Light sensor

Sometimes there are such situations when you need to turn on the light in the room every day at dawn, and turn it off at sunset. mimic daylight inside any enclosed space. This may be necessary, for example, when growing plants or keeping animals, where precise observance of the day / night regimen is necessary. Depending on the time of year, the time of sunset and sunrise is constantly changing, which means that the use of daily timers to switch on the lights will not cope with the task properly. Comes to the aid of a light sensor, or, more simply, a photo relay. This device records the intensity of sunlight falling on it. When there is a lot of light, i.e. the sun will rise, a log will be set at the output. 1. When the day comes to an end, the sun will go beyond the horizon, the output will be a log. 0, lighting lamps will turn off until the next morning. In general, the scope of the light sensor is very wide and is limited only by the imagination of the person who has assembled it.Often, such sensors are used to illuminate the cabinet when the door is opened.

Illumination sensor circuit

Light Sensor
The key element of the circuit is the photoresistor (R4). The more light it hits, the more its resistance decreases. You can use any photoresistor that you can find, because this is a rather scarce detail. Imported photoresistors are compact, but sometimes they are quite significant. Examples of imported photoresistors are VT93N1, GL5516. You can also apply domestic, for example, FSD-1, SF2-1. They cost much less, but they will also work well in this scheme. If it was not possible to get the photoresistor, but I really want to make the illumination sensor, then you can proceed as follows. Take the old, preferably a germanium transistor in a round metal case and cut down its top, thereby denuding the transistor crystal. The photo below shows just such a transistor with a sawed cover. "Light Sensor It is very important not to damage the crystal itself by opening the cover. Suitable for almost any transistors in such a round enclosure, Soviet germanium will work particularly well, for example, MP16, MP101, MP14, P29, P27. Sincenow the crystal of such a "modified" transistor is open, the resistance of the Q-E transition will depend on the intensity of the light falling on the crystal. Instead of the photoresistor, the collector and the emitter of the transistor are soldered, the output of the base simply bites off. The circuit uses an operational amplifier, you can apply any single, suitable for pinout. For example, widely available TL071, TL081. Transistor in the circuit - any low-power NPN structure, suitable BC547, KT3102, KT503. It commutes the load, which can serve as a relay, and a small segment of the LED strip, for example. It is desirable to connect a powerful load using a relay, the diode D1 stands in the circuit for damping self-induction pulses of the relay coil. The load is connected to the output labeled OUT. The supply voltage of the circuit is 12 volts. The nominal of the trimming resistor in this circuit depends on the choice of the photoresistor. If the photoresistor has an average resistance, for example, 50 kΩ, then the trimmer should have two to three times more resistance, i.e. 100-150 kΩ My photoresistor SFD-1 has a resistance of more than 2 megaohm, so I also took a trimmer 5 MΩ.There are also lower impedance photoresistors.
Light sensor

Assembling the light sensor

So, let's move from words to deeds - first we need to make a PCB. There is a LUT method for this, which I use. The file with the printed circuit board is attached to the article, you do not need to mirror before printing. Download fee: The board is designed for installation of a domestic photoresistor FSD-1 and a trimming resistor of type CA14NV. Several photos of the process:
Light Sensor
Light Sensor
Light Sensor
Now you can solder the details. First, resistors, a diode are installed, then everything else.
Light sensor
Lastly, the largest parts are soldered - a photodiode and a trimmer resistor, the wires can be pulled out through the terminal blocks for convenience. After the soldering is completed, it is imperative to remove the flux from the board, check the installation, ring the adjacent tracks to the circuit.Only then can power be supplied to the board.
Light Sensor
Light sensor

Sensor settings

When you turn on the LED for the first time, the LED on the board will either be lit or be completely extinguished. Gently rotate the trimmer - in some of its position, the LED will change its state. You need to install a trimming resistor on this edge between two positions, and closing or vice versa by illuminating the photoresistor to achieve the desired threshold.

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