Infrared Detector Sugestions

Thread Starter

JDR04

Joined May 5, 2011
356
I am looking for advice and suggestions on building a infrared breakbeam type circuit to trigger my camera. I have read a bit about it all but would appreciate clarification on.....

What is the best way to achieve at least 5m between your infrared detector and infrared transmitter.(In sunlight as well)

I'm told that the detector and the transmitter should oscillate at 38Khz. Is this correct, or, just a fallacy?

Would it be easier/better/more accurate to use a microcontroller to run the detector circuit and another one to run the transmitter circuit? (Both need to be independant of each other)

Would a microcontroller run circuit generally provide a longer battery life if using standard 9V PP3 type battery?

So, if anybody could give me a few ideas and suggestions to think about I would be most appreciative.

Thanks -JDR04
 

GopherT

Joined Nov 23, 2012
8,012
38kHz has become somewhat of a standard and off-the-shelf modules are available. The 38k hz is preferred vs DC because it removes background noise (sunlight ) in most cases.
You can use a break beam by putting each half (transmitter / receiver) on each side of the path. You can also use a "make beam" by allowing the IR to reflect when an object is present. That is sometimes easier because you don't need to run wires all over the place - a single module can do both.
 

MrChips

Joined Oct 2, 2009
23,849
38kHz is the most commonly used frequency in handheld remote controllers for encoding digital signals. You can purchase readily available IR detectors that are already sensitive to the 38kHz IR signal.

That may be the only particular reason to choose 38kHz. You may use a different frequency using a 555 oscillator for the transmitter and a 567 tone decoder circuit as the receiver.

As Gopher suggests, sometimes it is easier and simpler just to reflect the IR signal off a small mirror so that both receiver and transmitter is in the same unit. This is commonly done in beam breaker alarm systems.
 

THE_RB

Joined Feb 11, 2008
5,438
...
As Gopher suggests, sometimes it is easier and simpler just to reflect the IR signal off a small mirror so that both receiver and transmitter is in the same unit. This is commonly done in beam breaker alarm systems.
I'll argue with that one.

If you use a standard 38kHz IR receiver chip, it has auto gain. When the mirror-reflected beam is unbroken, it's gain will be low but it will still detect the beam.

If the beam is broken by an object passing between sender and mirror, that object will still reflect some IR, and the gain will crank right up and STILL detect the beam (as it is now reflected by the object, not the mirror).

If you use a mirror based system you need to fix the gain, so the mirror JUST reflects enough to detect the beam, but an object will not reflect as well, and will be seen as "beam broken".

One way to do that is to manually adjust the fixed gain when installed, or another way is use a large cap in the gain circuit (which you need to design) so it auto-adjusts to the mirror average signal, but any short term reduction in signal is sensed as "beam broken".
 

MrChips

Joined Oct 2, 2009
23,849
You're right. I was thinking more of a DIY transmitter/receiver with fixed gain at some arbitrary frequency (except line frequency and harmonics).
 

Thread Starter

JDR04

Joined May 5, 2011
356
Thanks guys.

So its pretty much decided that I should stick to a frequency of 38Khz.

I think I will go for an independant IR transmitter that will be set up opposite to the IR receiver.(No wire attachments etc)

Just so I understand this correctly, is it only the IR transmitter that needs to oscillate at 38Khz, or it the transmitter and receiver? If its both, how do you sync them?

In order to achieve flexibility, I'm thinking of using a microcontroller for both the transmitter and the receiver modules. What I mean by flexibility is things like activation/trigger on times, battery life and minimum amount of parts to use.

I was also wondering on how to achieve reliability in terms of about 5m in bright sunlight?

Comments are appreciated, thanks again - JDR04
 

MrChips

Joined Oct 2, 2009
23,849
The transmitter can simply be a 555 oscillator at 38kHz driving an IR LED.

The receiver does not oscillate to 38kHz. It is a circuit that responds to the 38kHz IR signal to give a logic output when the signal is detected.
 

GopherT

Joined Nov 23, 2012
8,012
First, you want to flash the IR emitter somewhere between 50% and max current to really have a signal.

You need to connect your phototransistor with a 10k or better resistor. Then connect that to an op amp with a capacitor to cut the background signal. The capacitor will only let CHANGES in the IR signal pass through, no constant signal will pass through. .

From the op amp, you need to run to a high pass filter that will cut off all low frequency signals (sunlight passing through tree leaves blowing in the wind). Cut off frequency should be 10k or so.
 

Thread Starter

JDR04

Joined May 5, 2011
356
Thanks guys, I think I got the idea of it now.

Can you suggest a circuit to try out??

Thanks -JDR04
 

THE_RB

Joined Feb 11, 2008
5,438
If you have having a transmitter at one end, and the receiver at the other end (break beam), you can use a typical 38kHz IR receiver chip.

The transmitted signal should be in pulses, as most receivers will fail if the 38kHz transmission is constant. So you transmit 1mS pulses of 38kHz, with (say) 5mS blank time between each pulse.

The receiver will then output a signal that is logic 0 for 1mS, and logic 1 for 5mS, repeating, when the beam is present. When the beam is broken the receiver will output a fixed logic 1.
 

Thread Starter

JDR04

Joined May 5, 2011
356
Thanks everyone.

Are there any IR detectors out there known to be better than the rest?? Same goes for IR LED to be used in the Transmitter.

It sounds as though it will be easier to at least use a microcontroller for the transmitter part as well.

Any other comments or suggestions will be appreciated. Thanks again to the AAC.
 

Thread Starter

JDR04

Joined May 5, 2011
356

Shagas

Joined May 13, 2013
804
Just a piece of advice from personal experience :

If you are going to use one of these 38khz modules (which you should) then make sure that
1) it's one which is able to give an active low with a continuous 38khz signal.It will make things easier
2)you check the graph out for LED wavelength versus sensitivity . 980nm is optimal , you can see that the sensitivity drops significantly as you stray from the 980nm
3)you check the graph for carrier frequency vs sensitivity . If you are going to use a 555 timer you will not get exactly 38khz. Combining points 2 and 3 you can see that if you don't stick to the parameters then you might get crappy results
 

bance

Joined Aug 11, 2012
315
Here is a link to an IR cordless headphone project, it doesn't use a microcontroller, but you may find other parts of the circuit useful.

There is a good explanation of how the circuit works.

HTH Steve

PS. here's a link to a supplier with a matched pair sender/reciever (better prices than e-bay and cheap shipping)
 

THE_RB

Joined Feb 11, 2008
5,438
You can pull an IR LED from any old (spare) remote control, that will work just fine with the 38kHz sensor.

There's no need to source a "sepcial" IR LED.
 
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