Circuit Lake

Electronic Project and Circuit Collection

Septic Holding Tank Monitor

03/31/2010 Category: AVR, Microcontroller, Monitoring, Project, Sensor

The purpose of the project is to monitor and display the water level in a 3000 gallon septic holding tank. The system consists of two main parts: the sensor unit and the display unit.

Waste Holding Tank

An ultrasonic sensor is used to detect the water level in the tank. It has an analog output that is proportional to the distance between the sensor and the water level. The sensor measuring distance in this case is about 8 1/2 feet when the tank is empty. An Atmel AVR ATmega8 microcontroller generates a trigger pulse for the sensor, allows the sensor to make the distance measurement, and then reads the sensor analog output voltage. The analog value is then transmitted via an RF link to the display unit.


The display unit uses an LCD to display a “percent full” reading, as well as a simple gas gauge type of indicator. A vertical marker moves between the E and F to show how full the tank is at a glance. See Photo 2. The float status is also shown (Full or Not Full). Other data is intended to be displayed such as time and date as well as an indication of remaining tank capacity (in days).

An RF receiver outputs the sensor data and float status to the serial port of the ATmega8 in the display unit. One character position on the display shows changing data to indicate that data is being received via the RF link. A real time clock can be included to display current time and date (not implemented at this time). The septic holding tank monitor is designed by Dave Filicicchia from U.S.