Power SuppliesUSB Circuit Diagrams

USB Switch Schematic Circuit Diagram

People engaged in experimenting or developing USB connected peripheral hardware often find it frustrating to repeatedly unplug and reconnect the USB plug to re-establish communication with the PC. This inconvenience arises, for instance, whenever the peripheral equipment is reset or when a new firmware version is installed. Apart from being tedious, this repetitive action eventually leads to excessive wear in the USB connector. The solution lies in constructing an electronic isolator, which, with a simple button press, disconnects the peripheral device. This method ensures a reduction in physical wear and tear, bringing tranquility back to the workspace. The circuit achieves this by employing a quad analog switch type 74HC4066, utilizing two of the switches within the package to isolate the data path.

USB Switch Schematic Circuit Diagram

Bistable Flip-Flop Configuration: Classic Design Integration

In this circuit, two of the switches from the 74HC4066 analog switch are configured into a traditional bistable flip-flop arrangement, a setup commonly built using transistors. A power MOSFET is employed to manage the power supply current flowing to the USB device.

Power Management and Initialization: Ensuring Defined State

Capacitor C2 plays a vital role in ensuring the flip-flop always starts up in a defined state when connected to the USB socket (‘B’ in the diagram). Consequently, the peripheral device linked to USB socket ‘A’ remains in the ‘not connected’ state until pushbutton S2 is pressed. This action toggles the bistable, activating both analog gates in the data lines and turning on the MOSFET. The PC can then recognize the USB device.

Disconnection Mechanism: Seamless Device Disengagement

Pressing S1 effectively disconnects the device. Unlike physical USB connectors, this circuit does not sequence the connections. However, the power supply connection strips are deliberately longer than the inner data-carrying strips, ensuring that power reaches the peripheral device before the data signals are established.

Advantages of Electronic Switch: Reliability and Construction Simplification

Unlike physical connectors, the electronic switch does not encounter the same contact problems. Hence, the measures required in the physical connector setup are unnecessary in this circuit. The simplicity of the design allows for easy construction on a small square of perforated stripboard.

Component Selection and Operational Capabilities: Optimal Device Compatibility

The design utilizes the 74HC(T)4066 type analog switch, chosen for its superior characteristics compared to the standard 4066 device. The USB switch is suitable for both low-speed (1.5 MBit/s) and full-speed (12 MBit/s) USB port applications. However, it’s important to note that the properties of the analog switches and perf-board construction do not support high-speed (480 MBit/s) USB operation. Additionally, the IRFD9024 MOSFET can effectively handle currents up to 500 mA for the connected peripheral device without any issues.


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