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Moisture Detector
This unit is designed to detect moisture (water), such as in basements, other household areas etc. Because of the 9 volt supply, It can be safely used around humans without fear of shock. The battery should last for about two years because during non activation, it is drawing zero current. An advantage of this circuit is that any number of detectors (the smaller PCB) can be connected in parallel to the input, thus monitoring many areas at once. The circuit itself, uses two 2N2222 transistors connected directly to one another. This is called a “Darlington Configuration”, and produces a tremendous amount of gain (sensitivity) . Actually any BJT transistors with a Beta (forward current gain) of 100 or more will work just fine in place of these. Regarding the 2N2222’s, if you use the newer versions which are plastic, they also will work just fine.

How it Works:
When any moisture contacts the sensor, a high resistance is detected which passes a small current through R2 to Q1. R2 and C1 ensure that no interference will enter Q1. When Q1 turns on, it immediately turns on Q2, which then activates the Piezo buzzer. During non activation, both transistors are off and there is zero current demand from the battery. When the buzzer is activated, the current is still only 6 mA. I would suggest that you tin (solder) all of the copper on the sensor PCB. This will reduce the effects of the copper oxidizing and becoming non conductive. Should you need an alarm that is louder, connect two buzzers in parallel and use two 9 volt batteries in series to produce 18 volts instead of just 9 volts. The discordant sound of the two buzzers and the increased voltage is quite noticeable. The main printed circuit board is 3 1⁄4 inches wide and 5 inches long. The smaller, “sensor” printed circuit board is 3⁄4 inches wide and 2 1/8 inches long. Also note that a push button test switch is included that connects a 470 K resistor to the input, thus simulating moisture.
Parts List:
R1, R3 470K Ohms Resistors SW1 (Push Button) SPST Switch
R2 15K Resistor SW2 (Toggle) SPST Switch
R4 22K Resistor B1 9V Battery
C1 .022 uF Capacitor T1, T2 Input Terminals
PB1 (RS273-059) Piezo Buzzer Q1, Q2 2N2222 Transistors
Printed Circuit Boards:
Home | High Selectivity Crystal Radio with Short Wave | High Sensitivity BFO Metal Detector
Battery Life Chart | Parabolic Microphone | Ultra High Gain Audio Amplifier | Audio Sine Wave Generator
One Tube DX Receiver | Telephone Line Use Monitor | Moisture Detector | Vacuum Tube Amplifier