Issue 148 November 2002
Ultrasonic Homing Device

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

SOFTWARE

Each synchronize-then-locate cycle begins by initializing the counters that keep track of how many times each ear has had a valid detection out of the total amount allowed for that cycle. The example we’re currently analyzing has it looking for four ping detections out of seven tries. After a delay of about 6 ms, the sync routine waits for detection by the left ear. When it’s received, it becomes the reference that the next detection is compared to.

After the sync pulse is detected, another 6-ms delay is executed (see Figure 1). After the blanking period, the ears become active and the software loops, waiting for an ear to hear a ping. However, it loops 256 times, as controlled in the chktim routine, so it doesn’t get caught in an endless loop in case the signal is lost. If no ping is heard and the 256 loops are used up, the program returns to the beginning of the cycle and resets.

If an ear hears the second ping, a pause is inserted, and the other ear is also checked to see if it too heard the ping. The pause allows the other ear to hear the ping, though not necessarily at the exact same time, but rather within a period determined by the pause. That way, if the transmitter is roughly—though not exactly—in front of the ears, both ears will hear the ping at approximately the same time. This is evaluated as equal.

If after the pause the other ear does not hear the ping, the ear that did hear it is determined to be closest to the transmitter and its ping-detected counter decrements down from four. If the count is not zero, it means that enough pings haven’t been detected during this cycle. Each loop cycle causes the loop total counter to decrement before the program runs another sequence. This repetition occurs until either four ear ping detections have been made, in which case the pings detected counter will count down to zero, or seven attempts were made after which the loop total counter will count down to zero.

If the loop total count reaches zero before the ping detection count, then there were not four successful detections in the allowed seven tries. The program then resets and recycles. However, if the ear does have four detections, it will indicate so by setting its I/O bit high and then returning to reset and recycle. You may download the software listing for this project from the Circuit Cellar ftp site.