Automatic power shut-off circuit

When you forget to turn off the power switch, power will be wasted and unsafe, which is never allowed. In order to ensure the safety of the circuit
and save power, this circuit can solve the problem of occasionally forgetting to turn off the power switch, as shown in the figure.
    This power supply mainly has two functions:
    1) It is a 3.7-8.7V adjustable power supply composed of a fixed integrated voltage stabilizing circuit ICl (7805) (the voltage drop of VD1 and VD2 is about 1.3V). Adjusting
the resistance of RP can change the output voltage. Every time the resistance of RP changes by 1000, the output voltage changes by 1V.
    2) When no load is connected, the power supply will automatically turn off the main power supply after a period of delay. This function
is realized by transistors VT1, VT2, diodes VD1, VD2 and capacitor C2. When the load is operating normally, there is a voltage drop of about 1.3V on the diodes VD1 and VD2, causing
VT2 and VT1 to conduct in saturation. Relay K works and the normally open contact is closed to ensure normal power supply to the power circuit. At the same time, capacitor G
is charged to 7-8V through VT2. When the load is disconnected, VT2 is cut off, but due to the discharge of C2. VT1 will continue to remain on: after a
delay, VT1 is cut off, Kl is disconnected, cutting off the entire power circuit, and realizing complete self-shutdown i. The delay time is controlled by
the capacity of C2. When the capacity increases, the delay time becomes longer and vice versa. If you want to restore power supply, just press Sl.
    When designing, choose a transformer with a secondary voltage of 0-12V and 250mA (-ri). When the output current exceeds 200mA according to the parameters shown in the figure, the integrated
voltage stabilizing circuit IC1 needs to add a heat sink. When the secondary side voltage of the transformer exceeds 12V, the parameters of RP and K need to be readjusted. For the selection of other components,
please refer to the markings in the figure.