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HP48SX InfraRed power.
Message #1 Posted by Tal on 8 June 1999, 9:10 a.m.

I would like to know if there is any way to increase the power of the HP48SX IR transmiter?

I will appreciate very much if you could tell me what components I should change in the calculator.

Best regards, Tal. email:

Re: HP48SX InfraRed power.
Message #2 Posted by Reinhard Hawel on 8 June 1999, 6:25 p.m.,
in response to message #1 by Tal

This link will help you:

There is also a text file here on the hpmuseum site: (the same text)

As you see in the schematics on page 6, the driver for the IR LED ia a simple switch. You might succeed by changing R1, or possibly R2, but you should watch the IR LEDs datasheet before trying.

It is well known in communications engineering, that rising transmission power has a low effect on transmission distance.

There is also the risk of killing the diode or overloading the OUT pin, which is (i guess) a CPU pin.

If you want to increase distance, you should raise the "antenna gain" (higher LED efficiency; narrower, more "concentrated" beam, ...)

Also, please consult the datasheet of the diodes mentioned in the technical interfacing guide.

The receive circuit is rather difficult to improve, because there are no filters. As I mentiones in an earlier post, a time-variant filter should do a rather good job, maybe together with a Markoff chain. Increasing the gain is not the right way to solve the problem (though it is a part of it).

The main problem in this transmission method is, that it isn't modulated, so that there can be no sharp passband filters in a receiver circuit. I heard, the Ericsson 888 mobile phone uses a similar (horrible) format transmitting short pulses for a RS-232 transmission. This saves battery, but it is a unpleasant way of transmitting data over more than 5 cm.

Are you an engineer, or does anybody else want me to explain some words used in this message? Feel free to ask here in the forum or via email.

Sorry, Tal, I haven't found my old schematics til now. Be assured, as soon as I find them, you will be the first who receives a copy.

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