More  Alarms  Water Level Monitors

Water Cistern Level Alarm -- April 25, 2014

I received an unusual request from a Discover Circuits visitor a while back.  The guy had a deep water cistern which has to be filled when nearly empty. The cistern was described as a deep cylindrical tube, holding 1000s of gallons of drinking water.  Periodically, the cistern is filled from a tanker truck, when the water inside reached a critical level.  The guy wanted a simple circuit he could build which would flash an LED when the water level reached a certain depth.  That flasher would tell him it was time to order more water.  The circuit I came up with is shown below. 

A simple water sensor is lowered into the cistern to the desired depth.  The sensor has two stainless steel electrodes encapsulated in a small plastic potting shell.  A few steel washers inside would weigh the device down, so it would remain immersed in the water.  A two conductor cable connects the sensor to a plastic box at the surface.  The cable length is carefully measured so it would turn on a flashing LED when the water level dropped below the sensor.

The circuit is powered by two AAA alkaline calls forming a 3v DC supply.  The circuit uses a single 74HCT14 hex Schmitt trigger inverter IC.  When water is sensed, a 0.5Hz oscillator is kept off.  When water is no longer sensed, the oscillator is activated.  The output of the oscillator triggers a 20ms pulse generator which feeds an n-channel FET which drives a super bright red LED.  The overall current during normal operation is very low so the battery should last for many years.

Deep Water Cistern	Serpac 211 Box with Battery Compartment
Water Tanker Truck

Click on Drawing Below to view PDF version of Schematic