DIY LIGHT SENSOR - ELECTRONICS

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Sometimes such situations arise when you need to turn on the light in the room on and off at dawn every day at dawn, i.e. to imitate daylight hours in any closed room. This may be required, for example, when growing plants or keeping animals, where precise adherence to the day / night regime 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 turn on the lighting will not cope with the task properly. An ambient light sensor, or, more simply, a photo relay, comes to the rescue. This device registers the intensity of sunlight falling on it. When there will be a lot of light, i.e. the sun will rise, the log will be established at the exit. 1. When the day comes to an end, the sun will go beyond the horizon, there will be a log at the exit. 0, the lights will turn off until next morning. In general, the scope of the light sensor is very wide and is limited only by the imagination of the person who assembled it. Often these sensors are used to illuminate the cabinet when the door is opened.

Light Sensor Circuit

The key link in the circuit is the photoresistor (R4). The more light hits it, the more its resistance decreases. You can use any photoresistor that you can find, because this is a fairly scarce part. Imported photoresistors are compact, but sometimes they are quite substantial. Examples of imported photoresistors are VT93N1, GL5516. You can also use domestic, for example, FSD-1, SF2-1. They cost much less, but will also work well in this scheme.
If it was not possible to get a photoresistor, but I really want to make a light sensor, then we can proceed as follows. Take the old, preferably a germanium transistor in a round metal case and cut off its top, thereby exposing the transistor's crystal. The photo below shows just such a transistor with a sawn-off cover.

It is very important in this case not to damage the crystal itself, tearing off the lid. Almost any transistors in such a round case will do, Soviet germanium ones, for example, MP16, MP101, MP14, P29, P27, will work especially well. Because Now the crystal of such a “modified” transistor is open, the resistance of the KE transition will depend on the intensity of the light incident on the crystal. Instead of a photoresistor, the collector and emitter of the transistor are soldered, the output of the base just bites off.
The circuit uses an operational amplifier, you can use any single, suitable for pinout. For example, the widely available TL071, TL081. The transistor in the circuit is any low-power NPN structure, suitable BC547, KT3102, KT503. It commutes the load, which can serve as a relay, or a small segment of an LED strip, for example. It is desirable to connect a powerful load using a relay, the diode D1 is in the circuit for damping the self-induction pulses of the relay winding. The load is connected to the output labeled OUT. The supply voltage of the circuit is 12 volts.
The value of the tuning resistor in this circuit depends on the choice of the photoresistor. If the photoresistor has an average resistance, for example, 50 kOhm, then the trimmer should have two to three times greater resistance, i.e. 100-150 kOhm. My SFD-1 photoresistor has a resistance of more than 2 megohms, which is why I also tuned in at 5 megohms. There are also lower resistance photoresistors.

Light Sensor Assembly

So, let's move on from words to deeds - first of all, we need to make a printed circuit board. There is a LUT method for this, which I use.
A file with a printed circuit board is attached to the article, you do not need to mirror before printing.
Download board:

pechatnaya-plata.zip 6.74 Kb (downloads: 214)

The board is designed to install the domestic FSD-1 photoresistor and a tuning resistor type CA14NV. A few photos of the process:

Now you can solder the parts. First, resistors, a diode are installed, then everything else.

Lastly, the largest parts are soldered in - the photodiode and the tuning resistor, wires for convenience can be output through the terminal blocks. After the soldering is completed, it is necessary to remove the flux from the board, check the installation, ring adjacent tracks for a short circuit. Only after that can you supply power to the board.

Sensor Setup

When you turn it on for the first time, the LED on the board either lights up or is completely off. Gently rotate the tuning resistor - in some position, the LED will change its state. It is necessary to install a tuning resistor on this line between two positions, and by closing or by flashing the photoresistor to achieve the desired threshold.

Visually, the operation of the light sensor is shown in the video. A shadow is created above the photoresistor, the light intensity decreases, the LED goes out. Successful assembly!