Electronic Design

Automatically switch between two serial input devices

Typically I prefer to use the standard serial mouse when working with most programs in Windows, such as Office95, but when drafting with programs like Autocad, I prefer using a trackball. The automatic switchover circuit presented here accomplishes this without requiring the user to shut down, change connectors, and reboot the PC.

It can be constructed in a small box with two sub-D connectors to accept the two mice. A third cable with a sub-D connector is connected to the PC serial port (COM1 or COM2).

The circuit is an automatic switch controlled by two signal detectors. When the first mouse is moved, U1A (a retriggerable flip-flop) disables the second mouse and vice versa. If both the mice are moved simultaneously, only one will gain access to the PC’s serial input. LEDs DS1 and DS2 indicate which mouse is currently connected.

The power supply is external from a wall transformer to avoid overloading the serial lines of the PC. To experiment with self-powered configurations, you must:

  • remove P1,P2,P3, to reduce the current used
  • remove VRI,VR2,C5,C6
  • remove the wall transformer
  • jumper VDD to C4
  • jumper VSS to C7.

See associated figure

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