How could I design a circuit for a solar panel so that it only produces output from a red laser?

Question

I'm designing a little laser tag system with some arduino's as a little project for Christmas break and I'm having trouble figuring out how to make my solar panel receive information from my laser only, and not from ambient light sources. Right now, I have been able to code the arduino to modulate the laser and carry a message. I can decipher this message with the solar panel and another arduino in complete darkness obviously, but I'm having trouble as to how to make my circuit sensitive to my laser in daylight. I've heard about band pass filters, but I'm not sure I really understand how I am suppose to implement them. I'm sorry if I sound like a total nube!

Answer 1

Firstly, "laser" tag doesn't (usually) use lasers - and for good reason:

Lasers are dangerous and can blind people. Use with caution.

Laser tag normally operates using a focused infra-red beam. Basically a powerful and focused infra-red remote control (like you have for the TV). This is usually backed up with normal focused white light (from a white LED) for targeting. Yes, a weak laser beam could be used instead of the white light, but it's not the laser that usually carries the information.

You transmit by using a pulsed square wave (the length of pulses and gaps, aka marks and spaces of the signal indicate 0 and 1).

The carrier frequency for the IR transmission varies, but on the laser-tag systems I repair on a regular basis (and have built extra parts for) runs at 50KHz. TV remotes run, usually, at 36KHz (the higher frequency was chosen so that you couldn't cheat by bringing a remote control with you to a game).

The receiver, which is made from a couple of infra-red photodiodes, uses a high-pass (or band-pass) filter tuned to the carrier frequency, along with an amplifier, to isolate the 50KHz (or whatever your chosen frequency is) light signal to recover the on-off pulses. Basic systems don't bother about encoding or decoding information - the mere presence of these pulses over a minimum time frame is interpreted as a "hit". More complex systems include the ID of the gun that has made the hit for score tallying and other statistics.

_{(Note: this is from first-hand experience of one system alone. There may be other systems out there that do things differently.)}