need help with infrared leds

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?

Reply to
vredsdfr
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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.

Reply to
John Popelish

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.

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.

Reply to
vredsdfr

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.

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.

Reply to
John Popelish

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