This simple humidity sensor circuit using a 555 timer, shows us the humidity level that a ground under test has. To achieve the goal, a couple of probes are used.

These probes are placed on the ground to be tested.

This Twilight switch circuit activates a light, a lamp, a motor, etc., at the time of sunset and performs the reverse process at dawn.

This is an ideal circuit for activating a security light that must be on all night.

This darkness detector circuit beeps when the light drops below a certain level.

The main elements of this circuit are the 555 integrated circuit, working as an astable multivibrator and an LDR which is used as darkness sensor.

Lie detector circuit with two transistors

This simple lie detector consists of two transistors and other additional components and can detect if a person does not tell truth (the person is lying), using two sensors placed directly on the skin.