Timing dimming low light lighting power saving circuit

On the light switches in corridors, doorways or stairwells, the words "Lights off when people leave" or "Turn off lights as soon as you leave" are often posted in order to save electricity.
However, people often ignore these requirements to turn off the lights, leaving the lights on all night long, which consumes electricity and shortens the life of the bulbs.
    The circuit shown in the figure can effectively solve the problem of "lights go out when people leave". It is a delay switch that automatically turns off the lights. When people
light up the lights in the corridor, doorway or stairway for 5 minutes (or 3 minutes), the lights will automatically go out. This time is enough for people to complete the distance,
including unlocking or Locked time is included. This automatic device can save more than 90% of electricity and is very convenient to use. It only needs to turn on the light once.
    Its working principle is: In Figure 10-20, SB1, SB2, and SB3 are the three switches of the three lights EL1, II, 2, and EL3 respectively located on the three-story staircase
. No matter which switch is pressed, three lights can be lit at the same time. When any switch is pressed to light up the lamp,
current also flows through the primary side of the transformer T. After the voltage is reduced, it is converted into DC through bridge rectification by VD1~VD4 to supply power to the circuit. At this time, because the voltage on cz
cannot change suddenly, the voltage on C2 is zero, so the transistor VT1 is not conducting, making the base of VT2 close to the ground potential and cut off. VT2 is cut off, and the collector
potential is close to the power supply voltage, so that the emitter output device VT3 is turned on, the relay K is closed, and its contact K is self-locking. At this time. The switches pressed in SB1-SB3
have been lifted, and the lights can still light up. As time goes by, when the power supply current charges to Ba through Shu and the voltage on C2 reaches
the turn-on voltage of rri, VT1 outputs a positive pulse to turn on VT2. VT2 is turned on, its collector potential is close to the ground potential, VT3
is cut off immediately, the relay K is released, its contact K is disconnected, the power supply to the loads EL1~EL3 is cut off, and the light goes out.
    In the figure, diode VD5 is an isolation diode, which is used to prevent the electric energy stored in c] from causing the relay K to always be in a conductive state and unable to cut off the
load circuit . Relay K can choose JRX-13F type relay. The bulb power of EL1-EL3 is generally 15W. By adjusting the value of Ri
, the length of lighting time can be adjusted. Generally, 3 to 5 minutes is more suitable.