Piezo crystal monitors liquid level

J Jayapandian, IGCAR, Tamil Nadu, India -- EDN, 3/30/2000

The simple and inexpensive circuit in Figure 1 monitors the liquid level in a container. The piezo crystal, carefully mounted at the bottom surface of the container, receives it activation from the 74HCT14 hex Schmitt trigger. The crystal generates stable clock pulses according to its specification (for example, 6.14 MHz) when it is in free air. The crystal-based clock drives the 0th counter in an 8254 programmable-counter/timer chip, programmed in Mode 0 as an event counter. The first counter of the 8254, programmed in Mode 1 as a retriggerable one-shot whose time period is 1 sec, controls the gate of the 0th counter. The first counter allows the 0th counter to count for a period of 1 sec. The counts in the air medium serve as a reference. Depending on the height of the liquid level, the pressure acting on the surface of the crystal increases, thereby reducing reference-crystal clock frequency. The variation in clock frequency with respect to the air-medium reference gives you the height of the liquid level. (DI # 2507)