Adjustable Power

Since I first built a working circuit for the LibreTag project, I always found that testing indoors proved to be without any need of aiming. My walls are white and so much IR is generated by the IR LED,that you can almost point the gun in any direction and score a hit.

I have been thinking of ways to be able to scale the power output of the guns down depending on where you intend to use them. My best idea so far was to use another FET to switch the IR LED either through a ~100 ohm resistor or straight to ground, to give a x1 or x1/10th power selection. However the problem with this is that it would never give you a chance to say scale from a large room to a small room.

Tonight I had a brain wave. After glancing over the data sheet for the 38khz IR receivers I noticed that they have a fairly high drop off as the frequency diverges from 38khz. I wondered if I could use this to my advantage by changing the PWM frequency of the transmitting IR LED.

IR LED response curve

I am happy to report it works wonders. I still need to implement a menu option to make the “strength” selectable, but with the freqency set to 50khz the sensitivity of the receiver, as per the graph, drops down to about 20%. It makes the gun usable indoors, with no sign of any reflections. It is maybe a little too under powered at 50khz, but this can be tweaked.

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