Ldr sensors are sensitive to light sticking. The stronger the light, the smaller the resistance value; the darker the light, the greater the resistance value. So we can use this characteristic to construct the photosensitive circuit, and cooperate with relay/triode/MOS transistor to realize the photoswitch. Next, the circuit of realizing photoelectric switch by using ldr sensor, transistor and relay is introduced.
The working principle of ldr sensor is based on the internal photoelectric effect. The ldr photocell is very sensitive to light. Without light, ldr sensor presents a state of higher resistance value. Under strong light, its resistance value decreases rapidly, and presents a lower resistance value. The following is the physical diagram and characteristic curve of the ldr sensors.
The circuit of the photoelectric switch is mainly composed of ldr sensor, sliding rheostat, transistor and relay. The ldr photoresistor and sliding rheostat are connected in series, and the common point is connected to the base of NPN triode through current limiting resistance. The collector of NPN is connected to the base of PNP through current limiting resistance, and the relay is connected to the emitter of PNP for switching output. In the dark situation, adjusting the sliding rheostat reduces the resistance until the relay is in action. When the illumination is very strong, the resistance of the ldr sensors begins to decrease. When it is small enough, the NPN transistor can not be turned on. Therefore, the base of the PNP transistor is in the high level, which leads to the cut-off situation of the PNP transistor, so the relay resets.
When the lighting lamp is connected to the output terminal of the relay, the light can be automatically lit when the light is dark, and the light can be automatically extinguished when the light reaches a certain intensity, thus realizing the function of the light-controlled switch.
In addition to using triodes, comparators can also be used. The comparators need to be compared between two input terminals.