Water Activated Alarm

Posted on Feb 5, 2014

It uses a 555 timer wired as an astable oscillator and powered by the emitter current of the BC109C. Under dry conditions, the transistor will have no bias current and be fully off. As the probes get wet, a small current flows between base and emitter and the transistor switches on. A larger current flows in the collector circuit enabling

Water Activated Alarm
Click here to download the full size of the above Circuit.

the 555 osillator to sound. An On/Off switch is provided and remember 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 veroboard. 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).

Leave Comment

characters left:

Related Circuits

  • New Circuits



    Popular Circuits

    14V Switching Power Supply
    5 Watt audio amplifier
    8-Amp regulated power supply
    Touch Switch Monostable/Timer with 555 IC
    High current output switching power supply Schematic Diagram
    mobile phone battery charger circuit
    second generation reciprocating
    Freescale unveils highly integrated TPMS device
    sine wave oscillator
    Lantern controller circuit diagram of a two-dimensional
    OCL yourself feedback amplifier analysis