This simple water alarm circuit is a simple circuit that used to indicate level of drying in some place. The simple water alarm is based on 555 timer IC and a transistor. The figure show how is the circuit work. 555 IC is used to be an unstable oscillator and the transistor powered the circuit using emitter current. Transistor that can be used is BC109C. This circuit can controlled by automatic if connected with microcontroller. Principle work of this circuit is when under dry conditions, the transistor will have no bias current and be fully off. As the probes get wet, a small current flow between base and emitter and the transistor switches on. A larger current flows in the collector circuit enabling the 555 oscillator to sound.
An On/Off switch has provided and remembered to use a non-reactive metal for the probe contacts. Gold or silver plated contacts from an old relay may be used, however a cheap alternative is to wire alternate copper strips from a piece of ferro board. These will eventually oxidize over but as very little current is flowing in the base circuit, the higher impedance caused by oxidization is not important. No base resistor is necessary as the transistor is in emitter follower, current limit being the impedance at the emitter (the oscillator circuit).
An On/Off switch has provided and remembered to use a non-reactive metal for the probe contacts. Gold or silver plated contacts from an old relay may be used, however a cheap alternative is to wire alternate copper strips from a piece of ferro board. These will eventually oxidize over but as very little current is flowing in the base circuit, the higher impedance caused by oxidization is not important. No base resistor is necessary as the transistor is in emitter follower, current limit being the impedance at the emitter (the oscillator circuit).
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