Long range IR signal

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:

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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 (

Reply to
Fritz Schlunder
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irradiance

Hmm... Maybe better modify that. Figure 6 indicates 5900K sunlight at a level of 8200 lux is roughly an irradiance of 10W/m^2 (at the wavelength of interest). But if you look at this site here on section 10 (How bright are natural light sources):

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It would have you believe direct sunlight is 130,000 lux. Since the TSOP1238 quickly loses functionality past 10W/m^2 and becomes worthless at

the range. The link suggest full daylight (not direct) is from 10k lux to

25k lux, so maybe it can still work provided the receiver is well shaded from sunlight. Perhaps if the receiver was mounted in its own non-internally IR reflecting barrel...
Reply to
Fritz Schlunder

What size of light source are you using? You want the smallest possible light source you can.

Alternatively, have you considered laser diodes? Take a 1mW laser pointer, add a diverging lens in order to make the spot size ~1m, and modulate it.

Reply to
Ian Stirling

I read in sci.electronics.design that Richard Hosking wrote (in ) about 'Long range IR signal', on Sun, 10 Apr 2005:

Parabolic reflectors at each end, very carefully adjusted for best parallel beam.

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Reply to
John Woodgate

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

Reply to
Richard Hosking

The carrier frequency (in this case 38kHz) should always be about 50% duty cycle. Assuming you are using a standard receiver IC, then you will not likely get any benefit from shrinking the carrier frequency duty cycle. In fact you may end up with less sensitivity even if you are pulsing the LED(s) with more current.

Normal receiver ICs expect the carrier to be modulated on and off at some lower frequency. If you continuously run the LEDs at 38kHz the receiver IC will eventually lose sensitivity. 38kHz is around the frequency of some compact fluorescent lamps, so they could easily interefere with your signal if the receiver IC was sensitive to continuous 38kHz. So instead devices like the TSOP1238 have automatic gain control elements that reduce sensitivity if the detected 38kHz light modulation is continuous (like it would be from fluorescent lamps).

Instead the data needs to be encoded somehow to avoid continuous transmission. The TSOP1238 datasheet suggests that the 38kHz carrier should be turned on for a minimum of ten 38kHz (50% duty cycle) cycles up to a maximum of seventy cycles. After that no signal should be transmitted for at least fourteen cycles of the carrier frequency before another pulse packet it sent.

Since by the sounds of your situation you don't need complicated data transfer, I suggest using ten or eleven cycle bursts of 38kHz @ 50% Duty to minimize LED stress and power consumption. A possible implementation might be something like follows: Have a free running continuous 38kHz +/-2% frequency tolerance 50% duty cycle oscillator. Have a switch debounce circuit for the trigger switch with the output feeding a one shot. The output of the one shot is programmed to last about ten (or better eleven)

38kHz cycles or in other words about 290us. Have the output of the one shot AND gated with the 38kHz 50% duty cycle oscillator. The output of the AND gate drives the LED (through suitable high current LED driver).

The net result is under normal operation the LED is not driven at all. Then when someone presses the trigger you get one set of ten or eleven LED pulses each lasting about 13us (with 13us gaps in between), and nothing else after that (until the trigger is manually pressed again).

This is the basic idea, but feel free to modify it to suit your needs better.

I don't understand your description of your constant current source, so I can't make comments as to its feasibility, but using a constant current source for driving the LED seems rather more complicated than needed. Assuming you have a regulated supply voltage (5V or more would be nice), what is wrong with using a simple resistor to limit the current?

What is your power supply (batteries? regulated? voltage? current capability?)? An eleven pulse packet consisting of eleven 13us 1A pulses requires 143 microcoulombs of charge (Q=I*t). If this is delivered entirely from a capacitor, then you can approximate the voltage droop for various capacitances using the formula Q=CV. For example if we allow a 250mV sag when drawing 143 microcoulombs, then we need 0.000143 = C * 0.25, so a capacitance C of 572uF. The ESR should be low enough that the output voltage of the capacitor doesn't sag unreasonably at 1A peak current. If you use a resistor to limit the current then you can simply make it smaller to compensate for whatever ESR the capacitor has to enable 1A peak current pulses. None of this is too precise, but neither does the LED need precise current levels. Any standard 470uF or 680uF capacitor or so from 5V should function quite nicely. If the power supply is particularly stiff and easily able to cope with fast 1A+ pulses with minimal droop, then no extra capacitance is strictly required (though a small amount may still be desired for minimizing inductance based overvoltage effects).

Reply to
Fritz Schlunder

Thanks for this At present I have a current source (PNP transistor) from + supply to diode to FET logic level switch to ground. Assuming I wanted a 1A pulse to the diode at about 5-10% duty cycle at 38 KHz, this would mean a 1-2 usec pulse. I presume you would have to use a fairly fast RF transistor with a suitable current rating as the current source with a low ESR capacitor (say ?4700uF) across the supply close to the circuit. The FET swich should handle the situation OK. I would use the PNP current source with emitter resistor of 0.6 ohm and two diodes to base from + supply

- would this work or is there a better solution? What devices would be suitable for this, and what other considerations?

Thanks for any ideas Richard

Fritz Schlunder wrote:

Reply to
Richard Hosking

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