Re: Long range IR signal


"Richard Hosking" <richardh@xxxxxxxxxxxx> wrote in message
news:425912d7$0$8392$5a62ac22@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> I am trying to make an IR (as in TV remore control) based gun game
> Commercial ones seem to have a range of about 100m, with reasonably
> tight focussing.
> I have tried a conventional IR diode with a tube to limit the field.
> This was receiveable over about 20m with good focus.
> The receiver is a module with a photodiode/amp/AGC/BPF and logic. The
> centre freq is 38KHz modulated to get rid of extraneous light sources.
>
> Cheap optics dont seem to improve things much.
> How do they achieve this sort of range?
>
> I would be grateful for pointers
>
> Thanks
> Richard


Well I don't know how other people do this sort of thing, but if it were me
I would place a TSOP1238 receiver by Vishay (available from Mouser) on the
target, and then mount one or more SFH4503 infrared LEDs (available from
Digikey) by OSRAM on the gun inside of a non-internally IR reflecting
barrel. I would try to shade the TSOP1238 from direct sunlight, but I would
not worry about any kind of special optics.

Datasheets for the parts here:

http://www.vishay.com/docs/82013/82013.pdf

http://www.osram.convergy.de/upload/documents/2003/12/10/11/58/sfh4501.pdf

The TSOP1238 is the most sensitive/robust/well documented/readily available
receiver I am currently aware of. The SFH4503 is the most powerful/small
beam angle/readily available LED that I am aware of that is suitable for
this application.

The TSOP1238 expects the incoming light to be modulated at 38kHz, but it
also expects to see pulse packets of 10+ cycles (though not too many) for
maximum reception strength. I would use a 10 cycle packet (IE: 38kHz
carrier at 50% duty cycle, but only produce 10 complete cycles of that
carrier for each trigger pull, assuming a "semi-automatic weapon"). If you
make sure the duty cycle is kept very small (<1% overall), then it should be
allowable to pulse the IR LED with high current in the range of 1.5A to 2A
during each ten cycle packet. You will probably need some large low ESR
capacitors to provide that pulse energy, depending upon your power source
and the wiring impedance.

The data*** is a bit sketchy about LED output at 1.5-2A, but something
maybe around 300mW typical (but much worse for low yielding LEDs) of total
radiant IR might be reasonable. Given the half power beam angle of +/-4
degrees, by my calculations at 100 meters it may be possible to achieve
something better than 1mW/m^2. Someone double check my math on this, I may
have made some error(s).

Figure 6 of the TSOP1238 data*** leads me to believe that this irradiance
may be enough to function under direct sunlight at 100 meters. I of course
haven't experimentally verified this to be the case, but I think optimism is
justifiable. Range would be much better at night time.



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