This week I’ve been experimenting with a very simple and cheap project for wireless transmissions, a lightwave AM transmitter and receiver based on Scott’s design, which was based on VK2ZAY’s design. In my final design I’ve increased the base biasing resistors to decrease the size of the coupling capacitor and also used a darlington transistor to get more current gain.
The transmitter is pretty straight forward, the input modulates the current passing by the LED, which modulates the intensity of light, if you’ve designed any class A amplifiers in the past you surely know how it works. The receiver is just a simple transimpedance amplifier, which is amplifying the signal quite a bit (~56x gain) since the transmitter will usually be a bit far from the receiver. You can do the same with a op-amp, but I much prefer a discrete circuit for these simple things.
You can put a buffer stage with a darlington emitter follower on the output of the receiver so you can drive a speaker directly. Something that I would recommend is to add a small (10x gain maybe?) pre-amplifier for the transmitter, that way you’ll get a bit more signal if you’re source isn’t very loud, specially if you want to drive some high power LEDs, since you have a lot of current headroom with those.
If you want to experiment with different values in a simulation, here is the LTspice schematic. The best way to choose the best LED + photodiode combination to maximize the range is to build some breakout boards that you can plug different LEDs and photodiodes until you have the perfect combination.