Nuts & Volts - May 2007

●●●●

Wouldn’t it be

great to detect a

flooding issue

before it’s

too late?

BUILD A FLOOD DETECTION

ALARM

Well, this device can

detect when rising

water occurs. The

circuit shown in

Figure 1 is a

schematic for a flood

detection alarm.

This project consists of a box containing circuitry and the water probe assembly. When water touches the probe assembly, an alarm will sound. A relay is provided to switch on a water pump. As long as the probe assembly is in contact with the water, the pump switch will remain on. The alarm can be shut off with a switch once the flooding has been detected. Optionally, a 12V lantern battery can be used as a back-up power source if the power from the wall transformer is interrupted.

Circuit Description

The circuit in Figure 1 consists of a 700 Hz oscillator, rectifier, threshold detector transistors, and drive transistors. U1, R1, R2, and C1 form the 700 Hz oscillator. This drives probe P1 on TB1. When water touches probe P1 and P2, P2 picks up the square wave. The signal at P2 is rectified by D1 and charges C4. Q1 is used as a threshold detector. If the charge on C4 reaches a threshold of approximately 600 mV, Q1 turns on. When Q1 is on, driver transistors Q2 and Q3 are also on.