I intend to design laser light barriers which are going to start a timer when a car passes trough the first one and stop it when the second one is passed.
My problem know is that this has to work outdoors, which means the ambient light will vary greatly. So my idea was to modulate the laser beam at a fixed frequency and make the receiver only react to said frequency.
But I'm kind of clueless how to do that. I know that there has to be a more or less simple solution to this since commercial products all do it that way. Even the IR receiver in DVD players an stereo system all work like this.
So basically, I need a circuit that takes in a square wave signal(the modulated laser beam) with varying DC offset plus noise (ambient light) and gives a logic signal based on whether said square wave is present or not. The modulation frequency is not yet fixed since I'm going to use an MCU to generate it.
Can somebody please point me in the right direction? Googling it did not reveal much useful information, probably because I did not use the right terms.
Thanks in advance,