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DPRG: IR stuff

Subject: DPRG: IR stuff
From: Clay Timmons ctimmons at asic.sc.ti.com
Date: Wed Jul 30 10:48:19 CDT 1997

> I have bought a Radio Shack Infrared Emitter and Detecor. (two LED alike)
> Does anyone have any simple circuit(the simplest) to test them ?
> I have also bought a IR Detection module but this one is a 3 pinner does 
> anyone have some simple circuit for that  too ? (That is after I would 
> make the first one work, ok ?
> P.S. Or maybo someone knows a WWW address where these circuits would be 
> shown ?
> Later,
> Cyborg

To test them just breadboard the following two circuits.  
Monitor the ouput voltage while you turn the transmitter on and off.
If everything is working you should see a big difference in the output.

The transmitter is just an LED with the standard current limiting resistor.
Adjust resistor value for supply voltage and desired drive current.

             R           LED

                         | /|
   V+  --/\/\/\/\/\/\----|< |------------- GND
                         | \|

Receiver is equally simple. The receiver is an NPN phototransistor.
It may have three leads or two.  In either case only two are needed,
the collector and emitter.  Add a pullup resistor to the supply
I use about 10K, with the phototransistor pulling down to ground
depending on the amount of IR light received.  Output is the
voltage between the resistor and phototransistor.  The output can
go directly into a microcontroller digital input (cheesy) or
to a schmidt trigger or comparator (better) or to a D/A converter

            R        Phototransistor
                    Collector  Emitter
   V+  --/\/\/\/\/\/\----\      /------------ GND
                          \    /
                           \  ^

The IR phototransistors are sensitve to visible light as well.
The best solution is to enclose them in a light shield like
opto-couplers and shaft encoders do.  For applications
where visible light is present an IR filter 
(red plastic found on remote controls) helps limit the visible 
light.  A much better approach to deal with the problem
of ambient light is to modulate the signal like remote
controls do with 40Khz pulses.  In that case you can use 
the little IR detection module from Radio Shack and a different
type of transmitter.

- -Clay Timmons   ctimmons at asic.sc.ti.com




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