SIMPLE INFRARED SENSOR

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The simplest infrared sensor, which will report the presence of an obstacle, can be done with just one transistor. This homemade product has more likely not practical application, but rather theoretical, demonstrating the operation of the infrared sensor of the presence of an obstacle. Of course, no one bothers to make a practical application, say, when building simple robots.

Infrared obstacle sensor circuit

The operation of the circuit is very simple. An infrared LED emits infrared radiation in a spectrum invisible to the human eye. If an object appears in the path of radiation, then infrared rays begin to bounce off the object and return back towards the LED. A trap for these rays is an infrared photo element (IR photodiode). When reflected rays hit it, its resistance decreases. As a result, the current in the base circuit of the transistor increases and the transistor opens. The load of the transistor is a blue LED that starts to glow. You can connect a buzzer to the output and hear a beep.
If there is no obstacle to the sensor, then the rays are not reflected and the transistor does not open.
The transistor can be taken any, the same structure, you can Soviet KT315 or KT3102.

Sensor assembly

The circuit is assembled by hinged installation. No configuration required - works right away. I am powered by a 3.7 V battery.

Infrared sensors use robotic vacuum cleaners, various control systems, in a conventional printing printer, there are always a couple of these, or even more, etc.