# Drive IR LED with maximal power (for IR barrier with max distance)

I'm making a light barrier and working on a "sender" part right now. The main thing I want to achieve is the maximum distance. I'm using Vishay TSSP4038 for receiving signal. In the datasheet it's stated that it can work on distances about 30m. I can't get even close. With 350mA (1W) LED it's about 3m max (outdoors).

Here is my current schematic:

T1 is NPN transistor. Q2 is N-channel mosfet. LED power schematic is taken from this instructable.

ATTINY25 turns the LED on/off 38000 times per second (kHz). So effectively I can double max current through LED (change R3 to 2.5). I also have some 1W chinese LEDs from ebay, which do work better (distance-wise), but I'm not sure how much more current I can give them without burning.If you think that the transmitter is good then should I provide receiver details ?

I would be very thankful if someone better than me in electronics would check the schematic and maybe give me some advice or tell if something is really wrong.

– Dave
Jan 2 '16 at 19:17
• Unfortunately, there is no info about it in 1W LED datasheet - shop.rabtron.co.za/datasheets/WW-P05IR1W.pdf In simple LED I got from local store, values are: 0.1A - 70mW/sr, 1A - 700mW/sr Jan 2 '16 at 19:24

As others have suggested, beam spread is likely to be your problem. Most digital cameras will detect IR to some extent. (Try pointing your TV remote into the lens of your camera and press a button.) You can try using this technique to see what's going on.

• Draw some concentric circles on a sheet of paper - maybe 1 cm apart - like a target.
• Set up your IR LED some distance from a sheet of paper - say 30 cm.
• Set up your camera / phone to view the sheet of paper. A quick test with my HiFi remote control showed that it does transmit through office paper so you could test from either side.
• Lights down, cool jazz music on.
• Using the camera, centre the IR beam on the target.
• Try putting any lenses you can get your hands on in between the LED and the target and see if you can focus the beam any better. Camera lenses, spectacles, magnifying glasses, etc. A quick web search for DIY concave lenses got many results including some for turning them out of acrylic rod in a drill chuck.
• The object of the exercise is to narrow the beam angle and reduce the diameter of the IR light spot.

Have fun.

• Thanks, I've ordered leds with narrower beam and will try if those will be working better. Jan 3 '16 at 16:25

The most obvious candidate is your LED. The LED specified in the TSSP4038 has a half-power angle of 17 degrees, and is being driven at 200 mA, with a nominal 25 m detection distance. If your LED, for instance, has a half-power angle of +/- 60 degrees and is being driven at 100 mA, with the same efficiency, you'd expect a transmission distance of about $$D = 25\times{(17/60)}^2\times{(.1/.2)} =1\text{ meter}$$ which, frankly, works out much more precisely than I expected when I picked the numbers. So I'd guess that you need a narrow-beam LED. And boosting the current won't hurt, either, unless you kill the LED.