Issue 148 November 2002
Ultrasonic Homing Device

Start • Reveiver • Ear Mounting • The Transmitter • Software • Options • Sources and PDF

Much like animals use stereo sound detectors (commonly called ears) to home in on a sound’s source, the simple and low-cost circuit described in this article can give your robot a pair of ultrasonic ears, allowing it the same ability.

The ultrasonic transmitter might be fixed at the robot’s home port, where it returns for battery recharging, or it might be part of a moving target that your robot can follow around. You could also impress your friends by sticking the transmitter in your back pocket so your robot, and maybe your dog, can follow you around.

The idea behind this device is a fairly simple one that takes advantage of the fact that sound travels slowly through air, certainly in comparison to light. At 74 µs per inch (i.e., sea level), it’s slow enough for simple timing circuits to measure how long it takes to get from the transmitter to the receivers in the left and right ears. Unless the transmitter is directly ahead of both receivers, wherein they would both receive the ping simultaneously, one receiver has to be closer. The robot can differentiate between the two.

Synchronization is difficult because the remote transmitter’s ping is not directly connected to the receiver’s timing circuits. The solution is to have the transmitter send pings at a regular interval, which are then used in groups of two, as shown in Figure 1. The first ping is detected by the left ear and used as the sequence start marker. After this synchronizing ping is detected, a blanking period occurs before a window is opened, allowing for the detection of the next ping by the receiver closest to the transmitter.

Figure 1—The left ear is used to synchronize with the transmitted pings. Then, whichever ear hears the next ping is the ear closest to the transmitter. If they both hear the ping at roughly the same time, then the transmitter is more or less directly in front of both ears.