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 555应用集锦 | | 全面的555电路应用电路。 | | 555新颖的长定时电路 | | 555新颖的长定时电路 | | 555太阳牌汽车报警器电路 | | 555太阳牌汽车报警器电路 | | 555机动车方向灯电子闪烁器电路 | | 555机动车方向灯电子闪烁器电路 | | 555高压延时接通电路 | | 555高压延时接通电路 | | 555电热毯温控器电路 | | 555电热毯温控器电路 | | 555时基电路构成的脉宽调制电路 | | 555时基电路构成的脉宽调制电路 | | 555时基电路构成的脉冲位置调制电路 | | 555时基电路构成的脉冲位置调制电路 | | 用555制作的多波形信号发生器 | | 用555制作的多波形信号发生器 | | Water Activated Alarm | | Water Activated Alarm
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).
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