE THE CALULATION OF A HALFWAVE DIAPOLE ANTENNA.

WHERE 1/2 WAVE(IN FEET) = 468/F MHZ... 468/3.9 MHZ - 120' FEET FOR A 1/2 WAVE DIAPOLE

SO WAVELENGTH DOES HAVE A DIRECT RECIPRICAL RELATION TO FREQUENCY.

73
SPENCE
K4KEP

THE 38 KHZ AND 36 KHZ IS THE MODULATION OF CHOPPER FREQUENCY BREAKING UP THE IR EMMISION AT A RATE WHICH MAKES IT POSIBLE TO DETERMINE WHICH IS THE IR RADIATION YOU WANT AND WHICH IS THE UNDESIRED SOURCE WHICH YOU WANT TO IGNORE.

Post Edited (SPENCE) : 8/15/2005 10:38:52 PM GMT