Taking advantage of the high input impedance of the operational amplifier 741 and its high gain we can elaborate a simple circuit of a humidity sensor as shown in the figure. When the resistance in the sensor is high, indicating that the ground or place where it is located is dry, one of the LEDs connected to the amplifier output remains on and the other is off. When the resistance in the sensor drops, in the presence of humidity, the circuit switches and the LED that was on, goes out while the LED that was off, lights up. The trigger point depending on the type of sensor used is set in the 10k ohms P1 trimpot. Note that this circuit does not need a symmetrical power supply.
