Moisture Alarm with the 741 IC (ART428E)

This circuit triggers when the humidity detected by a sensor causes its resistance to decrease, triggering the oscillator. Them it emits a sound in a speaker.

In the figure we then have the complete moisture-activated alarm circuit in which the sensors consist of no more than two wire screens separated by a piece of dry cloth. The screens should be kept tight on this dry fabric which should also be porous.

When water falls on the fabric, its resistance lowers rapidly by firing the circuit (figure 1).


Figure 1 - The circuit
Figure 1 - The circuit



In figure 2 we have the sensor that is made with screens and a piece of porous fabric.


Figure 2 - The sensor
Figure 2 - The sensor



CI - 741 or equivalent

R1 - 100k - trimpot

R2, R3 - 100k x 1/8 W- resistors (brown, black, yellow)

R4 - 100 ohms x 1/4 W - resistor (brown, black, brown)

C1 - 0.1 uF - polyester capacitor

C2 - 100 uF x 16 V - electrolytic capacitor

FTE - 8-ohm speaker

Miscellaneous: sensor (see text), printed circuit board, symmetrical source, wires, solder, etc.


Circuit Bench