The paper feed detection module uses weak current to supply power. The power used by the AC motor is strong current. When the shredder detects that there is paper loaded, the motor works. This process actually uses weak current to control the strong current. Relays are the most commonly used components for weak current control and strong current. In this design, the working state of the motor has three situations: forward shredding, reverse rewinding, and no rotation at shutdown, and the ON and OFF of a relay can only realize the change of two states, so this system needs to use 2 Relay to achieve the three-state control of the motor.

When the relays K1 and K2 are not engaged, the motor is stopped and the shredder does not work; when the relays K1 are not engaged and K2 is engaged, the motor is rotating and the shredder starts shredding; the relays K1 are engaged and K2 is not During pull-in, the motor reverses and the shredder retracts the paper.If the relays K1 and K2 are pulled in at the same time due to the aging of electronic components and short circuit, the motor is in a power-off state to avoid long-term heating of the motor. Dangerous situations occur.

When the shredder is in standby, the two relays K1 and K2 are not in the closed state. When the paper feed detection sensor detects that there is paper loaded, the forward signal is valid, the voltage comparator U2 outputs a high level, and the relay K2 is turned on. , FWD is connected to 220 V mains, and the drive motor rotates forward; when the overload detection module detects that more than 5 sheets are loaded at a time, the output terminal of the operational amplifier U1 sends out a reverse signal to pull the relay K1 on and K2 off On, REV is connected to 220 V mains and the drive motor is reversed.