Yes,this was my experience the few times that I have tried to use similar
modules (the Sharp ones that Rat Shack sells or used to sell). If you look
at a block diagram of the inside of the receiver,I think there may be a
DC-blocking capacitor AFTER the main 38 or 40kHz detector. This is why you
only see a short pulse when you apply an external continuous carrier.

SO, I suppose the best answer,if you must use such a detector,is to
modulate the carrier on/off perhaps 100 times per sec,and use your PIC to
detect that. However, I feel that other solutions may be best for simple
beam-obstruction people detectors (using IR leds and
Phototransistors,perhaps combined with an LMC567 (IIRC) tone decoder,which
could easily provide a digital signal from a 40kHz carrier,using only four
external components (two resistors,two caps).

For more info on the type of IR "cans" that you are asking about, Wirz
Electronics has some info (I'm not sure how much) at

http://www.hobbyrobot.com/info

In addition, if you want a ready-made solution and you don't mind a small
expense (about $25), you could use an IR distance sensor. Yes,its
overkill,but it is ready to go,if you just hook it to a PIC,and you can
sense a person by just looking for the reading to pass a certain threshold.
For more info on these, see

http://www.hobbyrobot.com/info/gp2d02/index.html

(Which is also a gratuitous attempt to plug an app note I wrote <G>)

Good luck!

Sean


At 10:14 PM 9/21/99 -0800, you wrote:
>Has this been the general experience?  My detector (Liteon
>LTM-97AS-38) seems to activate only on the first few msec of a
>constant 38 KHz source.  Some references suggest that most detectors
>want 'short bursts' of IR at the designated frequency (i.e.,
>modulation) and aren't designed for a continuous signal...
>
>Steve
>
|
| Sean Breheny
| Amateur Radio Callsign: KA3YXM
| Electrical Engineering Student
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