This circuit facilitates the automatic switching of a printer’s USB connection between a PC and a laptop. The challenge was to enable occasional printer use by a laptop. While maintaining a constant connection with the PC at all other times. Instead of manually disconnecting the printer from the PC and connecting it to the laptop. This circuit automates the USB connection switch. It features standard type-B USB sockets (K1 and K2) and a USB type-A socket (K3). The laptop’s USB lead is plugged into K2, the PC’s USB lead into K1, and a USB cable from K3 links the printer to the circuit. The PC cable remains connected permanently, whereas the laptop cable is only connected when printing is required.
Managing USB Connections with Relays
Under normal operation, the laptop remains disconnected from K2. The USB signal to the printer is routed from the PC via K1, utilizing the normally closed contacts of relay Re1, passing through K3, and then reaching the printer. When the laptop is connected, the 5-volt power signal on its USB port triggers Re1 to switch over, connecting the printer to both K2 and the laptop. Unplugging the laptop restores printer control to the PC.
Optimizing PCB Design for USB Signals
To ensure optimal functionality, the PCB traces for D+ and D– signals must be minimal in length and ideally, equal. Additionally, the relay employed should be of low power, featuring a 5 V coil current at less than 100 mA, and must have two changeover (c/o) contacts. Switch S1 is necessary only in scenarios where both computers are continuously present and connected to the circuit. It facilitates the selection of the computer granted access to the printer.