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need help with infrared leds

V

vredsdfr

Jan 1, 1970
0
I am playing with some infrared leds and phototransistors. My project
is a pinewood derby timer. It's computer based and I use a usb data
acquisition box so the circuitry is pretty simple. I am a bit confused
about the emitter and detectors I bought. I got the radio shack 276-142
pair. Now... with my tv remote I can aim it over my shoulder, without
looking, bounce the beam off the wall and it will work great. With
these leds I have to push the current to 100mA+ and even then they
have to be very close (not more than an inch) and almost perfectly
aligned. Does anyone know of a better emitter and detector to use?
 
J

John Popelish

Jan 1, 1970
0
vredsdfr said:
I am playing with some infrared leds and phototransistors. My project
is a pinewood derby timer. It's computer based and I use a usb data
acquisition box so the circuitry is pretty simple. I am a bit confused
about the emitter and detectors I bought. I got the radio shack 276-142
pair. Now... with my tv remote I can aim it over my shoulder, without
looking, bounce the beam off the wall and it will work great. With
these leds I have to push the current to 100mA+ and even then they
have to be very close (not more than an inch) and almost perfectly
aligned. Does anyone know of a better emitter and detector to use?

The problem is not with your emitter detector pair, but with
the signal passing between them. The remote control
communicates with the television by producing short bursts
of 40 kilohertz pulses to form a binary coded message. The
receiver can ignore all the steady light it receives and all
the light flickering at other frequencies from fluorescent
lights and TV screens, because the detector signal is passed
through a narrow band 40 kHz filter before being amplified.
Then the amplified 40 kHz carrier is demodulated
(rectified and averaged over several cycles) to reproduce
the pulses that make up the binary coded message. The
message may also have error correction bits added to it, so
that even if it is a little corrupted, the message is pretty
likely to be decoded.

If your emitter is putting out a steady blast of light as
its signal, that blast must overwhelm all other light picked
up by the detector for the signal to be recovered. You can
help this by adding a narrow pass optical filter over the
detector (if one isn't already molded over it) but it will
not help nearly so much as modulating the emitter signal
with a carrier frequency.
 
V

vredsdfr

Jan 1, 1970
0
John said:
If your emitter is putting out a steady blast of light as its signal,

It is.
that blast must overwhelm all other light picked up by the detector for
the signal to be recovered.

The blast of ir light is diluted by the other light sources? It seems
to me that the other light sources would be additive and cause the
transistor to saturate too quickly.

You can help this by adding a narrow pass
optical filter over the detector (if one isn't already molded over it)
but it will not help nearly so much as modulating the emitter signal
with a carrier frequency.

It seems odd to me that I can saturate the transistor easier with a
pulsed signal rather than a steady one. Obviously I am wrong but I
don't understand it.
 
J

John Popelish

Jan 1, 1970
0
vredsdfr said:
It is.


The blast of ir light is diluted by the other light sources? It seems
to me that the other light sources would be additive and cause the
transistor to saturate too quickly.

The receiver transistor will not practically saturate as it
receives the light signal, except for very short range
applications (slotted interrupters). IR receivers deal with
tiny signals that get amplifier by later gain stages.
It seems odd to me that I can saturate the transistor easier with a
pulsed signal rather than a steady one. Obviously I am wrong but I
don't understand it.

Give up saturating the receiver, and think about incremental
signals. How do you distinguish between lots of ambient
light, including variations at twice line frequency and TV
scan rates, but find the small additional signal from the
LED. This is a lot like building an AM band radio that
finds a tiny carrier signal swamped in wide band noise
hundreds or thousands of times larger except that the energy
is in the form of IR instead of low frequency EM waves.
 
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