Automatic emergency light control circuit design

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The automatic emergency light introduced in this example does not work when there is light during the day or night. It can automatically light up after turning off the light at night or during power failure. It will automatically turn off after a delay.

Circuit working principle: The automatic emergency light circuit is composed of a light control lamp circuit, an electronic switch circuit and a delay lighting circuit. When there is light during the day or night, the photodiode VLS is exposed to light and is in a low-resistance state. The VT is turned off, and the electronic switch inside the IC is turned off due to the voltage of the 5-pin 0V, and the EL is not lit. At this time, the consumption electrode of the whole machine is low. When the night light is weakened from strong to strong, the internal resistance of VLS also begins to increase slowly. VT turns from the off state to the conduction state, and the voltage on R2 also gradually increases. However, due to the DC blocking effect of C1, this slowly changes. The voltage still cannot make the IC's 5-pin voltage higher than 1.6V, so the EL will not light up. If the light suddenly becomes weak at night when the light is turned off or the power is turned off, the VLS is in a high-resistance state, and the VT is rapidly saturated and turned on, causing a large voltage drop on R2. Since the voltage on C1 cannot be abrupt, a trigger voltage greater than 1.6V is generated on the 5th pin of the IC, so that the electronic switch inside the IC is turned on, and the EL is energized. At the same time, the +4.8V voltage charges C2 through R3, VD1 and IC to ensure that even if the VT is turned off, the IC's 5 pin will still have a voltage of 1.6V or more, and the electronic switch inside the IC will remain on, EL still Keep lit. With the charging of C2, the voltage of the 5 pin of the IC gradually decreases. When the voltage is lower than 1.6V, the electronic switch inside the IC is turned off, EL is extinguished, and C2 is discharged through R5, EL, R4 and VD2, for the next work. ready. If S is turned on, the emergency light can be used for continuous illumination during a power outage.

Component selection and debugging: IC selects TWH8778 type electronic switch integrated circuit, VT selects 9015 or 8550 type silicon PNP transistor; VLS selects 2DU series photodiode; VD1 and VD2 select IN4007 or IN4148 type rectifier diode. C1 and C2 select aluminum electrolytic capacitors with voltage resistance above 10V, R1~R4 select ordinary 1/8 or 1/4W metal film resistors, R5 selects 1W metal film resistors, EL selects 3.8V, 0.3A flashlights with small electric beads , S selects the small toggle switch, GB is powered by battery. After all the circuits are installed according to the diagram, they can work normally without debugging.