Close the power switch K2 and press the button switch K1. At this time, the crystal diodes V1 and V2 are turned on and the relay is closed. At the same time, the power source charges the capacitor C. When K1 is turned off, since C has been charged, it will discharge through R and V1V2, so that the transistor will continue to conduct and the relay will still pull. After a period of discharge, when the voltage between the two poles of C drops to a certain value, it is not enough to maintain the transistor to continue to conduct, and the relay is released. The time interval from the disconnection of K1 to the release of the relay is called the delay time. It depends on the size of R and C. Generally, when the C is 100 microfarads, the adjustable resistor R can be adjusted to obtain a delay time of 10 seconds to 90 seconds. If C takes 1000 microfarads, the delay time can be more than 5 minutes.
The diodes connected in parallel on the relay protect the crystal triode from high voltage caused by self-inductance when the relay is de-energized.