USB Switch

before the data signals are

Rainer Reusch (Germany) connected. The electronic
switch does not suffer from
Anyone experimenting or the same contact problems
deve l o p in g USB p o r te d as the physical connector
peripheral hardware soon so these measures are not
becomes irritated by the required in the circuit. The
need to disconnec t and simple circuit can quite eas-
connect the plug in order ily be constructed on a small
to re-establish communica- square of perforated strip-
tion with the PC. This proc- board. The design uses the
ess is necessary for exam- 74HC(T)4066 type analogue
ple each time the peripheral switch, these have better
equipment is reset or a new characteristics compared to
version of the firmware is the standard 4066 device.
installed. As well as tiresome The USB switch is suitable
it eventually leads to exces- for both low-speed (1.5 MBit/
sive contact wear in the USB s) and full-speed (12 MBit/s)
connector. The answer is to USB ports applications but
build this electronic isolator the properties of the ana-
which disconnects the peripheral device at data lines and switching the MOSFET on. The logue switches and perf-board construction
the touch of a button. This is guaranteed to PC now recognises the USB device. Pressing will not support hi-speed (480 MBit/s) USB
reduce any physical wear and tear and restore S1 disconnects the device. operation.
calm once again to the workplace. The circuit does not sequence the connec-
tions as a physical USB connector does; the The IRFD9024 MOSFET can pass a current of
The circuit uses a quad analogue switch type power supply connection strips are slightly up to 500 mA to the peripheral device with-
74HC4066. Two of the switches in the pack- longer than the two inner data carrying strips out any problem.
age are used to isolate the data path. The to ensure the peripheral receives power (080848-I)
remaining two are used in a classic bistable
flip-flop configuration which is normally built IRFD9024
T1
using transistors. A power MOSFET switches
the power supply current to the USB device.
IC1.B
Capacitor C2 ensures that the flip flop always PC
USB
3 4 device
powers-up in a defined state when plugged
K2
into the USB socket (‘B’ in the diagram). The K1
D+
3
5
IC1.A +5V 1
peripheral device connected to USB socket ‘A’ 2 D– 2 1 D– 2
1 +5V D+ 3
will therefore always be ‘not connected’ until 13 GND 4
pushbutton S2 is pressed. This flips the bist- USB-B 4
R3 R4

USB -A
10k

10k

GND
able, turning on both analogue gates in the

9 IC1.D IC1.C 11
IRFD9024 C1 14
R1 R2 R5
6 12
1M

1M

1M

IC1
100n 7
8 C2 10
D1
S1 S2
IC1 = 74HC4066
1n
S
G
080848 - 11
D

64 elektor - 7-8/2009