The circuit was designed sensitive enough to trigger and activate the alarm upon sensing wetness, moisture or dampness on its probes.
Terminology 555 Timer – an 8-pin electronic device used in several mixtures of applications involving multivibration and timingOperating Modes:BC109C – an epitaxial NPN silicon planar transistor having the collector electrically connected to the TO 18 metal case Circuit Explanation
The operation of the circuit depends on the astable mode of the 555 timer, which is driven by the emitter current coming from BC109C transistor, since this transistor has high gain. Being in astable mode, the IC functions as an oscillator or as a free running mode. To have the 555 oscillator in full operation of sounding, it needs to be triggered or enabled by a larger current that flows in the collector circuit. This is made possible when the transistor is switched on by the small current that flows between the base and emitter because of the moisture or wetness sensed by the probes. On the other hand, the transistor will remain OFF and without any bias during dry conditions.
To control the conduction of the circuit, a switch for ON/OFF state is supplied. The probe contacts should be made of non-reactive metal so that there will be no capacitance or impedance present. It can be achieved by salvaging silver or gold plated contacts from old relays. A cheaper alternative is wiring alternate copper wires. Since the transistor is in emitter follower mode, the impedance at the emitter or the oscillator circuit acts as the current limit so placing a base resistor is no longer required.
When choosing the resistance rating of the loudspeaker, it is important to consider that an 8 ohm speaker can damage the 555 IC if directly driven by the IC with a 9 V supply because the maximum allowed output current is 200 mA. To avoid this, a 10 ohm resistor can be connected in series with the 8 ohm speaker or might as well use an 18 ohm loudspeaker.
In the market, one of the most effective electronic devices is the 555 timer. When it operates at monostable mode, it may be utilized in pulse width modulation (PWM), capacitance measurement, frequency divider, touch switches, bounce free switches, missing pulse detection, and timers. When operating in astable mode, it may be utilized in pulse position modulation, security alarms, tone generation, logic clocks, pulse generation, lamp flashers, and light emitting diode. When operating in bistable mode, it may be utilized in bounce free latched switches, pulse-type waveform processing, and railroad track designing.