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Two Keyboards on one PC Schematic Circuit Diagram

Inverted Functionality for Changeover Switch

Unlike typical changeover switches designed for use with two PCs and one keyboard, this circuit operates in reverse. Instead of connecting two PCs to one keyboard, this version allows a single PC to function with two keyboards. K1 is linked to the PC, while K2 and K3 connect to two keyboards. In the idle state, the data outputs of the keyboards remain high. However, when a key is pressed, the keyboard serially transmits the data, causing the data line to fluctuate periodically.

Two Keyboards on one PC Schematic Circuit Diagram 1

Two Keyboards on one PC Schematic Circuit Diagram 2

Signal Detection and Switching Mechanism

This circuit employs a flip-flop circuit centered around IC1 to detect and store the low-level signal received. If the signal originates from K3, pin 6 outputs a high signal. This high signal drives transistor T1 into conduction through resistor R5, activating the relay. Consequently, signals from K3 are routed directly to plug K1. This state persists until a signal from the data line of K2 is transmitted. When this occurs, the flip-flop reverses, and pin 6 goes low.

The low signal at resistor R5 prevents transistor T1 from conducting, returning the relay to its rest position. Now, signals from connector K2 are connected to plug K1. LEDs D1 and D2 serve as indicators, displaying which keyboard is linked to the PC. Due to the relay’s relatively slow switching speed, the initial keystroke may not be accurately transmitted, resulting in a lost first keystroke. Additionally, upon the PC’s initial startup, the flip-flop’s state is random, making it unclear which keyboard is initially connected to the PC.

Semiconductor Transistors: Versatile Electronic Components

Transistors, miniature semiconductors, play a multifaceted role in regulating, controlling, amplifying, and generating electrical signals. Acting as switches or gates, transistors manage the flow of current or voltage. These components typically consist of three layers or terminals made of semiconductor material, each capable of carrying electrical current.


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