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HP 50g/HP 49g+/HP 39gs Battle of the beams -- Increasing IR communication range
01-11-2018, 02:49 PM (This post was last modified: 01-11-2018 03:01 PM by Robert VM.)
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HP 50g/HP 49g+/HP 39gs Battle of the beams -- Increasing IR communication range
I've been rather lucky obviously to get a secondhand 49g+ and 50g for a reasonable price. I couldn't resist taking a look inside the 49g+ especially at the IR tranceiver. See if there wasn't an easy way to increase IR LED drive current. There seemed little hope as I couldn't find any reports of such modification while it's been around for more than 10 years.
Other people found out how to open: 49g+ cut the heads off the rivets in the battery compartment; 50g and 39gs screen cover has to be lifted to access the two screws underneath (a suction cup from e.g. a satnav device will do). Guide a guitarpick or other such implement between the bottom half and the side ridge to pry open the case.
The IR transceiver is a six pin device.
   
Pc board tracks give some hints as to pin function. Leftmost pin is connected to a large pcb area, the only wider track is connected to the next pin. Probably GND and supply positive.
I found three brands of IRDA transceiver with 6 pins, all having the same pinout consistent with pcb track layout. Sharp, zilog and Vishay.
   
Finally a resistor connected in series with the rightmost pin, kathode of IRED according to the datasheets, was enough hint to do a little experiment with the series resistor. To encourage the experimenter the series resistor is labeled R1 on the pcb (49g+). The HP 49g+ would serve as the guinea pig, 50g was next if everything went well.
   
(This is a 7 pin device (as used in the 39gs?) almost same pinout)
The resistor is EIA coded 30B (2KOhm). Supply is 3.3V, IRED volt. about 1.8V, gives 0.75 mA IRED current.
Considering the currents mentioned in the datasheets, I thought it safe to increase the current to 7.5 ma.
   
First experiment was with a 330 ohm resistor, was later changed to 200. The change was rather noticable visually (with a camera). While originally nothing would show on the camera during printscreen, IRED output is very visible now. (example fotograph on website: http://www.site2241.net/october2008.htm).
Increasing the current further would be more of a problem for the 3.3V PSU than for the IRED I guess.
IR communication range of the pair (49g+/50g) is now about 30 cm, near halogen desk lamp. (Both devices need to be modified for 2-way comm)
HP 50g: resistor is in slightly different place, same value. In the picture the resistor has been replaced.
   
HP 39gs: 7 pin IRDA transceiver, almost the same pinout (IRED39-01). I haven't been able to test this device except visually. I think this device is easier to open, starting from the top. While the 50g can do very well without the two screws, the 39gs opens too easily without them.
(39gs pic in next post)
Robert
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HP 50g/HP 49g+/HP 39gs Battle of the beams -- Increasing IR communication range - Robert VM - 01-11-2018 02:49 PM



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