I need some examples of simple circuits that use sensors for temperature, proximity, light, pressure, smoke, etc, and their explanation, thanks

 

Hello there. the examples come in a lot of flavors, that is... many versions of the same thing.
A sensor is a device that measures the physical quantity (i.e. Heat, light, sound, etc.) into an easily readable signal (voltage, current etc.).
The transducer is a device which, when actuates transforms the energy from one form to another.
The photo sensor senses the infrared or ultraviolet light.

In the left circuit, in the dark, the phototransistor is very high impedance, making Vout close to Vcc, and dropping as more light is detected. In the right circuit, Vout is close to GND, going up as more light is detected.

An optical smoke alarm (also called photo-electric smoke alarm) works using the light scatter principle. The alarm contains a pulsed Infra red LED which pulses a beam of light into the sensor chamber every 10 seconds to check for smoke particles. This optical smoke alarm (Fig. 1) is built around opto-interrupter MOC7811 (IC1) and dual op-amp LM358 (IC2).
When a fire breaks out smoke will enter the optical chamber through the opening vents. When thick black smoke enters in the gap (slotted area) of the opto-interrupter sensor (IC1), its internal photo transistor stops conduction. The smoke particles cause the Infra red light to be scattered onto the photodiode light receptor. As a result, output of the dual op-amp goes low. So transistor T1 stops conduction and T2 starts conducting. This makes the piezo buzzer (PZ1) sound an alarm and LED1 to glow.

Potmeter VR1 controls the intensity of the internal LED of MOC7811. Potmeter VR2 is controls the reference voltage at pin 3 of IC2.
The circuit works off a 9V battery