Sometimes sophisti-cated sensors based on video cameras, infra-red or ultrasound are not quite up to the job, and we have to resort to somewhat more primi-tive switch-type sen-sors. These work like an animal’s whiskers (or ‘antennae’ on insects), detecting nearby objects in the environment. When disturbed, a sen-sor sends a pulse to the robot to indicate that an object is present.
Sensors that are both sensitive and robust can be made usig sinle guitar strings. The material is very flexible as well as being conductive. The idea could not be more sim-ple: we pass the wire through a metal tube, and when the wire is in contact with an object in the environment it bends and the two make electrical contact. Responsiveness and sen-sitivity depend chiefly on the length and stiff-ness of the piece of gui-tar string used. Do-it-yourself construc-tion should not present great difficulties even to the most mechanophoic reader. See figure 1 we proceed as follows.
1. Cut a length of steel guitar string (8 an to 10 cm is enough), and saw off a length of about 2 cm of 4 mm diameter brass tube. Deburr the edges inside and out. 2. Solder a wire to one end of the brass tube and another wire to the end of the guitar string. Insulate the joint on the guitar string using heatshrink tubing. 3. Slide the string into the tube so that only about the first 10 mm from the end of the tube is insulated. Fix the string centrally within the tube using hot-melt glue. Be careful not to allow too much glue to run down inside the tube.
The result should look like Figure 2. Of course, you are free to experiment with variations on this construction! As we have described it this robot whisker is essentially just a simple switch contact. To ensure that even the gentlest collision does not pass unnoticed we recommend that you use the whisker to trigger a flip-flop as shown in Figure 3. The microcontroller in the robot can then read the state of the flip-flop at its leisure and then reset it.