Homemade light-controlled automatic switch circuit

During the holidays, enterprises and institutions all use colored lights to decorate and add to the festive atmosphere. The author was troubled by the problem of colored lights turning on late and turning off early. After reading the "Electronic News" and related books and looking for them in the market, all of them were time-delay control types. So I made a light-controlled automatic switch (without time delay). This switch has few components, simple circuit, and reliable operation. The circuit is shown in the attached figure.

Working Principle In the attached figure, the transformer secondary output 10V voltage is rectified and filtered, and then passes through the 7805 three-terminal regulator to obtain +5V working voltage. The photoelectric switch is composed of the photoresistor RH, the transistor 3DG6 and the fine-tuning resistor RP. During the day, RH is exposed to light and has a low resistance, 3DG6 is in the cut-off state, so the solid-state relay SSR does not work. At night, RH has a high resistance without strong light, 3DG6 obtains a suitable bias current to conduct, and the solid-state relay works.

3DG6 requires β>100, RH can use MG45 type non-sealed photoresistor, RP can use fine-tuning resistor. Under the condition of normal natural light irradiating RH in the room, adjust RP to make the solid-state relay in the critical state.