Infrared Detector Information

Thread Starter

crazyengineer

Joined Dec 29, 2010
156
From reading this located in the app note on the site, it seems like I need to send a frequency of 38.5khz to the IR led so it can be detected by the ir detector.

However, I'm a little confuse about this part

Since the IR detectors only see IR signals in the neighborhood of
38.5 kHz, the IR LEDs have to be flashed on and off at that
frequency. A 555 timer can be used for this purpose, but the 555
timer circuit is more complex and less functional than the circuit we
will use in this and the next chapter. F
Does this mean that using a 555 timer to pulse the IR LED will not work?
 

Adjuster

Joined Dec 26, 2010
2,148
No, it is clearly stated that a 555 circuit can be used, but whoever wrote this also claims that some other circuit described in the same document is simpler and has better performance.

I know from my own feeble efforts that understanding technical matters in a foreign tongue is not easy.
 

ErnieM

Joined Apr 24, 2011
8,168
You can think of the IR detector as a 3 terminal device: power, ground, and signal. If I am reading the data sheet correctly you need to give it 16 pulses of the 38.5 KHz for the output to respond, the output is low for a signal, and high for no signal.

For 16 pulses on or off, this is a data bit rate of 1.25KHz, or very small. That is great for things like a remote TV control, horrible for something like audio data.

If you get a 555 to run at the 38.5 KHz then you can drive an IR LED with that and get the receiver to "receive" it. When the 555 is running the detector outputs a low, stop the 555 and the receives outputs a high.

All you are doing then is a simple ON or OFF detection.
 
I have some questions please


1) Do i have to work with a frequency less than or equal 38KHz so that the receiver can detect it ?

2) I want to measure the distance between the IR and an object and am confused about the technique used so that i could program the IC.

THanks in advance.
 

ErnieM

Joined Apr 24, 2011
8,168
Answers (you're welcome)

1) Doesn't have to be exact, just make it as close as practicable. Say in the range of 37.5 to 38.5 KHz.

2) You can't use this receiver to do that, as it's output is very slow compared to what you are measuring (unless it's how far away the moon is).

To get an estimate the speeds you need take the distance you want to measure and divide that by the speed of light. Then if you want say 1% accuracy in your measurement divide that time by 100.

You'll get a very tiny number.
 

chejian6

Joined May 22, 2011
10
There are receiver modules that can detect modulated 38KHz signals, like IRM3638 from Everlight. It has only 3 pins, the sensitivity is extremely high. You can find it here:
IRM3638
About the transmitter, I don't suggestion to use 555 although it's very cheap. The RC oscillator is not stable. A ATtiny or a STC15F02 will work better.
 
I tried the 3 leged receiver but i guess they aren't suitable in that project because it senses the wave from any place even if they are back to back. as u said, the sensitivity is extremely high.

I am thinking of using an Ultrasonic sensor instead but i am afraid of something. i will put two sensors side b side. lets say 12 cm far from each others. if the first sensor transmitted it's wave so that the first reveiver can detect it. will the second receiver also detects it ?? and how to avoid this problem. i don't want the other sensor to detect the first's sensor transmitting wave.
 
Top