555 IC Water Level Sensor/Detector Alarm

This schematic diagram below shows a 555 IC water level sensor/detector alarm circuit. This circuit is powered through the emitter current of the BC109C. 555 timer is used as astable oscillator in this circuit. When this circuit is under dry conditions the transistor will be fully off because it have no bias current. when the probes get wet, a small current flows between emitter and base then the transistor switches on. The 555 osillator is enabled to sound because of a larger current flows in the collector circuit. Here is the schematic diagram of the circuit:

An On/Off switch is provided and use a non-reactive metal for the probe contacts. We can use Gold or silver plated contacts from an old relay. But, a cheap alternative is to wire alternate copper strips from a piece of veroboard. These will eventually oxidize over, but the higher impedance caused by oxidization is not important because very little current is flowing in the base circuit. the transistor is in emitter follower, so no base resistor is necessary. The current limit being the impedance at the emitter. [Circuit’s schematic diagram source: www.zen22142.zen.co.uk]