An ultrasonic system will be used to measure the level of grain from 1 to 9 m in a bin, as shown in given Figure. This will be done by measuring the time delay between transmitting a short ultrasonic pulse and receiving the pulse echo from the grain surface.
A 50-kHz transmitter can be triggered on and off by a logic high/low input, as shown. The transmitted pulse is to have a duration of , and the propagation speed is 300 m/s. The receiver is connected to a comparator that goes high when a signal is received. The receiver must be disabled while the transmitter is sending a pulse using a logic high input, as shown. An 8-bit counter will start counting when the pulse is sent and stop when the echo is received. Its count is thus a measure of the time delay and the grain level. A reading is to be taken every second.
a. Calculate the expected time delay for grain levels between 1 m and 9 m.
b. Since the ultrasonic system measures from the bin top, the time delay will be reversed (i.e., long time is low level and short time is high level). To account for this, the counter will count down from some preset value. Determine the preset value and the correct counter clock speed so that the digital display is equal to the grain level (i.e., a level of 4.5 m would produce a binary count of , so the display would show 45).
c. Use one-shot pulse generators, flip/flops, and any other digital logic devices to complete the design shown in Figure.