Practical and interesting button-controlled additional circuit

When starting up the computer, it is required to turn on the external devices (such as headphones, webcams, digital cameras, monitors, laser printers, etc.) first, and then turn on the host. When shutting down the computer, the host should be turned off first, and then the external devices. This is to ensure that the computer is not damaged by the impact current generated by the external device on/off and to protect data from loss. However, in actual work, users often make mistakes in the on/off sequence for many reasons, causing human failures. Here is a simple and practical additional circuit that can ensure that the computer will not have a wrong on/off sequence. If the on/off sequence is wrong, the circuit will not work, because it can avoid unnecessary human failures that cause data loss and other phenomena.

The principle of the button-controlled additional circuit is shown in the figure. KA1 and KA2 are two 220V AC small relays (any type is acceptable), and X1 and X2 are sockets for plugging in external devices and computers respectively. When starting the computer, first press the start button "SB1", KAl-1 is energized, and the external socket X1 is powered; then press "SB2", KA2-1 is energized, and the computer socket is powered.

Obviously, this device is only effective if "SB1" is pressed first and then "SB2". However, the power switches of the computer and external devices using this device should be in the on position in advance, and their startup is completed by the power buttons "SB1" and "SB2".

After KA2 is energized, its normally open contact KA2-2 is closed, short-circuiting the shutdown button SB3. Therefore, when shutting down, you must first press "SB3" to release KA2, power off the computer, disconnect KA2-2, and then press "SB4" to release KA1 and power off the peripherals. This achieves the purpose of turning on the peripherals first and then the computer; turning off the computer first and then the peripherals. Practice has proved that as long as the wiring is accurate and in accordance with the operating procedures, the additional circuit is impartial and fair.