水活性报警器

从国外淘来的，稍微用谷歌翻译了下

  该电路采用了555定时器作为一个非稳态振荡器有线和发射器的BC109C电流供电。在干燥条件下，晶体管的偏置电流和没有被完全关闭。但由于探针弄湿晶体管将行为和敲响警钟。
      一个开/关开关记得提供及使用探针接触非活性金属。黄金或从旧的继电器触点镀银可以使用，但一个廉价的替代方法是电线从veroboard一块备用铜带。这些将最终氧化超过，而且是非常小的电流是在基电路流动，较高的阻抗由氧化引起的并不重要。没有基电阻是必要的，因为在晶体管发射极跟随器，电流限制，即于发射器（振荡器电路）阻抗。
      The circuit 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. However as the probes get wet the transistor will conduct and sounding the alARM.
      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).