I've developed a 38 kHz IR beacon for the CheapBot-08. It uses a 556 timer IC to create and combine two square waves. Half the 556 generates a 38 kHz square wave and must be adjusted for the proper frequency (this accounts for the tolerances in the resistors and cap). The second half of the 556 generates a roughly 10 Hz square wave. Combined, the output is a 10 hZ square wave of the 38 kHz beacon. Shutting off the 38 kHz beacon every 0.1 seconds prevents the robot's 38 kHz detector from losing senstitivity to 38 kHz.
I thought I'd just put the 556 on the o'scope in class and tune it up (adjust thwe PCB's pot to get the proper 38 kHz pulse). But I had forgotten that the o'scope would trigger on any signal, whether it was from the 10 hZ of the 38 kHz square wave.
I finally tapped into pin 5 of the 556 to see just the 38 kHz signal. It looks like on the next design, I'll add a test point to the PCB to make tuning the 38 kHz side easier.