I'm building a laser-based beam break detector for sports timing applications. The idea is to use a modulated infrared laser, reflect it off a retroreflector, and receive the modulated return signal on a photodiode tuned to the same frequency.
However, I'm facing a few challenges and would love some input:
1. Finding a suitable laser module
I’m looking for a laser diode (preferably 850–950 nm) that supports external modulation—ideally square-wave up to a few kHz. It should be compact and reliable over 15–20 m range, and available from standard distributors like Mouser or Digikey. I haven’t found anything modular that meets these needs. → Any suggestions for specific modules or part numbers?
2. Optical setup
To maximize range and reliability, I’m planning to add:
A UV/IR filter to reduce ambient noise.
A collimating lens for the laser to reduce beam divergence.
A focusing lens in front of the photodiode to collect and concentrate the returning beam.
Is this a good optical stack for such a setup? Are there any better approaches for maximizing signal strength from a reflected beam at that distance?
3. Alignment issues
Since this system will be used outdoors and repositioned frequently, manual alignment becomes a major bottleneck. → Is there a trick or design approach to make alignment faster and easier?
Any ideas, component suggestions, or references would be hugely appreciated. Thanks!
All much more expensive than you may appreciate. Getting something like that going indoors over a shorter distance would be a big deal for someone in that line of work.
And "near IR", outdoors - a tweak bandpass filter for that - gosh.
Suggest you experiment with a regular laser Diode.
At the receiver, have the laser diode modulated beam feed down a 1-2 foot tube (black inside coating).
The beam strikes an LDR, output goes to an AC coupled Comparator demodulation circuit.
A laser is the wrong choice for this.
A common 5mm IR diode and a 3-pin receiver is much easier to use.
Most think that you need the narrow beam of a laser for a tripwire, which is not true. A common IR diode has the same narrow beam. Because it's about shadows, not light. This setup did more than 60m, without optics, with a beam no more than 5mm thick.
Leo..
Thanks for your setup, but I’m a bit confused. Your setup works with a retroreflector, right? How do the LED and receiver avoid interfering with each other, being so close and without any physical separation?
Retroreflector should work the same as two separate units, albeit with more losses.
Just mount LED and receiver close together in separate black tubes.
I could power my separate transmitter from a 1Ah Lipo for more than a day.
It was a simple NE555 (smd) 38KHz oscillator, followed by a 120mA (peak) constant current LED stage. Two vertical LEDs in series to also eliminate false (insect/dust) detection.
Leo..
It doesn't matter which LED you use for a tripwire.
They all result in the same 5mm diameter shade spot.
The advantage of a narrower beam Led is the higher efficiency (radiant intensity).
You get more distance with the same LED current.
The disadvantage is that aiming becomes a bit harder, albeit not as bad as with a laser.
Make sure the wavelenght (nm) is the same as the receiver.
Perfect tuning of the transmit frequency is also key for a good distance.
Leo..