How to increase range in an IR remote

inaki

I have built an IR remote with an SX28 and it works. However the range is pretty short.
I have checked with a video camera that the·light is pretty dimm compared to an actual IR remote (TV set, compact disc, etc).

I am using a verys simple circuit: the SX28 pulses the base pin on a transistor·with a frequency of·40Khz.
The transistor E/C pins are·connected to ground and 5V through a resistor.·
I have used different·leds, resistors·and transistors. Current I have·tested ranges bewteen 250-500mA. Transistor I have used are rated 500mA MAX.

Which is the typical current used in remote controllers for the IR led ?
To achieve more range is it a matter of increasing current or are there other factors to have into account ?

In other words, what can I do to increase range ?

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Beau Schwabe

The amount of current available to the IR LED is going to be one factor via the current limiting resistor. The allowable amount of current is going to depend on the
manufacturer specifications for the IR LED itself. Another factor will depend on what you are using to receive the signal with.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Phil Pilgrim (PhiPi)

inaki,

Be sure, if you're using a bipolar transistor, to include a series resistor in your base circuit. For this kind of circuit, 1K should be about right. Otherwise you risk too much base current and possible damage to the transistor. Next, check the datasheet for your IRED to see what kind of current it can tolerate. There will be two current specs: for continuous and for pulsed current. The latter will also specify a duty cycle. To be on the safe side, start with the continuous current spec. We'll call that I_c. Now check the datasheet again to see what the IRED's forward voltage is at this current. (It'll probably be in a graph.) We'll call that V_fwd. Finally, check your transistor's datasheet to see what its saturation voltage is at I_c. Call that V_sat.

From this information, you can determine the proper value for your collector resistor, as follows:

··R_coll = (V_dd - V_fwd - V_sat) / I_c

Try a resistor of this size and measure the voltage across it with the IRED "on" to make sure your calculations were right. Then see if you get enough range. If not, you can start to increase the current (i.e. decrease the resistor's value). But bear in mind that, if you do, you will be in pulsed current territory and could destroy the IRED with a programming error that keeps it on too long.

-Phil

Note: The same computations apply if you're using a MOSFET. Only, instead of a series base resistor, connect the logic output directly to the gate, and add a 10K resistor from gate to ground. The latter will keep the MOSFET from turning "on" when the output pin is tri-stated. Also, you may have to compute V_sat if the MOSFET's datasheet only gives a graph of "on" resistance vs. current. This would be V_sat = R_on * I_c.

inaki

Do I need to lower the base resistor value if I want more current to pass through the transistor ?
Currently I am using a 4.7K resistor for the base, is it too much ? (for example for a 2N2222).

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Phil Pilgrim (PhiPi)

inaki,

This all depends on the transistor's DC current gain, shown in the datasheet as h_FE. You should base your computations on the minimums, since "typical" specs cannot be trusted. With a gain of 35, for example, and 5V driving the base through a 4.7K resistor, you can depend on getting at least 35 * (5 - 0.6) / 4.7K = 32.7 mA LED drive. If this is too low, you'll need to decrease your base resistor to get more base drive.

-Phil