The picture above is a 2 Watt Blue Laser I built several months ago. It's way too powerful for robot communications around the house (it would start fires), but it looked like a good enough illustration for this page. :-)
All right, this is intended to be a somewhat comprehensive write-up concerning different ways to talk to your robot. Whether it will be all that comprehensive remains to be seen, but let's get started.
Okay, first up, let's investigate infrared. We humans cannot see infrared, but that doesn't prevent our robot from doing so. Some of the microcontroller chips already come equipped with the ability to decode infrared signals from something like a TV remote. (I'm thinking of picaxe here.)
However, what I want to show is simple schematics for circuits you can easily add to your robot. Let's call it modular design. First, here is a very simple sketch that shows how to drive an infrared LED. The input marked with the letter S must be driven with a different circuit such as a microcontroller. The benefit of using this circuit, is that it pulls much less current from the microcontroller output pin (Specifically, only half a milliamp).
But what if we want to make one or more of these infrared beacons, without using additional microcontrollers? Isn't there a cheaper way? Yes. I have drawn a couple different possibilities, which you could use on your robot. Here is a circuit using a 555 timer chip.
"How can I use that to send data to my robot?" That is an easy fix. One way would be taking the 555 reset lead off of V+ and switching it on and off with something capable of sending data, such as a microcontroller. (All you need is a small one like an 8 pin chip). The data or code coming out of a microcontroller would be input at the lead tied to pin 4 of the 555, with the 1 KΩ resistor there to limit the current. [Another alternative for inputting a modulation signal would be to leave pin four tied to the positive supply voltage and using pin five instead.] This was not done because removing the capacitor from pin 5 might affect the stability of the oscillator.
The 555 chip is nice inasmuch as it will drive several LEDs. What if you don't have a 555? Do you have a logic gate like a 7400, 7402, 7404 or others? There are a lot of different types such as 74LSxx, or 74HCxx and others. Don't let that confuse you. Any of them will work in the following circuit.
And to wrap up this page showing IR or Visible light signalling. here is one more pulser circuit and a detector circuit using the 38 kHz detectors which are available many places for low cost.