Re: 40kHz Infrared Serial Modulation?

On Wed, 11 Jan 2006 23:39:26 -0800, redbrickhat wrote:

> I'm trying to transmit serial messages at 1200 bps to an IR receiver
> such as a GP1U52X.
> The datasheet says that the 40kHz signal must be further modulated by a
> 100 to 1000 Hz signal, I think because after several cycles are
> received the AGC in the receiver reduces gain.
>
> I was wondering if this second modulation is necessary when sending
> serial information (since the 1200 bps signal itself is like a
> modulation), and what would be the ideal frequency for such a signal. I
> suppose one problem would be if the UART were to send several "1" bits
> in a row since the GP1U52X might only detect the first ones before the
> AGC reduces gain.
>
> I was also wondering if the duty cycle of the 40kHz signal has to be
> 50%. I want to try a much lower duty cycle to pump more current into
> the IR LED.
>
> Any feedback will be appreciated.
>

Well, TV remotes send some kind of data stream for the different
buttons, but I don't recall how fast - the point being, the receiver
can be made to respond reliably, in a reasonable amount of time -
try just turning the 40 KHz on and off at your 1200 BPS rate, and
see what you get!

Good Luck!
Rich

.

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