USB Switching Apparatus
The "USB Switching Apparatus" described on this page is a device which allows easy switching a USB device and the audio jack between two computers.
In my case , it means that thanks to this switch I can use one Dell USB keyboard ( with built-in USB hub ) and one pair of headphones with two computers.
Additionally , this design uses small electromagnetic relays instead of CMOS switches to inhibit the creation of the grounding loop when CRT monitor
with two inputs is used and to minimize the loss of audio signal.
The device consists of three distinguisahable parts : touch sensor , sequence timer and a boost converter for the NIXIE tube.
The touch sensor circuit is capacitive based one because the sensor contact is made from brass and has to be coated with protective , insulating lacquer.
The main part of the sensor is the astable multivibrator with operating frequency ( in untouched state ) greater than its maximal operating frequency.
This condition prevents it from working unless someone touches the brass electrode increasing the multivibrator capacitance and causing it to oscillate.
Signal from the multivibrator is rectified and fed through the schmidt trigger to the clock input of a flip-flop . The Set and Reset inputs
of the flip-flop are used to block the switching when only one of the USB inputs is powered.
The sequence timer is a circuit ensuring proper timing when disconnecting and connecting USB device to the bus. Every USB device has to be powered
few milliseconds before it is connected to the data lines of the bus. Normally it is done by using different length of the contacts in the
USB plug but sice the USB device here is connected using relays a timer assuring right timing has to be used. The timer is built using four
shmidt trigger circuits made from popular , quad voltage comparator - LM339 .
The boost converter produces anode voltage for the NIXIE tube. The NIXIE display tube needs about 160V starting voltage and consumes about 2 milliamperes when it is lit.
. The design of the converter is based on MC34063 switching regulator and external MOSFET.
The complete schematic of the switch in pdf format can be downloaded by clicking picture below :
The switch casing is made from five pieces of copper metalized laminate commonly used for making PCB's and covered with mahogany veneer ( the real mahogany not the plastic imitation ) . The corners of the box are covered with pieces of thin ( 0.3 mm) brass sheet. Both the veneer and the brass is glued with the epoxy resin. Front panel is made from thicker ( 1.5 mm ) brass sheet by cutting the contour with a hacksaw and files . The inscription "Select" was etched in the brass witch iron chloride.
Top and bottom of the casing is made from oak wood milled to fit into the box. The wood is coloured with mahogany wood stain to match the colour of the veneer. The whole casing is covered with a layer of protective , transparent lacquer.
Last pictures show the completed , working "USB Switching Apparatus".
